JP2002254145A - Immersion nozzle for continuous casting having gas blowing function - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an immersion nozzle for continuous casting having a gas blowing function, which has a long service life, has little concern about sudden breakage trouble in a nozzle neck or the like, and can prolong a casting time while preventing the casting defect (blow hole defect) of a steel by exerting the floating effect of inclusions in a molten steel and preventing the clogging of an immersion nozzle hole. SOLUTION: In order to minimize the lowering of back pressure of argon gas during casting even after the dissipation of silica while bringing out a spalling resistance and the effectiveness of silica in an immersion nozzle structure, a melted silica material having a grain diameter of 0.1-0.6 mm is blended in the gas permeable material at an inner hole side of a slit. Thereby, the lowering of gas flow resistance caused by the dissipation of silica can be restrained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas casting type continuous casting immersion nozzle for performing gas blowing by a slit used when pouring molten steel from a tundish into a mold in continuous casting of steel, and more particularly to a slit for casting. On the other hand, the present invention relates to an immersion nozzle for continuous casting, which is characterized by a gas-permeable material disposed on the side of an inner hole through which molten steel flows.

[0002]

2. Description of the Related Art In continuous casting of steel, for example, an immersion nozzle for injecting molten steel from a tundish into a mold plays an important role in preventing oxidation of the molten steel and controlling flow in the mold. Although there are various types of immersion nozzles for continuous casting (hereinafter also referred to as immersion nozzles), there are those which are referred to as gas-blowing immersion nozzles or slit-type nozzles. Usually, as shown in FIG. It is composed of a base material 1 main body, a slit 2 corresponding to a gas pool for gas injection, a permeable material 3 having a porosity of 15 to 40%, and a powder line component 4, and a discharge port 5 is formed at a lower end. An inert gas such as an argon gas is blown from the permeable material 3 toward the inner hole of the immersion nozzle in order to float the inclusions in the molten steel and prevent the immersion nozzle hole from being closed.

As for the base material 1 forming the immersion nozzle, alumina-graphite having low thermal expansion and excellent thermal shock resistance is usually used. The gas permeable material 3 is also made slightly porous for ventilation. However, if the physical properties of the base material and the gas permeable material, especially the thermal expansion coefficient, are significantly different, there is a problem as a structural body. It is considered that the difference in thermal expansion is the main cause, but the permeable material of the inner hole is peeled off from the base material, or the base material is pushed up by the permeable material,
The base material is made of a material similar to the base material because a crack is generated on the base material side and a trouble of neck break occurs.

On the other hand, considering the temperature distribution of the immersion nozzle during molten steel and steel passing, since the temperature on the inner hole side is high and the temperature of the base metal on the outer periphery is low, the inner hole is so formed as to eliminate the difference in thermal expansion between the two. It is desirable that the air permeable material has a lower expansion than the base material. For this reason, the air-permeable material 3 of the inner hole is a low-expansion raw material based on alumina-graphite, and is made of fused silica in order to improve spalling resistance to thermal shock due to rapid heating during molten steel injection. It is generally carried out. As described above, fused silica is an indispensable compounding component for the pore-permeable material.

[0005] On the other hand, however, there is a major problem due to the inclusion of fused silica in the vent material of the inner hole. That is, there is a problem due to the disappearance of the compounded silica during molten steel casting. Various mechanisms have been studied for the disappearance of silica. For example, in the casting of low-carbon aluminum killed steel, the aluminum in the molten steel reacts with the silica in the immersion nozzle to form silicon as shown in the following equation. It is said that there is a reduction reaction by titanium in the casting of low-carbon aluminum-killed steel that elutes in or contains titanium.

4Al + 3SiO ₂ → 2Al ₂ O ₃ + 3Si Ti + SiO ₂ → TiO ₂ + 3Si Further, when used for a long time at a high temperature, the following reaction occurs between silica and carbonaceous components contained in the gas-permeable material. May occur and may be scattered as SiO gas.

