Patent Application
20250205777
This invention relates to an immersion nozzle for pouring molten steel into a mold in continuous casting of steel, and a mold for a continuous casting machine, and a continuous casting method of steel using the immersion nozzle.
In continuous casting of steel, molten steel is poured into a mold by immersing an immersion nozzle into the molten steel. The flow of the molten steel discharged through a pair of right and left discharge ports in the immersion nozzle collides with the inner wall of the mold on its short side and diverges into an upward flow rising along the inner wall of the mold and a downward flow descending along the inner wall of the mold.
In such a case, especially when the discharge flow velocity is high, an uneven distribution of the flow velocity may occur at the top and bottom of the discharge ports. This may cause unbalanced flow speeds between the left and right sides in the upward and downward flows, or a locally strong discharge flow, resulting in a significant fluctuation of the flow. Such a fluctuation causes poor formation of solidification shell or an occurrence of defects resulting from trapping of bubbles or inclusion in a solidified shell.
In order to solve this problem, a continuous casting method has been studied that can prevent defects due to bubbles or inclusions by slowing down the flow of the molten steel in the mold as well as forming a uniform flow. Based on this insight, a 4-hole type immersion nozzle formed by providing discharge ports for molten steel at two levels, an upper and lower positions, in the vertical direction (4-hole nozzle) is proposed, for example, in Patent Literature below.
Patent Literature 1 discloses a nozzle having a larger area of an upper discharge port than that of a lower discharge port to reduce a maximum descending flow speed as much as possible, and a continuous casting method using the nozzle.
Although the technique disclosed in Patent Literature 1 achieves the reduction of the descending flow speed, the molten steel flow to the upper and lower discharge ports is easily biased by a law of gravity, resulting in a high pressure in the bottom of the nozzle. Therefore, the molten steel is stagnated in the bottom of the nozzle to cause the inclusion present in the molten steel to react and adhere to an inner tube of the immersion nozzle or to cause erosion of the inner tube. Further, the sectional area of each discharge port is relatively small as compared to the 2-hole nozzle, so that there is a problem that the flow of the molten steel is disturbed by the adhesion/erosion to easily obstruct the operation.
The invention is made to solve the above problem and aims to provide a technique of reducing the adhesion of the inclusion in the molten steel to the nozzle or the erosion of the nozzle while properly controlling the flow of the molten steel in the mold.
In order to solve the problem, the inventors studied the pressure distribution inside the nozzle to optimize the opening area ratio of upper and lower discharge ports in a multi-hole immersion nozzle and the flow rate of each discharge port, and as a result, arrived at the invention.
The immersion nozzle according to the invention for solving the problem is an immersion nozzle for supplying molten steel from a storage container of the molten steel to a mold in a continuous casting machine for continuous casting of steel, in which an end of a main body of the immersion nozzle to be immersed into the molten steel in the mold is closed, a pair of discharge ports having a central axis as a symmetry axis is provided in each of an upper and lower position of the main body of the nozzle to be immersed in the molten steel, and an area of an opening part of the lower discharge port is within 1.0 to 1.6 times, inclusive, of an area of an opening part of the upper discharge port.
The immersion nozzle according to the invention may have preferable solution means as follows:
The mold according to the invention is a mold for a continuous casting machine having any of the above immersion nozzles, in which the mold is configured to have an index K which is represented by the following equation (1) and affects a variation of a molten surface is within a range of 0.09 to 0.14:
The mold according to the invention may have preferable solutions to the problem, such as providing an electromagnetic stirring apparatus having a direct current coil and an alternating current coil capable of applying a superposed magnetic field of direct current magnetic field and alternating magnetic field to the molten steel in the mold, outside a long side of the mold positioned above the discharge ports of the immersion nozzle, and providing an electromagnetic braking apparatus having a direct current coil capable of applying a direct current magnetic field to the molten steel in the mold, outside a long side of the mold positioned below the discharge ports of the immersion nozzle.
Further, a continuous casting method according to the invention is characterized by adjusting the index K of a molten surface variation represented by the above equation (1) within a range of 0.09 to 0.14 in continuous casting of steel using any of the above immersion nozzles.
In the continuous casting method of steel according to the invention, the followings are preferable solutions to problem:
According to the immersion nozzle and continuous casting machine of the present invention, steel can be continuously cast without forming a high-pressure portion due to stagnation of a flow at the bottom of an immersion nozzle and without causing a negative pressure in the vicinity of a discharge port, by making the area of a lower discharge port of the immersion nozzle larger than that of an upper discharge port. This can prevent the adhesion of inclusions by reaction with a nozzle refractory or nozzle erosion, and an effect of reducing a risk of inhibiting operation is expected. The immersion nozzle according to the invention is preferably used in continuous casting method of steel.
