JP7616520B2 - Continuous casting method for steel - Google Patents

Description

The present invention relates to a method for continuous casting of steel in which molten steel is supplied into a mold through a submerged nozzle from a sliding nozzle provided at the bottom of a tundish.

A commonly used continuous steel casting method is to supply molten steel from a ladle to a mold via a tundish, which is an intermediate vessel. During casting, an immersion nozzle installed at the bottom of the tundish is immersed in the molten steel in the mold. For example, when pouring molten steel into a mold for casting a slab with a rectangular cross section, an immersion nozzle with one discharge hole on each side of the mold in the direction of the short side is usually used. A stopper installed in the tundish or a sliding nozzle installed at the bottom of the tundish is used as a device for controlling the flow rate of molten steel passing through such an immersion nozzle.

Generally, sliding nozzles have two or three plates. With two plates, the sliding direction is the width direction (parallel to the long side of the mold), so drift occurs in the width direction. With three plates, the sliding direction is the thickness direction (perpendicular to the long side of the mold), so drift occurs in the thickness direction. On the other hand, with stopper control, it is said that drift is less likely to occur because the molten metal falls evenly from the periphery of the stopper. However, since the stopper is a refractory material and has buoyancy in the molten steel, the axis of the stopper and the axis of the upper nozzle are not necessarily coaxially arranged, and drift cannot be prevented.

Various technologies have been developed to stabilize the discharge flow of molten steel from the submerged nozzle and produce slabs with good surface and internal quality. Patent Document 1 discloses a continuous casting method in which the angle in the horizontal plane between the sliding nozzle and the discharge flow is set to 80 to 90 degrees to prevent the unilateral flow phenomenon of molten steel in the mold. Patent Document 2 discloses an injection method in which the submerged nozzle has a rectangular cross section and the injection flow from the injection nozzle to the mold is maintained as a uniform, low-speed downward flow during casting. Patent Document 3 discloses a continuous casting method in which the discharge hole is shaped like a slit to disperse and homogenize the molten steel flow discharged from the submerged nozzle, thereby producing slabs without surface or internal defects.

Patent Document 4 discloses an immersion nozzle equipped with twisted tape-shaped swirling blades inside. Patent Document 5 discloses a continuous casting method that prevents drift in the molten steel flow from the discharge hole by introducing an inert gas into the immersion nozzle and controlling the internal pressure.

Patent Document 6 discloses a technology in which the cross-sectional shape of the inner bore of the submerged nozzle is a flat shape such as an ellipse, the long axis of which is parallel to the long side of the mold, and the sliding direction of the sliding nozzle is perpendicular to the long axis. This stabilizes the flow of molten steel inside the submerged nozzle.

JP 2002-301549 A Japanese Unexamined Patent Publication No. 58-74257 Japanese Patent Application Publication No. 9-285852 JP 2000-237852 A Japanese Patent Application Publication No. 9-225604 JP 2007-69222 A

However, even with the above-mentioned conventional methods, it has not been possible to stabilize the flow of molten steel discharged into the mold and sufficiently prevent drift. It has not been possible to sufficiently prevent surface defects caused by inclusions called slivers that occur on the coil surface after rolling, or bubble defects called blowholes caused by argon blown in from the submerged nozzle.

The present invention aims to provide a method for continuous casting of steel that solves the above-mentioned problems of the conventional method, and by stabilizing the discharge flow from the submerged nozzle, prevents the entrainment of non-metallic inclusions such as alumina that cause slivers and argon bubbles that cause blowholes, thereby producing slabs with excellent surface and internal quality.

The method for continuous casting of steel according to the present invention, which advantageously solves the above-mentioned problems, is characterized in that, when molten steel is supplied into a mold from a sliding nozzle provided at the bottom of a tundish through an immersion nozzle, the immersion nozzle has an outer cross-sectional shape of an elliptical or streamlined shape at a portion immersed in the molten steel in the mold, the length ratio _D1 /D2 of one axis _D1 of the cross-sectional shape of the inner bore that is approximately parallel to a long side of the mold and another axis D2 perpendicular to the one _axis is in the range of 1.00 to 3.00, the ratio _S1 / _S0 of the cross-sectional area _S1 at the minimum cross-sectional area part of the inner bore to the cross-sectional area _S0 of the nozzle hole of the sliding nozzle is in the range of 0.96 to _1.30 , and the ratio _N1 / _S1 of the area _N1 of one of the discharge holes to the cross-sectional area _S1 is in the range of 0.96 to 1.20.

