JP4256723B2 - Continuous casting method for molten steel - Google Patents
Continuous casting method for molten steel Download PDFInfo
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- JP4256723B2 JP4256723B2 JP2003160434A JP2003160434A JP4256723B2 JP 4256723 B2 JP4256723 B2 JP 4256723B2 JP 2003160434 A JP2003160434 A JP 2003160434A JP 2003160434 A JP2003160434 A JP 2003160434A JP 4256723 B2 JP4256723 B2 JP 4256723B2
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- 229910000831 Steel Inorganic materials 0.000 title claims description 44
- 239000010959 steel Substances 0.000 title claims description 44
- 238000000034 method Methods 0.000 title claims description 20
- 238000009749 continuous casting Methods 0.000 title claims description 13
- 238000007654 immersion Methods 0.000 claims description 38
- 238000003756 stirring Methods 0.000 claims description 30
- 238000005266 casting Methods 0.000 claims description 16
- 230000001186 cumulative effect Effects 0.000 claims description 10
- 230000002093 peripheral effect Effects 0.000 claims description 4
- 230000007547 defect Effects 0.000 description 8
- 239000002344 surface layer Substances 0.000 description 8
- 230000037303 wrinkles Effects 0.000 description 4
- PNEYBMLMFCGWSK-UHFFFAOYSA-N aluminium oxide Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 description 3
- 230000000694 effects Effects 0.000 description 3
- 239000000843 powder Substances 0.000 description 3
- 238000005096 rolling process Methods 0.000 description 3
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 2
- 238000005097 cold rolling Methods 0.000 description 2
- 238000005098 hot rolling Methods 0.000 description 2
- 229910018072 Al 2 O 3 Inorganic materials 0.000 description 1
- 229910001209 Low-carbon steel Inorganic materials 0.000 description 1
- 238000005452 bending Methods 0.000 description 1
- 238000004140 cleaning Methods 0.000 description 1
- 238000005336 cracking Methods 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 238000011835 investigation Methods 0.000 description 1
- 229910052742 iron Inorganic materials 0.000 description 1
- 238000012423 maintenance Methods 0.000 description 1
- 230000005499 meniscus Effects 0.000 description 1
- 239000000203 mixture Substances 0.000 description 1
- 230000000737 periodic effect Effects 0.000 description 1
- 238000005554 pickling Methods 0.000 description 1
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 1