In any case, SiO ₂ + C → SiO {+ CO} In any case, the pore diameter caused by the disappearance of silica and the gas permeability of the argon gas bubbled due to the increase in air permeability increase, and this causes casting defects in steel. Bring. For example, the steel sheet after hot rolling, or, annealing, on the surface of the cold-rolled steel sheet, often raised to a width of 1 to 4 mm, a few mm in length, or these several mm of ridges are continuously linearly , A so-called blister-like defect extending over several hundred mm. As a result, the quality of the steel sheet is reduced and the product yield is significantly reduced. It is said that the main cause of the blister defect is that the argon gas blown from the immersion nozzle is trapped inside the slab, and it can be said that the bubble defect due to the argon gas. Such a change in the bubbling function accompanying the disappearance of the silica produces a bubble defect.

The state of the change of the bubbling function accompanying the disappearance of the silica appears as a phenomenon of a decrease in back pressure by blowing argon gas at a constant flow rate during casting, and is monitored by constantly measuring the back pressure. You can do it. In order to avoid the risk of bubble defects, it depends on how to prevent and stabilize the back pressure during casting. And
Breathable materials containing silica have a reduced rate of bubbling due to reduced back pressure during casting, and the longer the casting time, the higher the rate of foaming defects in the steel. Since the occurrence of such a bubble defect is likely to occur as the casting time is prolonged, a situation may occur in which the life of the immersion nozzle itself must be shortened.

In order to solve such a problem originated from the disappearance of silica, it is conceivable to use a non-silica or low-silica air-permeable material. For example, Japanese Patent Application Laid-Open No. 5-1593 discloses that the amount of silica is limited to 5% by weight or less. Further, in order to compensate for a decrease in spall resistance that occurs when the amount of silica is limited to 5% by weight or less, a method of additionally using 5 to 15% by weight of silicon carbide is disclosed in Japanese Patent No. 2891757.

[0010] Although the reduction of silica is very effective in reducing bubble defects, it also causes an increase in the frequency of troubles such as peeling of the inner body of the immersion nozzle and breaking of the neck.

As described above, originally, the life of the immersion nozzle itself should be determined by the corrosion resistance of the base material constituting the immersion nozzle. Is often the case.

[0012]

SUMMARY OF THE INVENTION An object of the present invention is to improve the inner hole, which may be a factor in determining the life of a gas-blowing type continuous casting immersion nozzle, from the viewpoint of the material, and to prevent sudden neck breakage. In addition, the present invention achieves a longer casting time while preventing the casting defects (bubble defects) of the steel by the floating effect of the inclusions in the molten steel and the prevention of the clogging of the immersion nozzle holes, with less dangers.

[0013]

SUMMARY OF THE INVENTION According to the present invention, a permeable material having good back pressure stability due to a change in the permeable property is provided, without forcibly reducing silica from the constituent material of the permeable material. Solved the problem.

Various investigations were made to maintain the back pressure of a certain level or more after the disappearance of the silica while utilizing the spalling resistance and the effectiveness of the silica in the immersion nozzle structure. In order to minimize the decrease in the air permeability, by mixing a fused silica raw material having a particle size of 0.1 to 0.6 mm into the air permeable material of the inner body, it is possible to suppress a decrease in air flow resistance due to the disappearance of silica. Found that it is possible.

That is, the immersion nozzle for gas-blown continuous casting of the present invention has a particle diameter of 0.1 to 0.6 mm at 80%.
It is characterized in that the fused silica raw material occupying 5 to 25% by mass and the permeable material composed of alumina and graphite aggregate are disposed on the inner side of the slit with respect to the slit.

When the particle size of the fused silica raw material is less than 0.1 mm, the specific surface area is increased, thereby increasing the rate of disappearance of silica. In comparison, when the particle size is small, the number of the particles increases, and the structural deterioration of the gas-permeable material due to the disappearance of silica during casting extends throughout the entire structure. Is inferior in wear resistance.

On the other hand, the particle diameter of the fused silica raw material is 0.6 mm.
As described above, as described above, the large grains have a smaller number of grains under a certain unit usage amount, and the structural deterioration of the gas-permeable material due to the disappearance of silica during casting becomes localized, and the effect is reduced. Although it is thought to be smaller, the wear resistance of the molten steel flow is hardly reduced. However, the newly formed gas passage due to the disappearance of silica increases in proportion to the particle size of the disappeared silica, and the resistance to gas passage decreases,
At the same time as the back pressure of the argon gas during casting decreases sharply, the bubble diameter of the bubbling also increases, which causes defects such as blistering of the slab. The particle size range to be blended is 0.
1-0.6 mm is good, but 80% or more thereof is substantially
Within this particle size range, there is no significant difference in the effect.