The continuous casting method for steel according to the present invention comprises the steps of:
The submerged nozzle is arranged so that the direction of the one axis is approximately parallel to the long side of the mold, and at least a pair of discharge holes are provided on both side surfaces in the one axial direction so as to discharge molten steel toward the opposing short side direction of the mold;
b. The submerged nozzle is arranged so that the direction of the one axis is approximately parallel to the long side of the mold, and at least one pair of discharge holes is provided on both side surfaces so that the discharge hole discharges molten steel while being deflected at an angle of 60° or less with respect to the opposing long side of the mold;
c) The submerged nozzle is disposed so that the distance between the other shaft side outer surface and the long side inner wall of the mold is 80 mm or more;
d. Casting is performed while imparting a swirling force to the molten steel in the mold using an electromagnetic stirrer;
This is thought to be a more preferable solution.

According to the present invention, the outer shape of the submerged nozzle immersed in the molten steel in the mold is elliptical or streamlined, the length ratio of the major axis to the minor axis of the cross-sectional shape of the inner hole is specified, and the major axis is arranged parallel to the long side of the mold. This allows the submerged nozzle to be immersed without disturbing the flow of molten steel in the mold near the submerged nozzle, and ensures the surface flow speed of the meniscus. In addition, the ratio of the cross-sectional area of the inner hole of the submerged nozzle to the cross-sectional area of the nozzle hole of the sliding nozzle is specified, and the ratio of the area of the discharge hole of the submerged nozzle to the cross-sectional area of the inner hole is specified. This makes it possible to prevent drift without causing nozzle blockage due to air being sucked into the submerged nozzle.

Furthermore, it is preferable that the long axis of the immersed nozzle is arranged approximately parallel to the long side of the mold, and at least one pair of discharge holes is provided on both sides in the long axis direction. This makes it possible to prevent the molten steel discharge flow from penetrating deep into the unsolidified part from the meniscus.

It is also preferable that the submerged nozzle is arranged with the long axis of its immersed portion approximately parallel to the long side of the mold, and that at least one pair of discharge holes is provided on both sides so that the discharge holes are deflected within a specified angle from the opposing long side of the mold to discharge the molten steel. In this way, a swirling flow can be imparted to the molten steel in the mold. This makes it possible to prevent non-metallic inclusions and the like from being captured by the solidified shell, and to produce a cast piece with excellent surface properties.

Furthermore, it is preferable to optimize the distance between the outer surface of the short axis side of the submerged entry nozzle and the inner wall of the long side of the mold. By doing so, the molten steel can be cast while ensuring a sufficient flow rate of the molten steel near the submerged entry nozzle.

In addition, it is preferable to use an electromagnetic stirrer to give the molten steel in the mold a rotating motion while casting. This prevents nonmetallic inclusions from becoming trapped in the solidified shell, making it possible to produce cast pieces with excellent surface properties.

FIG. 1 is a vertical sectional view of a mold and a submerged nozzle suitable for a method for continuously casting steel according to one embodiment of the present invention. 1A and 1B are diagrams showing the cross-sectional shapes of the minimum cross-sectional area part of the submerged nozzle, in which (a) shows an elliptical shape and (b) shows an axisymmetric streamline shape. 1A and 1B are schematic top views showing the positional relationship between the mold and the submerged nozzle inside the mold, where (a) shows the case where the direction of the discharge hole is parallel to the long side of the mold, and (b) shows the case where the discharge hole is biased toward the long side.

The following is a detailed description of the embodiments of the present invention. Note that the drawings are schematic and may differ from the actual product. The following embodiments are illustrative of devices and methods for embodying the technical ideas of the present invention, and are not intended to limit the configuration to those described below. In other words, the technical ideas of the present invention can be modified in various ways within the technical scope described in the claims.

As a result of analyzing the flow of molten steel inside the submerged entry nozzle and the mold, the inventors gained the following insights, which led to the completion of the present invention.

In the case of conventional submerged entry nozzles, where the cross-sectional shape of the inner bore of the submerged entry nozzle is a perfect circle, when the sliding nozzle slides, a drift in the direction of the sliding nozzle movement occurs inside the submerged entry nozzle because the opening is biased to one side. This drift increases the variation in the flow rate of the molten steel from the submerged entry nozzle discharge hole, and increases the maximum discharge flow rate.

