JP4240916B2 - Tundish upper nozzle and continuous casting method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an upper nozzle constituting a part of a molten steel supply flow path from a tundish to a mold in continuous casting, and a continuous casting method using the upper nozzle.
[0002]
[Prior art]
The oxidatively refined molten steel is usually deoxidized with Al, and oxygen in the molten steel increased by oxidative refining is removed. Alumina (Al ₂ O ₃ ) particles, which are deoxidation products, are separated from the molten steel due to the density difference from the molten steel, but there is a limit to the separation due to the density difference alone, and the fine alumina particles are suspended in the molten steel. It remains in. In addition, in order to stably reduce oxygen in the molten steel, Al is dissolved in the molten steel after Al deoxidation, and this Al is injected into the tundish from the ladle and in the tundish with the atmosphere. When oxidized by contact, new alumina is produced in the molten steel. When these alumina particles suspended in the molten steel pass through the immersion nozzle made of alumina-graphite, they adhere to and accumulate on the inner wall of the nozzle, and the immersion nozzle is blocked.
[0003]
When the immersion nozzle is blocked, various problems occur in the casting operation and the slab quality. For example, the slab drawing speed has to be reduced, and not only the productivity is lowered, but in severe cases, the casting operation itself must be stopped. Also, the alumina deposited on the inner wall of the immersion nozzle suddenly peels off, becomes large alumina particles and is discharged into the mold, and if this is trapped by the solidified shell, it becomes a product defect, and further, the solidification of this part is delayed. Molten steel can escape and even lead to breakouts. For these reasons, various measures for preventing alumina adhesion have been implemented.
[0004]
As one of the measures for preventing the adhesion of alumina, an inert gas such as Ar is blown into the flow path from an upper nozzle that constitutes a part of the molten steel supply flow path from the tundish to the mold. That is, all or a part of the upper nozzle is composed of porous brick, and alumina particles suspended in the molten steel are trapped by gas bubbles by blowing gas from the porous brick, flow out into the mold, and float on the molten metal surface in the mold. It is said. In this case, there is an effect that the inner wall surface of the nozzle is cleaned by the turbulent flow caused by blowing the inert gas. However, it is important to adjust the size of the gas bubbles and to blow uniformly from the entire porous brick in order to prevent the alumina adhesion efficiently. For example, when the gas bubbles are too large, the trapped amount of alumina particles is small, the adhesion suppressing effect is not exhibited, and disturbance on the hot water surface in the mold is caused, causing mold powder to be involved.
[0005]
From such a point of view, Japanese Patent Laid-Open No. 2-307654 (hereinafter referred to as “Prior Art 1”) proposes an upper nozzle in which pores for gas passage of porous brick have an average diameter of 25 μm or less. According to the prior art 1, by setting the average pore diameter to 25 μm or less, the porous brick is densified, gas is blown uniformly from the entire upper nozzle, and nozzle blockage is prevented. Japanese Patent Laid-Open No. 2000-84647 (hereinafter referred to as “Prior Art 2”) discloses that the back pressure of gas blowing is 1.2 kg / cm ² G or more and the gas blowing flow rate is 5 to 10 Nl / min from the upper nozzle. A method of blowing an inert gas has been proposed. According to the prior art 2, by increasing the back pressure, even if there is some variation in the porosity in each part of the porous brick, it is possible to blow uniformly and prevent nozzle blockage.
[0006]
[Problems to be solved by the invention]
However, it is difficult to stably produce a homogeneous refractory having an average pore diameter of 25 μm or less as in the prior art 1, and the prior art 1 has a problem that the manufacturing cost of the upper nozzle increases. If the average pore diameter is too small, the gas bubbles will be too small, the gas bubbles will be prevented from floating and separating from the molten steel, and the alumina particles trapped in the gas bubbles will remain in the slab, resulting in casting. There was also a problem that the cleanliness of the piece deteriorated. In Prior Art 2, there is a limit to improving the variation in porosity of porous brick, and it is extremely difficult to stably prevent nozzle clogging.
[0007]
The present invention has been made in view of the above circumstances. The object of the present invention is to reduce the gas bubble diameter without requiring a special refractory molding method. It is an object of the present invention to provide a tundish upper nozzle that does not cause clogging of the immersion nozzle without deteriorating the cleanliness of the nozzle and a continuous casting method using the same.
[0008]
[Means for Solving the Problems]
The inventors of the present invention have made extensive studies to solve the above problems. The examination results are described below.