- 238000003466 welding Methods 0.000 description 1
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Description
【０００１】
【発明の属する技術分野】
本発明は、溶鋼の連続鋳造方法に関し、浸漬ノズル外周部に付着するアルミナやパウダー等の介在物厚みを低減し、 浸漬ノズルから剥離する介在物を微細で、 無害な大きさとすることにより、薄鋼板におけるスリバー疵等の介在物起因の表面欠陥が極めて少ない鋼を製造する技術に関する。
【０００２】
【従来の技術】
【特許文献１】
特開平06−226409号公報
【特許文献２】
特開平09−182941号公報
【特許文献３】
特開平10−296410号公報
【０００３】
薄鋼板における表面疵低減方法として、表面疵の原因となる数100 μｍ〜数mmサイズの粗大介在物を鋳片表層部から排除するため、(1) 連続鋳造の鋳型内電磁攪拌による凝固シェルへの介在物捕捉防止や(2) 鋳片表層部の手入れによる介在物除去等が知られている。
【０００４】
電磁攪拌による凝固シェルへの介在物捕捉防止方法として、例えば、特開平06−226409号公報では、連続鋳造における鋳型上部の溶鋼を水平方向に攪拌することにより、凝固シェルへの介在物の捕捉を防止して、鋳片表層介在物を低減させる方法が開示されている。また、特開平09−182941号公報では、電磁攪拌の攪拌方向を交互にして、 攪拌の反転周期を１〜10秒で、かつ旋回流速を適正範囲に制御することにより、良好な表層、内部品質を有する鋳片を得る方法が開示されている。しかしながら、これらの方法では、浸漬ノズルからの吐出流等の影響で溶鋼の攪拌が不十分な場合があり、薄鋼板における表面疵を確実に防止することができなかった。
【０００５】
一方、表面手入れによる鋳片表層介在物の除去の例として、特開平10−296410号公報では、鋳片表層のスケール生成量を増加させ、デスケーリング装置でスケールと一緒に表層介在物を除去する方法が開示されている。また、鋳片表層を溶削あるいは研削によって数mm除去することにより、表層介在物を除去する方法が一般的に実施されている。粗大介在物を含む溶鋼中の介在物は鋳型内での浮上によって鋳片の表層部に多く存在する。したがって、これらの方法により薄鋼板の表面疵はある程度改善される。しかし、粗大介在物を完全に除去することは困難であり、手入れした面に粗大介在物が存在すれば、表面疵はやはり発生する。また、当然のことながら鋳片の歩留まりや生産性は悪化する。
以上の技術は、いずれも鋳片表層部分に着目した粗大介在物の除去を狙いとしており、溶鋼中の粗大介在物の量を低減するための方法については述べられていない。
【０００６】
【発明が解決しようとする課題】
本発明は上記のような従来の連続鋳造法に見られる問題点を解決して、溶鋼中の粗大介在物の量を低減することによって、薄鋼板で発生するスリバー疵等の表面欠陥を低減するための連続鋳造方法の提供を課題としている。
【０００７】
発明者は上記課題を解決するため、薄鋼板のスリバー疵と操業条件との関係を調査した結果、▲１▼浸漬ノズル１本当たりの累積鋳造量が増加するとスリバー疵発生率も増加すること、▲２▼浸漬ノズルを交換するとスリバー疵発生率が低減することが分かった。さらに注意深く詳細な調査を重ねた結果、浸漬ノズル外周の付着物厚みが15mm以上になるとスリバー疵発生率が急激に増加することを発見した（図１）。また、この付着物はアルミナ、ハーシナイト（Al2O3 ・FeO)、あるいは鋳型パウダーと地鉄の混合物であり、スリバー疵部の介在物形態と良く一致することが分かった。
【０００８】
浸漬ノズル外周部の付着物と薄鋼板におけるスリバー疵発生は以下のような関係があると推定し、付着物厚みを減らすことで表面欠陥を防止できると考えた。すなわち、▲１▼連続鋳造の鋳型内には浸漬ノズルからの溶鋼吐出流等の流動が常に存在し、この流動により溶鋼中のアルミナや巻き込まれた鋳型パウダー等の粗大介在物が浸漬ノズル外周部に付着する。付着物厚みは鋳造時間、または鋳造量とともに増加する。▲２▼この付着物は溶鋼の湯面変動や浸漬ノズルからの吐出流の偏流が大きいと、浸漬ノズル外側から脱落して、 溶鋼中に侵入する。▲３▼脱落、侵入した浸漬ノズル付着物が鋳片表層の凝固シェルへ捕捉されると、圧延で表面に露出してスリバー疵等の表面欠陥につながる。したがって、浸漬ノズル付着物が厚いほど、表面欠陥になりやすい。
【０００９】
【課題を解決するための手段】
以上の種々の検討を行った結果、 本発明を完成するに至った。本発明は、鋳型外周に電磁攪拌装置を設置して、溶鋼湯面近傍の鋳型内溶鋼を鋳型内周面に沿って水平方向に回転する流れを発生させる溶鋼流動制御を行いながら連続鋳造を行なう溶鋼の連続鋳造方法において、同一攪拌方向の累積鋳造量が浸漬ノズル１本当たりの累積鋳造量の30％〜70％となるように、少なくとも２分以上の反転周期で攪拌方向を変えることにより、浸漬ノズル外周部の付着物厚みを、溶鋼湯面から浸漬長さの1/3 までの平均厚さで、15mm以下に調整して鋳造することを特徴とするものである。
【００１０】
【発明の実施の形態】
以下に本発明の好ましい実施の形態を示す。
本発明では、溶鋼の連続鋳造において、鋳型長辺に面する浸漬ノズル外周部の付着物厚みを、溶鋼湯面から浸漬長さの1/3 までの平均厚さで15mm以下とする。付着物厚みを前記した範囲に調整するには、 鋳型外周に電磁攪拌装置を設置して、溶鋼湯面近傍の鋳型内溶鋼を鋳型内周面に沿って水平方向に回転する流れを発生させる溶鋼流動制御方法において、同一攪拌方向の累積鋳造量が浸漬ノズル１本当たりの累積鋳造量の30％〜70％となるように攪拌方向を変えることにより行うものとする。さらに、このような方法で付着厚みを調整する場合、攪拌の反転周期を少なくとも２分以上とするものとする。