Increasing the maximum discharge flow velocity increases the depth to which the discharge flow penetrates into the unsolidified portion. As a result, inclusions such as alumina, which is a product of deoxidation, and continuous casting powder, as well as argon bubbles injected into the submerged entry nozzle, penetrate deep into the unsolidified portion of the molten steel and remain there without being able to rise to the surface. This leads to product surface defects and internal defects such as cracks. In addition, if the cross-sectional shape of the exterior of the submerged entry nozzle is a perfect circle, the resistance of the immersed part of the submerged entry nozzle to the flow of molten steel in the mold increases, and the meniscus surface flow velocity decreases. It was found that when the meniscus surface flow collides with the submerged entry nozzle, the flow velocity becomes uneven and vortices are formed in the surrounding area, cutting away the continuous casting powder and leading to surface defects.

To prevent these problems, it was found that it is effective to cast the nozzle bore with a flat or streamlined cross-sectional shape such as an ellipse or oval, with its major axis approximately parallel to the long side of the mold.

FIG. 1 is a schematic vertical cross-sectional view showing a schematic configuration of a continuous casting facility for carrying out a continuous casting method of steel according to this embodiment, as seen from the short side of a slab. A sliding nozzle 1 is provided at the bottom of a tundish (not shown). A submerged nozzle 2 is connected to the lower side of the sliding nozzle 1. Molten steel is poured into a mold 3 from the submerged nozzle 2. In the example of FIG. 1, an electromagnetic stirring coil 4 for stirring the molten steel in the mold is installed in the mold 3. The sliding nozzle 1 has a nozzle hole 11 with a cross-sectional area of _S0 . In the example of FIG. 1, the sliding nozzle 1 is a three-plate type that is sandwiched between an upper plate 5 and a lower plate 6, and the direction perpendicular to the long side of the mold 3 is the sliding direction SD. The solidified shell that is cooled and solidified in the mold 3 is pulled out in the casting direction CD.

In this embodiment, the sliding nozzle 1 has a nozzle hole 11 that is a perfect circle. The submerged nozzle 2 has an upper inner hole 21 that is a perfect circle. The submerged nozzle 2 has a lower inner hole 21 that is an ellipse as shown in FIG. 2(a) or an axially symmetric streamlined shape as shown in FIG. 2(b). Since the thickness of the nozzle refractory is almost constant, the cross-sectional shape of the inner hole governs the outer shape. Here, the ellipse includes an oblong shape and does not include a perfect circle. Alternatively, an ellipse may be used, in which the short side of a rectangle is replaced with an arc, resulting in an ellipse having a parallel portion. The streamlined shape refers to a shape that, when placed in a flow, does not generate vortexes around it and is composed of curves that receive the least resistance from the flow. In a uniform flow, the shape is rounded at the front and pointed at the rear.

As shown in Fig. 2(a), an ellipse or oval has a major axis having a length (major axis) _D1 and a minor axis having a length (minor axis) _D2 perpendicular to the major axis. In the axisymmetric streamlined shape shown in Fig. 2(b), the length of one axis parallel to the flow _D1 and the length of a perpendicular line drawn from the peripheral point farthest from the one axis to the one axis are defined as half the length _D2 of the other axis. As will be described later, it is preferable that the molten steel between the immersion nozzle installed at the center (in the width direction and thickness direction) of a rectangular mold in a top view and the opposing long side of the mold be a swirling flow, that is, molten steel flows in opposite directions on either side of the mold center, and by making the outer shape of the immersion part of the immersion nozzle axially symmetric and streamlined, the generation of vortexes on the surface of the molten steel in the mold can be reduced.

In this embodiment, the submerged nozzle 2 is immersed in the molten steel in the mold in an arrangement as shown in Figure 3 (a) or (b). The cross-sectional shape of the minimum cross-sectional area portion 23 of the submerged nozzle 2 is elliptical. In this embodiment, the long axis direction LA of the submerged nozzle 2 is arranged parallel or substantially parallel to the long side of the mold 3 as one axis direction, and the short axis direction SA is the other axis direction. When a horizontal swirling flow is applied to the molten steel in the rectangular mold in top view, the molten steel near the submerged nozzle flows parallel to the long side, so if it is installed as shown in Figure 3 (a) or (b), the flow resistance can be reduced. In this case, the short axis direction SA is perpendicular or substantially perpendicular to the long side of the mold. Here, "arranged substantially parallel to the long side" means that the installation error accuracy of the equipment and devices is allowed, and a deviation of within 5° is allowed.