[0009]
As the gas bubble diameter of the inert gas blown from the upper nozzle is smaller, the fine alumina particles suspended in the molten steel are trapped in the gas bubbles and the alumina adhesion on the inner wall of the immersion nozzle is prevented. That is, in order to prevent the alumina from being attached efficiently, the gas bubble diameter needs to be small. Therefore, a method of reducing the gas bubbles to be blown was examined. In the prior art 1, the pore size of the porous brick is adjusted to 25 μm or less mainly by reducing the average particle size of the raw material powder. However, there is a problem in using the raw material powder having a small average particle size. There was a cause of cost increase.
[0010]
The present inventors assumed that there should be some correlation between the porosity of the porous brick, that is, the bulk density, and the pore diameter of the porous brick, that is, the gas bubble diameter, and considered changing the bulk density of the porous brick. . The bulk density depends on the compression molding force when the porous brick is compression molded from the powder raw material, and the bulk density increases as the compression molding force increases. In this case, the porosity decreases as the bulk density increases, and the porosity increases as the bulk density decreases. The bulk density can be obtained by measuring its external dimensions and mass, and can be easily measured as compared with the porosity. Therefore, it is very convenient for evaluating porous bricks.
[0011]
FIG. 1 shows the result of examining the average diameter of pores in the porous brick after changing the bulk density of the porous brick made of an alumina-based refractory. The average pore diameter of the porous brick was determined as follows. That is, when the porous brick was buried in mercury and a pressure corresponding to a specific pore diameter was applied, the amount of mercury sucked into the porous brick was determined, and the ratio of the corresponding pore diameter was determined from this mercury amount. By carrying out this operation under various pressures, the overall pore size distribution was determined, and then the average diameter was determined.
[0012]
The porous brick was made of a refractory having 90 mass% alumina (Al ₂ O ₃ ) and the balance silica (SiO ₂ ), magnesia (MgO), and chromia (Cr ₂ O ₃ ). Incidentally, the true density of these oxides, alumina 4.0 g / cm ^3, silica is 2.6 g / cm ^3, magnesia 3.5 g / cm ^3, chromia is 5.0 g / cm ^3, if alumina When the true density of the porous brick having the above composition containing 90 mass% or more is calculated, it is about 3.9 to 4.1 g / cm ³ . In addition, the bulk density of the porous brick which consists of a conventional alumina type | system | group refractory is about 2.80 g / cm < ³ > or less so that the prior art 1 may also be seen.
[0013]
As a result, as shown in FIG. 1, it can be seen that the average diameter of the pores decreases as the bulk density of the porous brick increases, and accordingly, the average diameter of the gas bubbles blown by controlling the bulk density of the porous brick. Was found to be adjustable. Further, as apparent from FIG. 1, it was also found that when the bulk density is 3.0 g / cm ³ or more, the average pore diameter is 20 μm or less. That is, it was found that the bulk density of the porous brick must be 3.0 g / cm ³ or more in order to obtain gas bubbles having an average diameter of 20 μm or less. In this case, it is necessary to increase the compression molding force as compared with the conventional molding method.
[0014]
On the other hand, if the gas bubbles become too small, the floating / separation of the gas bubbles in the mold is impaired, and the alumina particles trapped in the gas bubbles remain in the slab and the cleanliness of the slab deteriorates. Therefore, too small gas bubbles are not preferable in terms of cleanliness of the slab. Therefore, the distribution of pore diameters when the bulk density was changed was investigated. In this case, the pore distribution in the porous brick according to the prior art 1 was also investigated for comparison. The bulk density of the porous brick according to Prior Art 1 was 2.80 g / cm ³ . The distribution of pores was measured by the method using mercury described above.
[0015]
2, 3 and 4 show the pore size distribution in porous bricks when the bulk densities are 2.96 g / cm ³ , 3.01 g / cm ³ and 3.23 g / cm ³ , respectively. 5 shows the pore size distribution in the porous brick according to Prior Art 1.
[0016]
As shown in FIG. 2 to FIG. 4, it was found that fine pores of less than 10 μm increase when the average diameter of the pores decreases as the bulk density increases. In particular, it was found that the porous brick having a bulk density of 3.23 g / cm ³ has a high frequency of pores of less than 10 μm.
[0017]
On the other hand, as shown in FIG. 5, in the conventional example, it was found that the pore diameter distribution range is wide, and therefore, for example, even if the average pore diameter is 20 μm, the frequency of fine pores having a diameter of less than 10 μm is high. It was also found that in the conventional example, a large number of fine pores of less than 10 μm adversely affects the cleanability of the slab. Compared with the conventional example shown in FIG. 5, the pore size distribution range is narrower in porous bricks whose bulk density is increased to 2.96 g / cm ³ , 3.01 g / cm ³ and 3.23 g / cm ^3. I understood.