【００１１】
浸漬ノズル外周部の付着物厚みを15mm以下とするのは、図１に示すように、15mm超になると薄鋼板表面のスリバー疵発生率が急激に増加するためである。付着物厚みの測定位置を鋳型長辺に面する側としたのは、浸漬ノズル周方向の付着物厚みがこの部分で最大であったためである。また、付着物は溶鋼湯面から浸漬長さの1/3 部分に多く、付着物厚みは浸漬深さ方向でばらつきが大きかったため、溶鋼湯面から浸漬長さの1/3 部分の平均値を付着物厚みとした。
【００１２】
付着物厚みを減少させるために、 鋳型外周に電磁攪拌装置を設置して、同一攪拌方向の累積鋳造量が浸漬ノズル１本当たりの累積鋳造量の30％〜70％となるように攪拌方向を変えるのは、同一攪拌方向の累積鋳造量が30％未満、および70％超では付着物厚みを減らす効果が小さく、スリバー疵発生率もあまり減少しなかったためである。攪拌方向は周期的に変更するものとし、その反転周期を少なくとも２分以上とするのが好ましい。１サイクルの時間が２分未満では鋳片短辺の湯面の波立ちが大きく、コーナー部で割れが発生するためである。
【００１３】
このように本発明では、鋳型内電磁攪拌を適正に付与して、浸漬ノズル外周部の付着物厚さを減少させることによって、溶鋼の湯面変動や浸漬ノズルからの吐出流の偏流が大きい場合にも、付着物が浸漬ノズル外周部から溶鋼中に脱落して粗大介在物となるのを防止し、圧延でのスリバー疵等の表面欠陥を大幅に低減したものである。
【００１４】
【実施例】
次に本発明の実施例について説明する。
垂直曲げ型連続鋳造機により、鋳片寸法が245mm 厚×1600mm幅、鋳造速度1.7m/min、タンディッシュ内溶鋼温度が1570℃、横出し２孔タイプの浸漬ノズルを使って鋳型両短辺方向へ供給しながら極低炭素鋼の鋳片を製造した。１鍋当たり280 トンの溶鋼を４〜７鍋連続で鋳造した。その後、熱間圧延、酸洗、さらには冷間圧延を実施し、コイル表面品位の調査をした。付着物の影響が大きいノズル最終チャージのスリバー発生率で付着物厚さの影響を比較した。熱間圧延後の板厚は2.5 〜７mm、冷間圧延後の板厚は0.2 〜1.8mm であった。
【００１５】
鋳造方向の有効磁場長さ400mm の電磁攪拌コイルを用い、鋳型内溶鋼を攪拌した。電磁攪拌の最大電流値は600Aとし、コイル上端と溶鋼メニスカス位置を一致させた。電磁攪拌をする場合、攪拌方向は連続攪拌、あるいは交番攪拌とした。交番攪拌の条件は一定時間で攪拌方向を変える周期攪拌とした。
【００１６】
浸漬ノズル外周部の付着物厚さ、スリバー発生率や鋳片コーナー割れ発生状況は表１に示す通りであり、本発明がスリバー発生率を大幅に低減して優れた生産性を示すものであることが確認できた。
【００１７】
なお、表１における＊１、＊２の意味は下記の通りである。
＊１：鋳型長辺に面する側の付着物厚さを、溶鋼湯面位置から浸漬深さの1/3 までを10mm間隔で測定し、 全データを平均した。なお、各浸漬深さ位置での付着物厚さは下記の（１）式により算出した。
付着物厚さ＝（鋳造後の浸漬ノズルの直径―鋳造前の浸漬ノズル直径）／２ (1)＊２：スリバー発生率（％）＝スリバー疵が発生したコイル本数／観察したコイル本数×100 。
１本当たりのコイル重量は約10トン。付着物の影響が大きいノズル最終チャージの結果を使用した。
【００１８】
【表１】
【発明の効果】
本発明によれば、浸漬ノズル外周部の付着物脱落によって生成する粗大介在物が少ない鋳片が得られるので、圧延で発生するスリバー疵等の表面欠陥が少ない薄鋼板が確実に製造できる。また、歩留まり向上によるコスト改善、手入れ量低減による生産性向上を図ることができる。
【図面の簡単な説明】
【図１】本発明の浸漬ノズル外周部付着物厚みとスリバー発生率の関係を説明するための図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for continuously casting molten steel, by reducing the thickness of inclusions such as alumina and powder adhering to the outer periphery of the immersion nozzle, and making the inclusions peeled from the immersion nozzle fine and harmless, thereby reducing the thickness. The present invention relates to a technique for producing steel with extremely few surface defects due to inclusions such as sliver rods in a steel plate.
[0002]
[Prior art]
[Patent Document 1]
Japanese Patent Laid-Open No. 06-226409 [Patent Document 2]
JP 09-182941 A [Patent Document 3]
Japanese Patent Application Laid-Open No. 10-296410