If the amount of the fused silica raw material in the gas-permeable material exceeds 25% by mass, the rate of back pressure reduction and the rate of occurrence of slab defects at a constant flow rate during casting and the local structural deterioration , The wear resistance of the molten steel flow is significantly reduced. On the other hand, when the content is less than 5% by mass, the spalling resistance of the self-rest is greatly reduced, and at the same time, from the neck portion such as the nozzle neck caused by the difference in the coefficient of thermal expansion between the various base materials and the air permeable material. Break trouble rate increases. After considering these factors comprehensively, the amount of the molten silicic material in the gas-permeable material is preferably 5 to 25% by mass, more preferably 1 to 25% by mass.
0 to 25% by mass.

It has been found that the air permeability of the air-permeable material thus obtained can be improved if the change in the bubbling function and the back pressure can be controlled. First, the bubble diameter expansion index a
A = r / r '= 1.0 to 2.1 as the average gas bubble diameter r in a state where the silica has disappeared and the average gas bubble diameter r' in a state where the silica component is still healthy after the reduction firing. Preferably, by setting the range of 1.0 to 2.0 as an allowable range, defects due to swelling of the slab are suppressed and the floating of inclusions is promoted.

The back pressure drop index b is defined as b = p / p '= 0.3 to 1 where p is the back pressure after hot metal heating and p' is the back pressure after reduction firing, which is the state after use. (2) More preferably, in the range of 0.4 to 1, the defect phenomenon due to the swelling of the slab is suppressed. That is, the gas bubbling function reduces defects such as swelling of the slab and contributes to the reduction of inclusions when the two conditions a and b are within the standard.

Further, in order to check the abrasion properties of the molten steel, the ratio m / m 'of the BS wear value m after the disappearance of the silica and the BS wear value m' before the disappearance of the silica is referred to as a structural deterioration index c. Then, c = m / m ′ = 1 to 1.7 (3) More preferably, by setting the range of 1 to 1.6 as an allowable value, the life of the immersion nozzle is increased.

That is, by satisfying the above conditions (1) to (3), a long-life immersion nozzle can be obtained.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the present invention will be described below with reference to examples.

As a gas-permeable material, the expansion of gas bubbles during use, reduction of back pressure, deterioration of tissue,
The molten steel flow wear resistance and the thermal shock resistance of the entire nozzle were evaluated as follows.

The base material is a very commonly used alumina-graphite material, and a trial production was carried out based on a graphite content of 20% by mass. As shown in Table 1, various kinds of air-permeable materials having different particle sizes and amounts of fused silica were blended, and as shown in the schematic diagram of FIG.

In the table, alumina was used as fused alumina having a purity of 99%, and graphite was used as natural scaly graphite having a purity of 98%, but is not limited thereto. As alumina
In addition to electrofused alumina and sintered alumina, those obtained by electrofusing bauxite (alumina component ≧ 85 wt%), although the purity is somewhat low, can also be used. In addition, other carbonaceous raw materials, for example, pitch, coke, anthracite, artificial graphite, carbon black, mesophase pitch, etc., can be used as a part of graphite.

The fused silica is relatively high-purity amorphous silica. However, in consideration of economy, the amorphous silica crucible used for pulling the silicon single crystal can be used.

The production method is well known in the industry and is the same as that of a conventional immersion nozzle, so details thereof will be omitted. However, the base material, the air-permeable material, and the respective components are kneaded with an organic binder such as a phenol resin. Later, with a hydrostatic press machine,
It is molded into a nozzle shape, reduced and fired, and then used as a product of an immersion nozzle.

Further, in order to investigate the elimination characteristics of silica, etc. of the breathable material itself. 40 under the same conditions as the immersion nozzle
A small sample having a size of × 40 × 160 mm was prepared, and the ventilation characteristics and abrasion resistance before and after the heat treatment assuming actual use were examined.