[0018]
From these results, by increasing the compression molding force during compression molding and adjusting the bulk density of the porous brick to 3.0 g / cm ³ or more, the pore diameter, that is, the average diameter of gas bubbles becomes 20 μm or less, and the immersion nozzle The knowledge that the alumina adhesion in can be suppressed was obtained. In this case, there is also a finding that the cleanness of the slab is not deteriorated by setting the upper limit of the bulk density to 3.2 g / cm ³ or less so that the frequency of generation of fine gas bubbles of less than 10 μm is reduced. Obtained.
[0019]
The present invention has been made on the basis of the above knowledge, and the tundish upper nozzle according to the first invention constitutes a part of the molten steel supply flow path from the tundish to the mold in continuous casting, and the inside of the molten steel supply flow path A tundish upper nozzle that blows an inert gas into the molten steel flowing down, and is connected to the gas supply means for supplying the inert gas for blowing, and the inert gas is introduced into the molten steel. blown comprising a porous brick, and the porous bricks made alumina-based refractories containing alumina as a main component, and a bulk density of 3.0 g / cm ³ or more on the third porous bricks. It is characterized by being 2 g / cm ³ or less.
[0020]
Moreover, the continuous casting method according to the second invention uses the tundish upper nozzle according to the first invention, and from the tundish upper nozzle into the molten steel flowing down in the molten steel supply channel from the tundish to the mold. It is characterized by casting while blowing an inert gas.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 6 is a view showing an embodiment of the present invention, and is an enlarged view of an upper nozzle according to the present invention. FIG. 7 is a schematic longitudinal section of a continuous casting tundish and mold having the upper nozzle according to the present invention. FIG.
[0022]
The upper nozzle 3 according to the present invention includes a refractory material composed of a porous brick 22 and a nozzle base material 23, an iron skin 24 surrounding the refractory material, and a gas introduction pipe 25 communicating with the porous brick 22. . The opening portion of the shaft core portion of the upper nozzle 3 becomes a part of the molten steel supply channel 11 from the tundish to the mold. The nozzle base material 23 is usually made of high alumina brick in many cases, but is not limited to high alumina brick and may have other compositions. The iron skin 24 serves as a reinforcing material for the porous brick 22 and the nozzle base material 23 and serves to prevent leakage of the inert gas blown into the porous brick 22 to the back surface.
[0023]
The porous brick 22 is a high-alumina refractory containing 80 mass% or more of alumina. This high alumina refractory has high temperature strength and excellent wear resistance against molten steel, and is suitable as a material for the porous brick 22. The porous brick 22 is made of silica, magnesia, chromia or the like in addition to alumina. As these refractory materials, alumina oxide, bauxite, silica stone, quartz, magnesia clinker, chromium oxide and the like can be used.
[0024]
These refractory raw materials are mixed and mixed at a predetermined mixing ratio, and compression molded into the shape of the porous brick 22 for the upper nozzle 3. The refractory material used for the porous brick 22 does not need to be pulverized to a fine powder, and a normal particle size is sufficient. The compression force is adjusted so that the bulk density of the compression-molded porous brick 22 is 3.0 g / cm ³ or more. However, if the bulk density exceeds 3.2 g / cm ³ , fine gas bubbles less than 10 μm increase and the cleanliness of the slab deteriorates. Therefore, the bulk density is preferably 3.2 g / cm ³ or less. .
[0025]
A gas introduction pipe 25 is attached to the molded porous brick 22, and the porous brick 22 to which the gas introduction pipe 25 is attached and a refractory material constituting the nozzle base material 23 in a predetermined position in the mold for molding the upper nozzle 3. And the upper nozzle 3 is molded by compression molding them. Then, the molded upper nozzle 3 is baked to finish as a continuous casting tundish upper nozzle 3.
[0026]
Another manufacturing method of the upper nozzle 3 is that a refractory raw material in which alumina oxide, bauxite, silica, quartz, magnesia clinker, chromium oxide and the like for constituting the porous brick 22 are mixed in a predetermined mixing ratio, and a nozzle mother. Preparing a refractory material constituting the material 23 and a gas introduction pipe 25, placing them at a predetermined position in a mold for molding the upper nozzle 3, compression molding to mold the upper nozzle 3, and In this method, the molded upper nozzle 3 is fired and finished as a continuous casting tundish upper nozzle 3.