As a method of reducing surface flaws in thin steel sheets, to remove coarse inclusions of several hundred μm to several mm in size that cause surface flaws from the surface of the slab, (1) To solidified shells by electromagnetic stirring in the mold of continuous casting It is known to prevent inclusion inclusions and (2) to remove inclusions by cleaning the surface of the slab.
[0004]
As a method for preventing inclusion trapping in the solidified shell by electromagnetic stirring, for example, in Japanese Patent Laid-Open No. 06-226409, trapping inclusions in the solidified shell is performed by stirring the molten steel in the upper part of the mold in continuous casting in the horizontal direction. A method for preventing and reducing slab surface layer inclusions is disclosed. In JP-A-09-182941, the stirring direction of electromagnetic stirring is alternated, the reversal period of stirring is controlled within 1 to 10 seconds, and the swirling flow rate is controlled within an appropriate range, thereby achieving good surface layer and internal quality. A method of obtaining a slab having the following is disclosed. However, in these methods, the molten steel may not be sufficiently stirred due to the influence of the discharge flow from the immersion nozzle and the like, and surface flaws in the thin steel sheet cannot be reliably prevented.
[0005]
On the other hand, as an example of removal of slab surface layer inclusions by surface care, Japanese Patent Laid-Open No. 10-296410 discloses that the scale generation amount of the slab surface layer is increased and the surface layer inclusions are removed together with the scale by a descaling device. A method is disclosed. Further, a method of removing surface layer inclusions by removing several mm of the slab surface layer by means of welding or grinding is generally carried out. Many inclusions in the molten steel including coarse inclusions are present in the surface layer portion of the slab due to floating in the mold. Therefore, the surface defects of the thin steel plate are improved to some extent by these methods. However, it is difficult to completely remove coarse inclusions, and surface flaws are still generated if coarse inclusions exist on the cleaned surface. Of course, the yield and productivity of the slab deteriorate.
All of the above techniques aim to remove coarse inclusions focusing on the surface portion of the slab and do not describe a method for reducing the amount of coarse inclusions in the molten steel.