In the example shown in FIG. 3(a), the submerged nozzle 2 has two discharge holes 22, one on each side of the longitudinal axis LA, so that molten steel can be discharged toward the short side of the opposing mold. At least one pair of discharge holes 22 is sufficient, and two pairs are also acceptable. This arrangement can prevent the molten steel discharge flow from penetrating too far into the casting direction CD from the meniscus.

In the example of FIG. 3(b), a pair of discharge holes 22 are provided so that the molten steel is discharged by deflecting it at an angle θ with the opposing long side of the mold, so that a swirling flow can be formed in the molten steel in the mold, and the solidified shell near the meniscus can be washed to prevent inclusions from adhering. There should be at least one pair of discharge holes 22, and two pairs may be used. The deflection angle θ is preferably 60° or less. More preferably, it is 30° or less. If the deflection angle θ is too large, the discharge flow from the submerged nozzle collides with the solidified shell on the long side, which may cause operational accidents such as internal cracks and breakouts due to delayed solidification of the solidified shell. In addition, the deflection angle θ does not include 0°, and it is more preferable that it is 5° or more from the viewpoint of effectively generating a swirling flow of the molten steel in the mold.

In this embodiment, it is preferable that the sliding direction of the sliding nozzle 1 is perpendicular to the longitudinal direction LA of the inner hole of the submerged nozzle 2. The width of the direction in which the molten steel drifts inside the submerged nozzle 2 can be reduced, allowing the molten steel to flow evenly in the longitudinal direction LA. Therefore, it is possible to reduce the drift of the molten steel inside the submerged nozzle 2 that occurs when the sliding nozzle 1 slides.

In the submerged nozzle 2 having the inner hole 21 of the above-mentioned shape, the length ratio D ₁ /D ₂ between one axial length D ₁ and the other axial length D ₂ must be in the range of 1.00 to 3.00 at the minimum cross-sectional area portion 23 directly above the discharge hole 22. If the length ratio D ₁ /D ₂ is less than 1.00, the flatness of the inner hole 21 is reversed, and it is not possible to effectively prevent the occurrence of drift in the sliding direction of the sliding nozzle 1. In addition, there is a risk that the inner hole cross-sectional area S ₁ of the submerged nozzle 2 becomes too small compared with the nozzle hole cross-sectional area S ₀ of the sliding nozzle 1. On the other hand, if the length ratio D ₁ /D ₂ exceeds 3.00, the shape becomes too flat, increasing the risk of the submerged nozzle being broken, and in the event of alumina clogging occurring during casting, it is difficult to ensure a sufficient molten steel flow rate, making it impossible to control the molten steel level in the mold, and there is a risk that stable operation cannot be continued due to fluctuations in the molten steel level. In the case of the submerged nozzle 2 having a plurality of discharge holes 22 on the top and bottom, the minimum cross-sectional area portion 23 is located at the upper end position of the uppermost discharge hole 22.

As described above, the inner hole 21 of the submerged nozzle 2 not only changes its shape from the top to the bottom, but also reduces its cross-sectional area. The ratio S ₁ /S ₀ between the cross-sectional area S ₁ of the minimum cross-sectional area portion 23 directly above the discharge hole 22 and the cross-sectional area S ₀ of the nozzle hole 11 of the sliding nozzle 1 must be set in the range of 0.96 to 1.30. If this ratio S ₁ /S ₀ is less than 0.96, the supply of molten steel will be insufficient when the opening of the sliding nozzle increases in the non-steady state portion where the height of the molten steel in the tundish decreases. For this reason, the molten steel level in the mold cannot be controlled, and there is a risk that a situation will occur in which stable operation cannot be continued due to fluctuations in the molten steel level. On the other hand, if the ratio S ₁ /S ₀ exceeds 1.30, the effect of the flatness of the inner hole will be reduced, and the effect of suppressing drift in the sliding direction will be reduced. At the same time, there is a concern that the nozzle itself will become larger, increasing the running cost. Furthermore, negative pressure may be generated at the step between the lower nozzle 6 and the submerged nozzle 2, which may cause air to be sucked in. In addition, as the sliding nozzle 1 slides, there is a high possibility that drift may occur in the molten steel inside the submerged nozzle.