[0027]
Also in this case, the compression force is adjusted so that the bulk density of the porous brick 22 after compression molding is 3.0 g / cm ³ or more and preferably 3.2 g / cm ³ or less. In this case, since the bulk density of only the porous brick 22 in the upper nozzle 3 cannot be measured, the upper nozzle 3 is formed by variously changing the compression force in advance, and the porous brick 22 is cut out from the formed upper nozzle 3. The bulk density of the porous brick 22 is measured, and the compressive force and the bulk density are adjusted so that the bulk density of the porous brick 22 is 3.0 g / cm ³ or more and preferably 3.2 g / cm ³ or less. The upper nozzle 3 having a predetermined bulk density can be obtained by performing the compression molding process based on the above relationship.
[0028]
Although continuous casting of molten steel is performed using the upper nozzle 3 having such a configuration, as the continuous casting equipment using the upper nozzle 3 according to the present invention, for example, a continuous casting equipment as shown in FIG. 7 can be used. .
[0029]
In FIG. 7, the outer shell is covered with an iron skin 15 at a predetermined position above the mold 2 constituted by the opposed mold long side 13 and the opposed mold short side 14 housed in the mold long side 13. The tundish 1 enforced by the refractory 16 is arranged, and the upper nozzle 3 according to the present invention is provided at the bottom of the tundish 1 so as to be fitted to the refractory 16. Although not shown in FIG. 7, the gas introduction pipe 25 attached to the upper nozzle 3 is connected to a gas supply main pipe provided with a flow meter, and an inert gas such as Ar supplied from the gas supply main pipe. Is blown into the molten steel supply flow path 11 from the upper nozzle 3. A sliding nozzle 4 including an upper fixing plate 5, a sliding plate 6, a lower fixing plate 7, and a rectifying nozzle 8 is disposed in contact with the lower surface of the upper nozzle 3, and further contacts the lower surface of the sliding nozzle 4. An immersion nozzle 9 having a pair of discharge holes 10 is disposed in the lower portion, and a molten steel supply channel 11 from the tundish 1 to the mold 2 is formed. The sliding plate 6 is connected to the reciprocating actuator 12, and the reciprocating actuator 12 is operated to move between the upper fixing plate 5 and the lower fixing plate 7 while being in contact with these fixing plates. The amount of molten steel passing through the molten steel supply flow path 11 is controlled by adjusting the opening area formed by the plate 6 and the lower fixing plate 7. The immersion nozzle 9 is used with its tip immersed so that the discharge hole 10 is embedded in the molten steel 17 in the mold 2.
[0030]
Using the continuous casting equipment configured as described above, the sliding steel 4 is injected into the molten steel 17 injected into the tundish 1 from a ladle (not shown) with an inert gas such as Ar from the upper nozzle 3. While adjusting the molten steel flow rate, the discharge flow 18 is injected into the mold 2 from the discharge hole 10 toward the mold short side 14 through the molten steel supply flow path 11. The injected molten steel 17 is cooled in the mold 2 to form a solidified shell 19 and is continuously drawn below the mold 2 to form a slab. Mold powder 21 is added onto the molten steel surface 20 in the mold 2 for casting.
[0031]
The flow rate of the inert gas blown from the upper nozzle 3 is not particularly limited, and may be a normal blown amount of about 5 to 15 Nl / min, for example. By making the bulk density of the porous brick 22 of the upper nozzle 3 in the range of 3.0 to 3.2 g / cm ³ , the average diameter of the inert gas bubbles becomes 12 to 20 μm and fine gas bubbles of less than 10 μm. Occurrence frequency can be suppressed. Therefore, the alumina particles suspended in the molten steel 17 flowing down in the molten steel supply flow path 11 are captured by the inert gas bubbles blown from the porous brick 22 and flow out into the mold 2, and gas together with the inert gas bubbles. It floats on the molten steel surface 20 by the buoyancy of the bubbles and is absorbed by the mold powder 21.
[0032]
As a result, the alumina adhesion on the inner wall surface of the molten steel supply flow path 11 starting from the inner wall surface of the immersion nozzle 9 is suppressed, the nozzle blockage by alumina is prevented, and the castable time can be greatly extended. . Further, since the alumina particles flowing into the mold 2 float and are absorbed by the mold powder 21, the cleanability of the cast slab is not deteriorated. Furthermore, since the coarsening due to the adhesion / deposition of alumina particles on the inner wall of the immersion nozzle 9 can be prevented, the large inclusions in the slab caused by the exfoliation of the coarsened alumina can be greatly reduced.