[0006]
[Problems to be solved by the invention]
The present invention solves the problems found in the conventional continuous casting method as described above, and reduces the amount of coarse inclusions in the molten steel, thereby reducing surface defects such as sliver flaws generated in the thin steel plate. Therefore, it is an object to provide a continuous casting method.
[0007]
As a result of investigating the relationship between the sliver rod of the thin steel sheet and the operating conditions in order to solve the above-mentioned problems, the inventors have found that (1) when the cumulative casting amount per submerged nozzle increases, the sliver rod generation rate also increases. (2) It was found that the occurrence rate of sliver wrinkles is reduced when the immersion nozzle is replaced. As a result of further careful and detailed investigations, it was discovered that the incidence of sliver wrinkles rapidly increases when the thickness of the deposit on the outer periphery of the immersion nozzle exceeds 15 mm (Fig. 1). In addition, this deposit was alumina, hercinite (Al 2 O 3 · FeO), or a mixture of mold powder and ground iron, and was found to be in good agreement with the inclusion form of the sliver collar.
[0008]
It was estimated that the deposit on the outer periphery of the immersion nozzle and the occurrence of sliver wrinkles in the thin steel plate had the following relationship, and it was thought that surface defects could be prevented by reducing the thickness of the deposit. That is, (1) there is always a flow of molten steel discharge flow from the immersion nozzle in the continuous casting mold, and due to this flow, coarse inclusions such as alumina and molten mold powder in the molten steel are placed on the outer periphery of the immersion nozzle. Adhere to. The deposit thickness increases with casting time or casting amount. (2) This deposit will fall off the outside of the immersion nozzle and enter the molten steel if there is a large fluctuation in the molten steel surface or a large deviation in the discharge flow from the immersion nozzle. (3) If the immersion nozzle deposits that have fallen off and enter are trapped in the solidified shell of the slab surface, they are exposed to the surface by rolling and lead to surface defects such as sliver flaws. Therefore, the thicker the immersion nozzle deposit, the more likely it becomes a surface defect.
[0009]
[Means for Solving the Problems]
As a result of the various studies described above, the present invention has been completed. In the present invention , an electromagnetic stirring device is installed on the outer periphery of the mold, and continuous casting is performed while performing molten steel flow control for generating a flow in which the molten steel in the mold near the molten steel surface rotates in a horizontal direction along the inner peripheral surface of the mold. In the continuous casting method of molten steel, by changing the stirring direction at an inversion cycle of at least 2 minutes or more so that the cumulative casting amount in the same stirring direction is 30% to 70% of the cumulative casting amount per immersion nozzle, The thickness of the deposit on the outer periphery of the immersion nozzle is adjusted to an average thickness from the molten steel surface to 1/3 of the immersion length to 15 mm or less, and cast .
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, preferred embodiments of the present invention will be described.
In the present invention, in the continuous casting of molten steel, the thickness of the deposit on the outer periphery of the immersion nozzle facing the long side of the mold is 15 mm or less in terms of the average thickness from the molten steel surface to 1/3 of the immersion length. In order to adjust the deposit thickness to the above-mentioned range, an electromagnetic stirring device is installed on the outer periphery of the mold, and the molten steel in the mold near the molten steel surface generates a flow that rotates horizontally along the inner peripheral surface of the mold. In the flow control method, the stirring direction is changed so that the cumulative casting amount in the same stirring direction is 30% to 70% of the cumulative casting amount per immersion nozzle. Further, when the adhesion thickness is adjusted by such a method, the inversion period of stirring is set to at least 2 minutes or more .
[0011]
The reason why the thickness of the deposit on the outer periphery of the immersion nozzle is set to 15 mm or less is that, as shown in FIG. The reason for measuring the deposit thickness on the side facing the long side of the mold is that the deposit thickness in the circumferential direction of the immersion nozzle was the maximum at this portion. In addition, the amount of deposits was large in the 1/3 part of the immersion length from the molten steel surface, and the thickness of the deposits varied greatly in the immersion depth direction, so the average value of 1/3 part of the immersion length from the molten steel surface was calculated. It was set as the deposit thickness.
