JP2004098082A - Method for casting molten stainless steel performing electromagnetic stirring - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for casting molten stainless steel performing an electromagnetic stirring by which the yield of good cast slab is improved and the cast slab excellent in the quality can stably be produced by preventing defect caused by blow hole and inclusion developed on the surface of the cast slab and inclusion defect in the inner part of the cast slab when the molten stainless steel is cast by using a vertical-bending type continuous caster. <P>SOLUTION: In the method for casting the molten stainless steel by using the vertical-bending type continuous caster, by which the molten steel in a tundish is poured into a mold through an immersion nozzle by using the bending type continuous caster having the vertical part, the molten steel poured into the mold is cast while performing the electromagnetic stirring. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a stainless steel that performs electromagnetic stirring that can improve the quality of a slab by reducing surface defects or internal defects of the slab when casting molten stainless steel using a vertical bending type continuous casting machine. The present invention relates to a method for casting molten steel.
[0002]
[Prior art]
Conventionally, molten stainless steel produced in a refining furnace such as a converter or electric furnace is cast from a tundish into a mold through an immersion nozzle, and cooled by the mold and a cooling zone arranged below the mold. A method of producing a slab using a so-called continuous casting apparatus that performs continuous casting while the casting is being used. (Cited Document 1) and (Cited Document 2). However, inclusions such as oxides and slag in molten stainless steel (molten steel) or mold powder are trapped by solidified shells (solidified shells) during the solidification process of slabs, resulting in surface defects of the slabs, This is a factor that causes surface flaws and cracks during processing. In addition, when molten steel is poured from a tundish into a mold, inclusions adhere to the interior of the immersion nozzle, causing a change in the pouring amount, interruption of the pouring due to nozzle clogging, and the like.
[0003]
In order to prevent the inclusion of inclusions inside the immersion nozzle, casting is performed while blowing an inert gas such as argon gas into the immersion nozzle. However, the injected argon gas, etc., is discharged from the discharge port of the immersion nozzle into the mold, forming bubbles of various sizes, large ones float, and small fine bubbles accompany the discharge flow of molten steel. The slab may penetrate deep into the slab or become trapped by the solidified shell, which is solidifying and increasing in thickness, resulting in cellular defects.
Further, oxide inclusions accompanying the discharge flow also penetrate deep into the slab, and are caught by the solidified shell or form an accumulation zone inside, which causes surface or internal defects.
[0004]
As a countermeasure, as described in Patent Document 3, the casting condition of a molten stainless steel is adjusted by using a vertical bending type continuous caster and adjusting the casting conditions by changing the angle of the discharge port of the immersion nozzle upward from 5 ° to downward. °, the immersion depth is 150 to 300 mm below the meniscus and poured into the mold, and casting is performed at a casting speed of 0.8 to 1.8 m / min, resulting from inclusions and argon gas bubbles. It has been proposed to improve productivity by high-speed casting while preventing such defects.
[0005]
[References]
(A) Reference 1 (Japanese Patent Publication No. 61-39144)
(B) Reference 2 (Japanese Patent Application Laid-Open No. 3-174962)
(C) Reference 3 (Japanese Patent Laid-Open No. 6-262302)
[0006]
[Problems to be solved by the invention]
However, in the method described in Patent Document 3, the casting conditions such as the angle of the discharge port of the immersion nozzle and the immersion depth from the meniscus of the immersion nozzle, and the casting speed are set to be in a specific range. In normal continuous casting operation, the low-speed region immediately after pouring from the immersion nozzle into the mold through the tundish, the stable casting region where pouring and drawing are stable, and the low-speed region at the end of casting Always occurs.
[0007]
Under the conditions described in the cited document 3, in the low speed region, in the early stage of casting, at the end of casting, or in the unsteady portion where slow casting such as breakout or minor equipment trouble occurs during casting, molten steel from the immersion nozzle is Since the discharge flow is reduced, the shell-washing effect of the inner surface of the solidified shell due to the discharge flow of molten steel is reduced, bubbles and inclusions are trapped in the solidified shell, the amount of which increases, and the quality of the slab is deteriorated. There's a problem.
[0008]
Furthermore, even in a region where casting is stable, the gap between the mold and the immersion nozzle becomes narrower around the immersion nozzle immersed in the mold, causing stagnation in the flow of molten steel. The inclusions and argon gas bubbles are trapped in the thinned part (surface layer) of the solidified shell due to a decrease in temperature and a decrease in the shell washing effect. There is a problem that yield is worsened.