An embodiment of the invention will be specifically described below. It should be noted that each drawing is schematic and may be different from reality. Also, the following embodiments illustrate the apparatus or method for embodying the technical idea of the invention, and the configuration thereof is not limited to the following. That is, the technical idea of the present invention can be modified within a technical scope disclosed in the claims.
In this embodiment, a sectional area of a lower discharge port 2 is set to 1.0 to 1.6 times a sectional area of an upper discharge port 1. The reason for this is described below.
When using a multi-hole immersion nozzle with upper and lower discharge ports, the focus is typically on how to obtain a reducing effect on the discharge flow rate in order to reduce defects in a cast slab.
However, the inventors have obtained the following knowledge: The flow of molten steel tends to be biased downward due to gravity. Consequently, the pressure at the bottom 3 of the inner tube in the immersion nozzle increases, often resulting in the formation of a stagnant region. Additionally, negative pressure occurs near the discharge port. These two factors contribute to the reaction between the inclusion in the molten steel and the refractory of the immersion nozzle. As a result, inclusions may adhere to the immersion nozzle or cause erosion of the refractory, making stable operation difficult.
In an immersion nozzle having an upper discharge port 1 and a lower discharge port 2, an opening area of the lower discharge port 2 is made larger than that of the upper discharge port 1, whereby the flow between the upper and lower discharge ports is rectified to reduce a retention portion, or stagnant portion formed in the bottom 3 of the immersion nozzle. The size of the stagnant portion causes the change of the area balance between the upper and lower discharge ports and the inner diameter of the nozzle main body in the vicinity of the discharge ports, which is a factor determining the flow of the molten steel, and such a factor exerts on the “continuity” of the flow field of the molten steel, so that it is difficult to predict the influence by such a factor.
In order to control the formation of the stagnant portion resulting from the local high pressure portion or negative pressure portion using the size balance between the upper and lower discharge ports, the influence of the area ratio between the upper and lower discharge ports on the stagnant portion is evaluated by numerical calculation.
The inventors also considered that the stagnant portion in the bottom of the nozzle can be controlled by directing a part of the molten steel flow to collide with the refractory disposed between the upper and lower discharge ports and thereby forcibly directing the part of the molten steel flow to the upper discharge port, and consequently examined the influence of the change in the inner diameter of the nozzle main body in the vicinity of the discharge port on the stagnant portion by numerical calculation.
As shown in
First, 4-hole immersion nozzles No. 1 to No. 5 were subjected to numerical calculation. Note that each nozzle has a straight shape with an inner diameter R of 150 mm and is provided with upper discharge ports and lower discharge ports with an opening shape as shown in Table 1. In the analysis evaluation, a general-purpose thermal-fluid analysis solution STAR-CCM+ was used, and the total pressure distribution at a steady state was determined under the conditions that a pressure near the outlet side of the discharge port was 0 and a maximum flow rate in the nozzle was 3.0 m/s. In Table 1, “longitudinal” represents a vertical direction, and “lateral” represents a horizontal direction.
Among the analytical results of Table 1, the relationship between a maximum pressure inside the immersion nozzle and a ratio SL/SU of the sectional area of the upper discharge port 1 to the sectional area of the lower discharge port 2 is shown in the graph in
Further, among analytical results of Table 1, a relationship between a minimum pressure near the discharge port and a ratio SL/SU of the sectional area of the upper discharge port 1 to the sectional area of the lower discharge port 2 is shown in the graph of
Another analysis was performed on an immersion nozzle with SL/SU of 1.0, specifically, on the relationship between a ratio r/R of an inner diameter r from the upper end of the upper discharge port to the bottom end of the immersion nozzle with respect to an inner diameter R up to the upper end of the upper discharge port of the immersion nozzle and the maximum pressure in the flow path inside the nozzle. Table 2 shows the results of the analysis. Note that the maximum pressure inside the nozzle is normalized to be 1.0 when r/R is 1.0. The relationship between the normalized maximum pressure and the inner diameter ratio r/R is shown in the graph of
As seen from Table 2 and
An actual continuous casting of steel using an immersion nozzle can be performed by mixing an inert gas such as an Ar gas into molten steel through the tundish upper nozzle. This causes the molten steel to be subjected to buoyancy effect of the bubbles, and the formation of the high-pressure area at the bottom 3 of the immersion nozzle can be alleviated.