[0012]
In order to reduce the deposit thickness, an electromagnetic stirrer is installed on the outer periphery of the mold, and the stirring direction is set so that the cumulative casting amount in the same stirring direction is 30% to 70% of the cumulative casting amount per immersion nozzle. The reason for the change is that when the cumulative casting amount in the same stirring direction is less than 30% and more than 70%, the effect of reducing the thickness of the deposit is small, and the occurrence rate of sliver flaws does not decrease so much. The stirring direction is periodically changed , and the inversion period is preferably at least 2 minutes. This is because if the time for one cycle is less than 2 minutes, the hot water surface of the short side of the slab is large and cracks occur at the corners.
[0013]
As described above, in the present invention, when the electromagnetic stirring in the mold is appropriately applied and the thickness of the deposit on the outer periphery of the immersion nozzle is reduced, the molten steel surface fluctuation and the deviation of the discharge flow from the immersion nozzle are large. In addition, the deposits are prevented from dropping into the molten steel from the outer peripheral portion of the immersion nozzle to become coarse inclusions, and surface defects such as sliver ridges in rolling are greatly reduced.
[0014]
【Example】
Next, examples of the present invention will be described.
With vertical bending type continuous casting machine, slab size is 245mm thickness x 1600mm width, casting speed is 1.7m / min, molten steel temperature in tundish is 1570 ℃, both sides of mold are short-sided using submerged 2-hole type immersion nozzle Slabs of ultra-low carbon steel were manufactured. 280 tons of molten steel per pan was cast continuously for 4-7 pans. Thereafter, hot rolling, pickling, and cold rolling were performed to investigate the coil surface quality. The influence of the deposit thickness was compared with the sliver generation rate of the nozzle final charge, where the effect of the deposit was large. The thickness after hot rolling was 2.5 to 7 mm, and the thickness after cold rolling was 0.2 to 1.8 mm.
[0015]
The molten steel in the mold was stirred using an electromagnetic stirring coil having an effective magnetic field length of 400 mm in the casting direction. The maximum current value of electromagnetic stirring was 600 A, and the upper end of the coil was matched with the molten steel meniscus position. In the case of electromagnetic stirring, the stirring direction was continuous stirring or alternating stirring. The condition of alternating stirring was periodic stirring in which the stirring direction was changed over a fixed time .
[0016]
The deposit thickness, sliver occurrence rate, and slab corner cracking occurrence situation on the outer periphery of the immersion nozzle are as shown in Table 1, and the present invention greatly reduces the sliver occurrence rate and exhibits excellent productivity. I was able to confirm.
[0017]
The meanings of * 1 and * 2 in Table 1 are as follows.
* 1: The thickness of the deposit on the side facing the long mold side was measured from the molten steel surface position to 1/3 of the immersion depth at 10 mm intervals, and all data were averaged. In addition, the deposit thickness in each immersion depth position was computed by the following (1) formula.
Deposit thickness = (Diameter of immersion nozzle after casting−Diameter of immersion nozzle before casting) / 2 (1) * 2: Sliver generation rate (%) = Number of coils with sliver wrinkles / Number of observed coils × 100 .
Each coil weighs about 10 tons. The result of nozzle final charge with a large influence of deposits was used.
[0018]
[Table 1]
【The invention's effect】
According to the present invention, a slab with less coarse inclusions generated by dropping off the adhering matter on the outer periphery of the immersion nozzle is obtained, so that a thin steel plate with few surface defects such as sliver flaws generated by rolling can be reliably produced. Further, it is possible to improve the cost by improving the yield and improve the productivity by reducing the maintenance amount.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining the relationship between the thickness of a deposit on the outer periphery of an immersion nozzle according to the present invention and the occurrence rate of a sliver.
Claims (1)
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