[0009]
The present invention has been made in view of such circumstances, and defects caused by bubbles and inclusions generated on the surface of the slab when casting molten stainless steel using a vertical bending continuous machine, and the inside of the slab An object of the present invention is to provide a method for casting stainless steel molten steel that performs electromagnetic stirring and that can prevent inclusion defects and increase the yield of good cast slabs and stably produce cast slabs of excellent quality.
[0010]
[Means for Solving the Problems]
In accordance with the present invention, there is provided a method for casting molten stainless steel with electromagnetic stirring according to the present invention, which comprises using a bending type continuous casting machine having a vertical portion to pour molten steel in a tundish into a mold through an immersion nozzle. In a method of casting molten stainless steel using a casting machine, molten steel poured into a mold is cast while being electromagnetically stirred. According to this method, the discharge flow of the molten steel from the immersion nozzle is reduced in the unsteady portion and the steady portion where the slow casting is performed, and the shell washing effect on the inner surface of the solidified shell due to the discharge flow is reduced. By forming an agitated flow, the shell washing effect on the inner surface of the solidified shell can be enhanced, bubbles and inclusions trapped in the solidified shell can be removed, and a slab with a good quality surface layer can be manufactured. it can.
[0011]
Here, it is preferable that the angle of the discharge port of the immersion nozzle is 5 degrees upward to 35 degrees downward, and the immersion depth is 150 to 300 mm for casting. As a result, it is possible to suppress the downward flow in which the discharge flow from the immersion nozzle hits the solidified shell and intensifies the downward flow, and suppresses bubbles and inclusions accompanying the molten steel from penetrating deep into the slab. it can. In addition, it is possible to prevent the fluctuation of the molten metal level due to the upward flow formed by inversion and the entrainment of the powder.
[0012]
If the angle of the discharge port of the immersion nozzle exceeds 5 degrees upward, the upward flow becomes strong, and this upward flow causes entrainment of the powder. On the other hand, when the angle of the discharge port exceeds 35 degrees downward, the downward flow becomes strong, and bubbles and inclusions accompanying the molten steel penetrate into the deep part of the slab and form bubbles and inclusions on the surface layer and inside of the slab. The resulting defects are likely to occur. On the other hand, when the immersion depth is less than 150 mm, fluctuations in the molten metal level and powder entrainment due to the discharge flow occur. On the other hand, if it is deeper than 300 mm, the number of bubbles and inclusions accompanying the downward flow increases, and the quality of the inside of the slab is impaired.
[0013]
Further, the casting speed is preferably set to 0.8 to 1.8 m. As a result, the amount of molten steel discharged from the discharge port is suppressed, the discharge flow of molten steel is adjusted to an appropriate flow, and bubbles entering the deep part of the slab accompanying powder entrainment or downward flow caused by upward flow And inclusions can be suppressed. In addition, the casting speed is set to 0.8 to 1.8 m for high-speed casting, thereby promoting heat supply to the vicinity of the meniscus in the mold to generate the deckle due to a decrease in the temperature of the molten steel and to reduce the pinching and pinching due to insufficient melting of the powder. Generation of holes and the like can be suppressed. When the casting speed is less than 0.8 m, the productivity is deteriorated due to a decrease in the casting speed. On the other hand, when the casting speed exceeds 1.8 m, the discharge rate increases, the fluctuation of the molten metal level due to the discharge flow of molten steel, the entrainment of powder, and the penetration of bubbles and inclusions into the slab become deeper, and the inside of the slab becomes This leads to an increase in defects.
[0014]
Preferably, a swirling flow of molten steel is provided below the meniscus in the mold. Thereby, the swirling flow of the molten steel along the inner peripheral wall of the mold can be formed to further enhance the cleaning effect of the solidified shell, and the solidified shell is prevented from being trapped by bubbles and inclusions. In addition, stagnation of the molten steel that occurs in a portion where the gap between the immersion nozzle and the mold is narrow is suppressed, and defects that occur in the surface layer portion of the slab are suppressed, so that a clean slab can be manufactured.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, embodiments of the present invention will be described with reference to the accompanying drawings to provide an understanding of the present invention.
FIG. 1 is an explanatory view of a continuous casting apparatus used for a method of casting stainless steel with electromagnetic stirring according to an embodiment of the present invention. FIG. 2 is a sectional view of an immersion nozzle of the continuous casting apparatus. It is a graph showing the relationship of a product failure index. As shown in FIG. 1, a continuous casting apparatus 1 includes a tundish 3 for pouring and storing a molten steel (molten steel) 2 from a ladle (not shown) and a molten steel 2 attached to a lower portion of the tundish 3 in a mold 7. An immersion nozzle 4 for pouring is provided.