At the start of casting, the molten steel poured from the ladle fills the tundish and drops in temperature as it is poured into the mold through the submerged entry nozzle, losing heat to the various refractories. If the temperature drops significantly, it may fall below the freezing point, causing the molten steel to solidify inside the submerged entry nozzle, resulting in a so-called hanging state. In such cases, "manual intervention at the start of casting" is carried out, for example by inserting an oxygen pipe through the outlet hole of the submerged entry nozzle and blowing oxygen onto the solidified surface to melt it. This manual intervention at the start of casting is not only dangerous to workers, but also hinders stable operation of continuous casting, as it is undesirable in terms of quality.

In addition, the ratio N ₁ /S _{1 of the area N 1 of} one of the discharge holes 22 to the cross-sectional area S ₁ must be in the range of 0.96 to 1.20. When the discharge holes 22 are provided in pairs on both sides of the submerged nozzle 2, the area N ₁ of one of the discharge holes 22 is the total area of the discharge holes 22 on one side. When the discharge holes 22 are provided on the bottom of the submerged nozzle 2, the area N ₁ of one of the discharge holes 22 is calculated by adding up the areas of the discharge holes 22 on the bottom. When the surface quality of the product is important, it is preferable to not provide the discharge holes 22 on the bottom of the submerged nozzle, but to provide the discharge holes 22 only on the side. When the ratio N ₁ /S ₁ is less than 0.96, nozzle clogging is likely to occur, the molten steel level fluctuations become large, and the amount of molten steel discharged may be insufficient relative to the required amount, making it difficult to perform stable operation. On the other hand, when the ratio _N1 / _S1 exceeds 1.20, the balance of the discharge rates between the opposing nozzles is lost, and the flow rate of one of the nozzles becomes larger, which is called drift. This drift may send inclusions and the like deep into the unsolidified part of the molten steel in the casting direction CD. Furthermore, nozzle clogging may occur in the part of the discharge port where the flow rate is reduced, increasing the fluctuation of the molten steel surface. In addition, if the drift becomes extremely large, there is a risk of an operational accident such as a breakout.

As shown in Fig. 3(a), the distance L _S between the outer surface on the minor axis side of the submerged nozzle 2 and the inner wall on the long side side of the mold 3 is preferably 80 mm or more. Here, the distance L _S is set to the smaller of the distances to the two opposing long sides of the mold. If the distance L _S is less than 80 mm, a sufficient molten steel flow rate cannot be obtained when a swirling flow is generated in the mold, for example, when the molten steel is electromagnetically stirred, and there is a high possibility that the solidified shell will capture inclusions that cause surface defects.

As shown in Figure 1 and Figures 3(a) and (b), it is preferable to install an electromagnetic stirring coil 4 for electromagnetic stirring in the mold 3. Casting can be performed while imparting swirl to the molten steel in the mold 3. By electromagnetically stirring the molten steel, it is possible to prevent inclusions and the like from being captured by the solidified shell, and to produce a cast piece with excellent surface properties.

Next, examples of the present invention will be described. Note that the present invention is not limited to the following examples.