[0033]
In the above description, the example in which the two locations of the upper nozzle 3 are the porous bricks 22 has been described. However, the entire upper nozzle 3 may be the porous brick 22, or one or two or more locations may be the porous brick 22. . Further, the individual apparatuses of the continuous casting machine are not limited to the above, and for example, the function may be the same so that a sliding nozzle having a two-plate structure may be used instead of the sliding nozzle 4 having a three-plate structure. Any device may be used.
[0034]
【Example】
The results of test casting in which the continuous casting equipment shown in FIG. 7 is used and the bulk density of the upper nozzle shown in FIG. 6 is changed in the range of 2.83 to 3.23 g / cm ³ will be described. The composition of the porous brick of the upper nozzle was an alumina-based refractory containing 90 mass% alumina, 6 mass% silica, 2 mass% magnesia, and 2 mass% chromia.
[0035]
10 Nl / min Ar was blown from the upper nozzle, and continuous casting was performed for 4 hours or more. The used immersion nozzle was collect | recovered after casting, and the alumina adhesion amount of the immersion nozzle inner wall in the position of 200 mm from the immersion nozzle lower end was measured. The measurement results are shown in FIG.
[0036]
As is apparent from FIG. 8, when the bulk density of the porous brick is 3.0 g / cm ³ or more, the amount of alumina deposited is substantially measured although a very small amount of alumina is partially deposited on the inner wall of the nozzle. There were very few results. On the other hand, in the range where the bulk density is smaller than 3.0 g / cm ^3, it was found that the amount of adhered alumina increases as the bulk density decreases.
[0037]
Moreover, as a result of rolling the cast slab into a thin steel plate and investigating inclusion physical property defects in the thin steel plate, the cast slab cast with the bulk density of the porous brick in the range of 3.00 to 3.23 g / cm ³ It was found that no surface defects due to the piece of alumina occurred. That is, it was found that the cleanability of the slab was extremely high.
[0038]
【The invention's effect】
According to the present invention, since the gas bubbles to be blown are reduced by setting the bulk density of the porous brick made of the alumina-based refractory to 3.0 g / cm ³ or more, it is stable without using a special refractory molding method. The blown gas bubbles can be reduced to 20 μm or less. As a result, alumina adhesion on the inner wall surface of the submerged nozzle is suppressed, nozzle clogging with alumina is prevented, the casting time can be greatly extended, and the alumina particles flowing into the mold float. Therefore, the cleanliness of the cast slab is not deteriorated. Furthermore, since the coarsening due to the adhesion / deposition of alumina particles on the inner wall of the immersion nozzle can be prevented, the large inclusions in the slab caused by the exfoliation of the coarsened alumina can be greatly reduced.
[Brief description of the drawings]
FIG. 1 is a diagram showing the relationship between the bulk density of a porous brick and the average pore diameter.
FIG. 2 is a diagram showing a pore size distribution in a porous brick having a bulk density of 2.96 g / cm ³ .
FIG. 3 is a diagram showing a pore size distribution in a porous brick having a bulk density of 3.01 g / cm ³ .
FIG. 4 is a diagram showing a pore size distribution in a porous brick having a bulk density of 3.23 g / cm ³ .
FIG. 5 is a diagram showing a distribution of pore diameters in a conventional porous brick.
FIG. 6 is a diagram showing an example of an embodiment of the present invention, and is an enlarged view of an upper nozzle according to the present invention.
FIG. 7 is a schematic longitudinal sectional view of a continuous casting tundish and mold having an upper nozzle according to the present invention.
FIG. 8 is a diagram showing measurement results of the amount of alumina adhered to the inner wall of the immersion nozzle in the example.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Tundish 2 Mold 3 Upper nozzle 4 Sliding nozzle 9 Immersion nozzle 11 Molten steel supply flow path 16 Refractory 17 Molten steel 22 Porous brick 23 Nozzle base material 24 Iron skin 25 Gas introduction pipe

Claims (2)

A tundish upper nozzle that forms part of the molten steel supply flow path from the tundish to the mold in continuous casting and blows inert gas into the molten steel flowing down the molten steel supply flow path. A gas supply means for supplying a gas, and a porous brick that communicates with the gas supply means and blows an inert gas into the molten steel, and the porous brick contains alumina as a main component. And the bulk density of the porous brick is 3.0 g / cm ³ or more and 3.2 g / cm ³ A tundish upper nozzle characterized by: Continuous casting, wherein the tundish upper nozzle according to claim 1 is used for casting while blowing an inert gas from the tundish upper nozzle into the molten steel flowing down in the molten steel supply flow path from the tundish to the mold. Casting method.

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