[0016]
Further, a mold 7 for cooling the molten steel 2 poured from the discharge port 5 of the immersion nozzle 4 to form a solidified shell 6 and a slab cooled by the mold 7 to support a slab 8 in a molten state. It has a support segment 9 in which a plurality of watering nozzles for cooling 8 are arranged. The slab 8 is drawn at a predetermined speed by a pinch roll (not shown) while increasing the solidification thickness by cooling. Powder 10 is added to the molten steel 2 in the mold 7 on the meniscus (fluid surface), and part of the molten steel 2 is melted by heat of the molten steel 2 flowing out from the discharge port 5 to form a molten layer. Casting with good lubrication is performed by flowing into the gap between the mold 7 and the solidified shell, and electromagnetic stirring devices 11a and 11b for stirring the molten steel 2 in the mold are provided outside the mold 7.
[0017]
As shown in FIG. 2, the immersion nozzle 4 has two discharge ports 5 symmetrically below the immersion nozzle, and the angle Q of the discharge ports 5 is 5 ° above the axis x of the immersion nozzle 4. It is formed so as to satisfy the range of 35 ° downward, and the immersion depth L is set to 150 to 300 mm. The supply of the argon gas to the immersion nozzle is blown from an upper nozzle arranged above the immersion nozzle 4 (not shown).
[0018]
Next, a method for casting molten stainless steel by electromagnetic stirring using the continuous casting apparatus 1 according to the embodiment of the present invention will be described.
Pour stainless steel molten steel (molten steel) 2 into tundish 3 and when molten steel 2 in tundish 3 has accumulated about 20 to 25 tons, attach it to the bottom of tundish 3 while continuing pouring from a ladle. The pouring from the dipped nozzle 4 to the mold 7 is started. When the molten steel 2 is poured into the mold 7, the molten steel 2 is temporarily held for about one minute, and then, while pouring the molten steel 2, the meniscus of the molten steel 2 is raised to a position about 100 mm below the upper end of the mold 7. After that, a solidified shell is sufficiently formed, and pulling out of the dummy bar is started to perform casting.
[0019]
The state of pouring into the mold 7 is such that the casting speed is infinitely low or the casting speed is less than 0.6 ton / min from the start of casting until the steady speed is reached. Inclusions mixed in the molten steel 2 and bubbles of argon gas blown into the immersion nozzle 4 are captured in the solidified shell generated by cooling the mold 7. Then, a surface defect of the slab occurs, which greatly impairs the quality of the slab.
On the other hand, even during casting at a steady casting speed of, for example, 0.8 to 1.8 m / min, a risk of breakout may occur due to poor lubrication of the powder 10 or fluctuation of the molten metal surface. In this case, the casting speed is adjusted within a range of less than 0.6 m / min from the state where pouring is almost stopped.
[0020]
Furthermore, in the case of continuous casting (for example, ladle replacement and continuous casting), the amount of pouring from the immersion nozzle 4 is extremely reduced due to the relation with the ladle replacement time, and casting is performed. It is necessary to carry out the operation of continuously pouring the next charge. Further, even during casting at a steady casting speed, a drift of the discharge flow occurs due to clogging of the left and right discharge ports 5 of the immersion nozzle 4, and the flow in the mold 7 becomes unstable due to the drift of the discharge flow. Become. Particularly, in the vicinity of the immersion nozzle 4 immersed, the gap between the immersion nozzle 4 and the inner wall is narrow, so that the flow of the molten steel 2 at this location is reduced, and the uniform flow of the slab 8 in the width direction is reduced. Cannot be obtained, and stagnation due to fluctuations in flow occurs, and it is found that the above-described casting, in which bubbles and inclusions are trapped in the solidified shell 6 at the stagnation portion, impairs the quality of the slab 8. Was.
[0021]
Based on these findings, the results of research on how to solve the above-mentioned problems of the unsteady part and the steady part that always occur in the casting process while maintaining high-speed casting show that the unsteady part and the steady part In any of the regions, it was found that it was necessary to positively form a stirring flow (swirl flow) of the molten steel and clean the inner surface of the solidified shell with the swirl flow of the molten steel. Therefore, the electromagnetic stirrers 11a and 11b are arranged in the mold 7, and the electromagnetic stirrers 11a and 11b located in a range of 200 to 350 mm below the meniscus are energized and the molten steel 2 which turns the inner peripheral wall of the mold 4 is energized. To form a flow. By this swirling flow, the surface of the solidified shell can be washed by the flow of the molten steel 2, and a good solidified shell with few bubbles and inclusions can be formed. As a result, the quality of the cast slab could be improved in any of the above-mentioned unsteady part and steady part.