300 tons of molten steel of ultra-low carbon steel was produced in the converter-RH degassing process. The temperature of the molten steel in the tundish was set to 1560-1580°C, and the molten steel was poured into the mold using a three-layer sliding nozzle, a two-layer sliding nozzle, and a submerged nozzle, to produce a slab of 250 mm in thickness and 1200-1600 mm in width at a casting speed of 1.6-2.0 mm/min. The molten steel was rotated horizontally by electromagnetic stirring during casting. The steel plate was a cold-rolled steel plate of 0.7-1.2 mm. The effect was evaluated based on the coil defect contamination rate (number of defects/coil length (m)) and the presence or absence of nozzle clogging during casting. The results of tests performed by continuous casting under various conditions are shown in Tables 1-1 and 1-2. In the table, the shape of the submerged nozzle inner hole is the shape of the minimum cross-sectional area part 23 directly above the upper discharge hole, and is elliptical except for treatment No. 19, and is elliptical except for treatment No. 20. 19 has a streamlined shape. _D1 is the length of one axis parallel to the long side of the mold (mm), _D2 is the length of the other axis perpendicular to the one axis (mm), _S1 is the cross-sectional area of the minimum cross-sectional area part 23 of the submerged nozzle 2 ( ^mm2 ), _S0 is the cross-sectional area of the nozzle hole 11 of the sliding nozzle 1 ( ^mm2 ), θ is the angle (°) between the direction of the submerged nozzle discharge hole and the long side of the mold, n is the number of discharge holes 22 of the submerged nozzle 2, _N1 / _S1 is the ratio of the area of one of the discharge holes to the cross-sectional area of the minimum cross-sectional area 23 of the submerged nozzle 2, _L1 is the distance (mm) between the outer surface of the short axis side of the submerged nozzle 2 and the inner wall of the long side of the mold, and ηD is the coil defect occurrence rate, which represents the percentage (%/m) of the number of defects per coil length (m). The success rate at the start of casting is the manual intervention rate at the start of casting, and is expressed as a percentage of the number of manual interventions to the total number of casting starts. The smaller the manual intervention rate, the more stable the operation. The rate of abnormality caused by molten metal level fluctuations is expressed as a percentage of the number of molten metal level fluctuations relative to the number of castings. In Tables 1-1 and 1-2, SD indicates the relationship between the sliding direction of the sliding nozzle 1 and the long side of the mold, "orthogonal" indicates a three-layer sliding nozzle, and "parallel" indicates a two-layer sliding nozzle.

For Process Nos. 1 to 19, these are the results of casting tests carried out within the scope of the present invention. The coil defect occurrence rate ηD was also 1.9% or less, and the quality results were very good. In addition, no nozzle blockages occurred during casting. The manual intervention rate at the start of casting was low, and the rate of abnormalities due to fluctuations in the molten metal level was also low, resulting in stable operation.

In the case of the process No. 20, the nozzle flatness ratio _D1 / _D2 was large. In this case, it is estimated that the refractory itself was difficult to manufacture and easily cracked, and the resistance to the passage of molten steel was large, which caused the nozzle to be clogged. In addition, the _D1 / _D2 ratio was too large, so that the shape of the inner hole became flat. When the alumina clogging occurred during casting, the molten steel flow rate was not sufficient and the occurrence rate of abnormalities due to the fluctuation of the molten steel level was high.
Process No. 21 is a case where the ratio _S1 / _S0 was small. In this case, clogging of the nozzle occurred when pouring into the mold. It is presumed that this is because the nozzle inner diameter cross-sectional area was small compared to the nozzle hole cross-sectional area of the sliding nozzle 1. Even if _S1 becomes small, that is, _S0 is fully open, the flow resistance of molten steel is large in the _S1 portion, the molten steel flow rate decreases, and the required amount of molten steel is not supplied. In other words, a hanging state occurs in the _S1 portion, and not only the controllability of the molten steel level fluctuation is significantly reduced, but also the molten steel temperature drop at the start of casting is significantly large, and the manual intervention rate at the start of casting is extremely high, making it impossible to perform stable operation.
In the case of the process No. 22, the ratio _S1 / _S0 was large. In this case, the flow straightening effect was not obtained, and the occurrence rate of abnormalities due to the fluctuation of the molten metal level during casting was high. As a result, the coil quality deteriorated and the nozzle was clogged at the end of the casting process.
In the case of process No. 23, the nozzle flatness _D1 / _D2 is smaller than 1.00, and the length of the major axis and the minor axis are reversed. In addition, the ratio _S1 / _S0 is small. As a result, the resistance of the flow caused by the submerged nozzle is large, and the quality is deteriorated. When _S1 is small, that is, even when _S0 is fully open, the flow resistance of the molten steel is large in the _S1 portion, the molten steel flow rate is reduced, and the required amount of molten steel is not supplied. In other words, a hanging state occurs in the _S1 portion, and not only the controllability of the molten steel level fluctuation is significantly reduced, but also the molten steel temperature drop at the start of casting is significantly large, and the manual intervention rate at the start of casting is extremely high, and stable operation cannot be performed.
In the case of No. 24, the area _N1 of the discharge hole was too small compared to the minimum cross-sectional area _S1 of the nozzle. The coil performance improved, but nozzle clogging during casting was observed. When _N1 was smaller than _S1 , the flow resistance of the molten steel at the discharge hole was large, the flow rate of the molten steel decreased, and the required amount of molten steel was not supplied. As a result, not only was the controllability of the molten steel level fluctuation significantly decreased, but the drop in the molten steel temperature at the start of casting was also significantly large, and the manual intervention rate at the start of casting was extremely high, making it impossible to perform stable operation.
In the case of the process No. 25, the area _N1 of the discharge hole was too large compared to the minimum cross-sectional area _S1 of the nozzle. Because the discharge hole was large, the flow straightening effect was not confirmed, and the occurrence rate of abnormalities due to fluctuations in the molten metal surface during casting was also high. Therefore, the coil quality was not improved.