However, if an excessive amount of the inert gas is mixed, the floatability of the flow discharged through the immersion nozzle will be increased in the meniscus inside the steel casting mold, causing the large molten surface variation, which hinders the operation. Therefore, the amount of the gas to be blown should be properly adjusted.
In a continuous casting method using the above immersion nozzle, an Ar gas or the like may be blown into the nozzle to suppress clogging of the nozzle caused by adhesion of alumina or the like. In particular, bubbles blown out of the upper discharge port 1 together with the molten steel may float up and cause the molten surface to vary.
The possibility of actual execution and the effect of the invention configured as described above will be described with reference to the following examples.
Casting was conducted in a vertical bending type continuous casting machine using the nozzle according to the invention, specifically a nozzle and a casting method described in Table 3. As an indicator of operation stability in Table 3, an eddy current sensor was installed just above a molten surface at a central position of the thickness biased from the short side in the widthwise central direction only by ¼ of the distance W between the short sides of the mold (casting width). The time-varying change of the molten surface level was measured by the eddy current sensor. In this case, the degree of the molten surface level variation in each treatment was represented by an index when the degree of the molten surface level variation in the treatment No. A1 is 100. An average value between the first half and the last half of the casting was used in the evaluation as an index of the operation stability. Note that all the upper and lower discharge ports were opened in a direction opposite to the short side of the mold and the center of the discharge flow was parallel to the long side of the mold.
Table 3 shows that all Invention Examples demonstrate good results as compared to Comparative Examples. When comparing treatments with the same ratio QAr/TP regarding the Ar gas flow blown in from the tundish upper nozzle, the treatments Nos. A2 and A4, each with the inner diameter ratio r/R of a suitable range, show better results than the treatments Nos. A1 and A3, each with the inner diameter ratio r/R of 1.0. When comparing treatments with the same inner diameter ratio r/R, the treatments Nos. A3 and A4, each with the ratio QAr/TP regarding the Ar gas flow blown in from the tundish upper nozzle in an appropriate range, show better results than the treatments Nos. A1 and A2. In particular, the treatment No. A4 shows the lowest average value of the index of the molten surface level variation and develops a high operation stability.
Table 4 shows the index of the molten surface level variation when a treatment was conducted under the conditions of the treatment No. A1 of Example 1, in which the upper discharge port of the immersion nozzle was shifted by an angle θ with respect to the short side of the mold in the circumferential direction of the nozzle. In the treatments Nos. C2 to C4, in which the angle θ was 3 to 10°, the improvement of the molten surface variation was observed as compared to the treatment No. A1. The treatment No. C5 resulted in a slight increase in the molten surface variation. This is considered due to the fact that the influence of reverse flow, which collided with the long side to reach the molten surface, was increased by excessively shifting the discharge port to the long side.
Table 5 shows the index of the molten surface level variation when a treatment was conducted under the conditions of the treatment No. A4 of Example 1 by shifting the upper discharge port of the immersion nozzle by 7° with respect to the short side of the mold in the circumferential direction of the nozzle. In the treatment No. D1, the improvement of the molten surface variation is observed as compared to the treatment No. A4.
Table 6 shows an index of the molten surface level variation when the treatment was conducted in the continuous casting machine of Example 1 using the immersion nozzle with a different opening area ratio SL/SU of the upper and lower discharge ports, with a K value of the equation (1) varied. Note that the inner diameter ratio r/R of the immersion nozzle was set 1.00 and the ratio QAr/TP regarding the Ar gas blown in from the tundish upper nozzle was set to 1.50. When the K value fell within the range of 0.09 to 0.14, the molten surface variation remarkably increased. Meanwhile, when the K value was too small, the effect of suppressing the molten surface variation was small due to the influence of excessive molten steel passing mass, too narrow casting width, or too shallow immersion depth of the nozzle. Also, when the K value was too large, the effect of suppressing the molten surface variation was small due to the influence of too small molten steel passing mass, too wide casting width, or too deep immersion depth of the nozzle. The inventors believe that it is effective to suppress the molten surface variation keeping a proper distance for reducing the flow speed of the molten steel discharged through the immersion nozzle.
An electromagnetic stirring apparatus and an electromagnetic braking apparatus were installed at an upper part and at a lower part, respectively, in the mold in the continuous casting machine of Example 1 as shown in
In this description, “L” as a unit of volume means 10−3 m3, and “t” as a unit of mass means metric ton=103 kg, and “N” as a symbol representing a volume of a gas means a volume at a standard state that a temperature is 0° C. and a pressure is 101325 Pa.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2022-061668 | Apr 2022 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2023/011462 | 3/23/2023 | WO |