[0022]
Further, the discharge port 5 of the nozzle 4 for immersing the molten steel 2 into the mold forms an upward flow and a downward flow in which the molten steel flow from the discharge port 5 hits the solidification shell 6 and reverses. The strength changes with the angle of 5. Therefore, as shown in FIG. 2, the angle Q is set in the range of 5 degrees upward to 35 degrees downward with respect to the axis x of the immersion nozzle 4, and the immersion depth L of the immersion nozzle is set to 150 to 300 mm. By doing so, formation of an excessive upward flow or downward flow can be prevented.
[0023]
Then, appropriate supply of heat to the vicinity of the meniscus becomes possible, and promotion of melting of the powder and generation of deckle can be suppressed. Furthermore, it is possible to suppress the inclusions from penetrating into the deep portion of the slab, and to prevent the surface layer and internal defects of the slab. When the angle of the discharge port 5 is larger than 5 degrees upward, when the pouring amount (ton / min) is high, the powder 10 is entangled due to an increase in the flow rate of the molten metal caused by the upward flow. On the other hand, when the angle of the discharge port 5 exceeds 35 degrees downward, the downward flow increases, and the above-mentioned bubbles and inclusions penetrate into the deep part of the slab, and are trapped inside to deteriorate the quality. The immersion nozzle 4 may be a commonly used split type or an integrated type attached to a tundish.
[0024]
Further, an argon gas can be blown into the immersion nozzle 4. In this case, inclusions adhere to the inside of the immersion nozzle 4 by blowing 4 NL / min or less from an upper nozzle disposed above the immersion nozzle 4. And the floating of inclusions mixed in the molten steel 2 in the mold 7 can be promoted. When the blowing amount of the argon gas exceeds 4 NL / min, the number of bubbles of the argon gas increases, and the number of bubbles captured by the solidified shell 6 also increases, thereby impairing the quality of the slab 8.
[0025]
Further, in the present embodiment, an immersion nozzle in which the content of either or both of carbon and silica is reduced to zero or less than 5% by mass, or a dolomite component (a main component of which is CaO-MgO) is used. Nozzles can be used. In the case of this hardly adhering immersion nozzle, since there is little adhesion of inclusions on the inner surface of the immersion nozzle and the clogging of the discharge port 5 does not occur, it is possible to perform casting without blowing argon gas, which is caused by argon gas. Since bubble defects can be prevented, more preferable results can be obtained.
[0026]
As described above, by performing casting so as to satisfy the above-described conditions, formation of a clean solidified layer by washing the surface of the solidified shell generated in the unsteady portion or the steady portion, insufficient melting of powder and generation of deckle Casting extremely high quality slabs with high yield and industrially stable, while preventing the surface layer and internal defects of slabs generated by bubbles and inclusions penetrating deep into the slabs be able to.
[0027]
【Example】
Next, an example of a method for casting stainless steel molten steel that performs electromagnetic stirring according to an embodiment of the present invention will be described.
In a mold having a thickness of 250 mm and a width of 1200 mm, the size of the discharge port is set to 65 °, and the immersion nozzle is immersed so as to be 200 mm below the meniscus. Casting was performed by forming a swirling flow of molten steel along the inner wall of the mold. As a result, as shown in FIG. 3, the angle of the immersion nozzle is changed from 5 degrees upward to 35 degrees downward, and an electromagnetic stirrer is arranged in the mold to provide a swirling flow to the molten steel in the mold (MD-EMS). (●), it was possible to cast a high quality cast piece with few surface defects and internal defects, and an extremely good result with a product rejection index of 0.22 or less was obtained. And the product quality of the steel material which roll-processed this slab was few, and the steel material excellent in quality was able to be manufactured.
[0028]
On the other hand, when the angle of the immersion nozzle is changed from 5 degrees upward to 35 degrees downward, the electromagnetic stirring device is not arranged in the mold, and the molten steel in the mold is not stirred (○), the casting speed is 0.7 m. / Min and in the region of 1.6 m / min or more, the product rejection index which is extremely deteriorated due to surface or internal defects as a whole is 0.3 to 1.0. And the variation became large, and the quality of the cast slab was deteriorated. Also, in the case of the present invention, when the casting satisfying the above conditions was performed and an argon gas of 2 NL / min was blown into the immersion nozzle, good rejection due to air bubbles was small and good results were obtained. .