From the above results, it is clear that stable operation of continuous casting can be achieved from the start to the end by setting all of the characteristics of the present invention, namely, the ratio _D1 / _D2 of the lengths of the perpendicular axes of the cross-sectional shape of the submerged nozzle and the inner bore, the cross-sectional area ratio _S1 / _S0 of the inner bore of the submerged nozzle to the nozzle hole of the sliding nozzle, and the area ratio _N1 / _S1 of the discharge hole of the submerged nozzle to the inner bore, to appropriate values. If any of the conditions of the characteristics of the present invention are not met, stable operation is possible for a short period of time, but any of the factors that affect stable operation, such as controllability during molten metal level fluctuations, starting stability, and alumina clogging, will deteriorate, and stable operation of continuous casting will be impaired.

In this specification, the unit of mass "ton" represents 10 ³ kg.

The continuous steel casting method of the present invention allows stable operation without clogging of the submerged nozzle and reduces the occurrence of defects in the product, making it industrially useful.

REFERENCE SIGNS LIST 1 sliding nozzle 2 submerged nozzle 3 mold 4 electromagnetic stirring coil 5 upper plate 6 lower plate 11 nozzle hole 21 inner hole 22 discharge hole 23 minimum cross-sectional area portion SD sliding direction CD casting direction LA one axial direction (longitudinal direction)
SA Other axis direction (minor axis direction)
_D1: One axis (length), major diameter _D2 : Other axis (length), minor diameter L _S: Distance between the outer surface of the submerged nozzle and the long side of the mold

Claims (6)

When molten steel is supplied into a mold from a sliding nozzle provided at the bottom of a tundish through a submerged nozzle,
The submerged nozzle has an outer cross-sectional shape of a portion that is immersed in the molten steel in the mold, the outer cross-sectional shape being elliptical or streamlined,
a length ratio _D1 / _D2 of one axis _D1 approximately parallel to the long side of the mold in a cross-sectional shape of the inner hole to another axis _D2 perpendicular to the one axis is set in a range of 1.00 to 3.00;
a ratio S ₁ /S ₀ of a cross-sectional area S ₁ at the minimum cross-sectional area portion of the inner hole to a cross-sectional area S ₀ of the nozzle hole of the sliding nozzle is set in a range of 0.96 to 1.30;
A method for continuous casting steel, wherein a ratio N ₁ /S ₁ of an area N ₁ of one of the discharge holes to the cross-sectional area S ₁ is set in the range of 0.96 to 1.20. The method for continuous casting steel according to claim 1, wherein the submerged nozzle is arranged so that the direction of the one axis is approximately parallel to the long side of the mold, and at least one pair of discharge holes is provided on both sides in the direction of the one axis so as to discharge molten steel toward the opposing short side of the mold. The method for continuous casting steel according to claim 1, wherein the submerged nozzle is arranged so that the direction of one axis is approximately parallel to the long side of the mold, and at least one pair of discharge holes is provided on both sides so that the discharge holes are deflected within 60° from the opposing long side of the mold to discharge molten steel. The method for continuous casting of steel according to any one of claims 1 to 3, wherein the submerged nozzle is positioned so that the distance between the outer surface on the other axis side and the inner wall on the long side of the mold is 80 mm or more. A method for continuous casting of steel according to any one of claims 1 to 3, in which casting is performed while imparting swirling force to the molten steel in the mold using an electromagnetic stirring device. 5. The method for continuous casting of steel according to claim 4, wherein casting is carried out while imparting a swirling force to the molten steel in the mold by an electromagnetic stirrer.

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