[0029]
Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and all changes in conditions without departing from the gist are within the scope of the present invention. For example, the electromagnetic stirrer may be arranged to stir the molten steel in the mold, or may be arranged to stir the molten steel in the mold, or may apply the swirl flow to the molten steel below the meniscus, in addition to the case where the swirl flow is given to the molten steel below the meniscus. Stirring of molten steel can be used together. Furthermore, an electromagnetic stirrer can be placed in the support segment below the mold to stir the unsolidified portion of the slab to prevent internal defects such as improved solidification structure and formation of center porosity. The argon gas can also be blown from a dipping nozzle or a tundish nozzle.
[0030]
【The invention's effect】
As described above, in the casting method of stainless steel molten steel that performs electromagnetic stirring according to claims 1 to 4, the molten steel in the tundish is cast into a mold through a submerged nozzle using a bending-type continuous casting machine having a vertical portion. In the casting method of molten stainless steel using a vertical bending continuous casting machine that pours molten steel, the molten steel poured into the mold is electromagnetically stirred, so that the washing effect of the solidified shell in the unsteady part and the steady part can be stably exhibited, It is possible to cast a high quality cast piece with few surface and internal defects, and to increase the product yield of the good cast piece.
[0031]
In particular, in the casting method of molten stainless steel that performs electromagnetic stirring according to claim 2, since the angle of the discharge port of the immersion nozzle is 5 degrees upward to 35 degrees downward and the immersion depth is 100 to 300 mm, the casting is performed. It is possible to prevent the discharge flow from the immersion nozzle from hitting the solidified shell and intensify the reversed discharge flow, and it is possible to prevent fluctuations in the molten metal level, entrapment of powder, and bubbles and inclusions accompanying the molten steel from entering the deep part of the slab. And the quality of the slab can be stably improved.
In the casting method of molten stainless steel for performing electromagnetic stirring according to claim 3, since the casting speed is set to 0.8 to 1.8 m, the discharge flow of molten steel from the discharge port is controlled. The flow can be made appropriate, the quality of the slab can be improved, and the casting can be performed with high productivity, and the manufacturing cost can be reduced.
[0032]
In the casting method of molten stainless steel with electromagnetic stirring according to claim 4, since the swirling flow of the molten steel is provided below the meniscus in the mold, a swirling flow is formed along the inner peripheral wall of the mold to form the solidified shell. The cleaning effect can be further improved, bubbles and inclusions are not trapped in the solidified shell, and the stagnation of molten steel that occurs at the narrow gap between the immersion nozzle and the mold is prevented. Thus, clean slabs can be stably manufactured without bubbles and inclusion defects caused by the above.
[Brief description of the drawings]
FIG. 1 is an explanatory view of a continuous casting apparatus used in a method for casting stainless steel molten steel that performs electromagnetic stirring according to an embodiment of the present invention.
FIG. 2 is a sectional view of a continuous casting apparatus immersion nozzle.
FIG. 3 is a graph showing a relationship between a casting speed and a product inconsistency index.
[Explanation of symbols]
1 Continuous casting equipment 2 Stainless steel molten steel (molten steel)
Reference Signs List 3 tundish 4 immersion nozzle 5 discharge port 6 solidified shell 7 mold 8 slab 9 support segment 10 powder 11a, 11b electromagnetic stirring device

Claims (4)

In the casting method of stainless steel molten steel using a vertical bending continuous caster in which molten steel in a tundish is poured into a mold through a submerged nozzle using a bending type continuous casting machine having a vertical portion, the molten steel poured into the mold is A method for casting molten stainless steel that performs electromagnetic stirring, characterized by electromagnetic stirring. The method of claim 1, wherein the angle of the discharge port of the immersion nozzle is 5 degrees upward to 35 degrees downward, and the immersion depth is 150 to 300 mm. A method of casting stainless steel with electromagnetic stirring. 3. The casting method for molten stainless steel with electromagnetic stirring according to claim 1 or 2, wherein the casting speed is 0.8 to 1.8 m. The method for casting molten stainless steel with electromagnetic stirring according to any one of claims 1 to 3, wherein a swirling flow of the molten steel is provided below the meniscus in the mold. Casting method.

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