In the heat treatment assuming a real furnace, a small-sized sample was immersed for 2 hours in a high-frequency induction furnace in which pig iron was heated and melted at 1600 ° C. and heat-treated. The product after reduction firing and the state after hot metal heating were compared and examined in detail, and the effect of the disappearance of silica was investigated in detail.

First, the evaluation of the expansion of the gas bubble diameter during casting was performed by setting the small sample described above in a water model device,
The average gas bubble diameter was measured by an underwater bubbling test with the surface of 40 × 160 mm facing upward.

After the hot metal was heated, the fused silica had almost disappeared, but the average gas bubble diameter / silica component disappeared.
Table 1 shows the ratio of the average gas bubble diameter in a state where the silica component is still healthy after the reduction firing, as a bubble diameter expansion index. The smaller the value, the closer to 1, the smaller the degree of expansion of the bubble diameter of gas bubbling, and the better the stability of the bubbling function.

At this time, assuming fluctuations in the back pressure monitored during casting, the back pressure was measured while flowing a constant gas flow rate using this water model. As in the case of the cell diameter expansion index, Table 1 shows the ratio of back pressure after hot metal heating / back pressure after reduction firing as the back pressure reduction index. This number is large,
The closer to 1, the smaller the fluctuation of the back pressure during casting and the better the stability of the bubbling function.

Depending on the degree of disappearance of silica during casting, the structural deterioration progresses and the molten steel flow wear resistance is reduced. The evaluation of the molten steel flow wear resistance is performed by blowing abrasive grains of silicon carbide with a high-speed airflow. B to measure the volume of the dent caused by wear on the specimen
The S wear test was employed. As a tendency, it is considered that a material having a large amount of fused silica is slightly inferior in abrasion, but is not a large difference at a practical level. Rather, the reduction in wear resistance after the disappearance of silica is more problematic. Therefore, in each sample, the ratio of the BS wear value after heating the hot metal / the BS wear value after the reduction firing, based on the BS wear value after the reduction firing, is shown in Table 1 as a structural deterioration index. It is judged that the smaller the value is, the closer to 1, the less the structural deterioration is and the better the resistance to molten steel flow wear is.

As described above, the evaluation as the immersion nozzle structure was performed with the actual nozzle shape. In actual use, evaluation of thermal shock resistance is important in order not to cause cracks, neck breaks, or peeling of the inner body.

As a method of evaluating thermal shock resistance, assuming the use condition of the actual machine, a flame of about 2000 ° C. was directly injected into the nozzle bore using an ultra-high temperature burner while a predetermined gas flow rate was flowing through the bore. To rapidly heat the nozzle body and confirm the occurrence of cracks in the nozzle body due to a sudden thermal expansion difference between the nozzle body and the bore material, or the presence or absence of peeling of the bore body.
Naturally, if the nozzle body does not crack or the inner body does not peel, the thermal shock resistance is good (（). If the crack or peeling occurs, the thermal shock resistance is lacking (×). , Is determined not to withstand actual use. Although this test is considered to be too severe rather than the operating conditions of the actual furnace, it can be evaluated that the safety factor in the actual furnace is high by passing this test.

Table 1 shows the mixing ratio of the air-permeable material for the inner hole and various evaluation results.

[0038]

[Table 1] In Table 1, when the bubble diameter expansion index satisfies 2 or less (以下 1) and the back pressure reduction index satisfies 0.4 or more, the gas bubbling function is marked as ○. Similarly, the microstructure deterioration index was 1.6 or less, and the molten steel flow wear resistance was evaluated as ○.

Examples 1, 2, 3, and 4 are within the scope of the present invention. The gas bubble diameter expansion index is 1.2 to 2.0, the gas back pressure reduction index is 0.4 to 0.7, and the structure is deteriorated. The index is 1.1 to 1.
6. The thermal shock resistance is good. In addition, as a result of an actual furnace test, it exhibited stable back pressure and bubbling state, good resistance to molten steel abrasion and thermal shock resistance, and could withstand 1.0 use.

Comparative Example 1 used a coarse fused silica raw material having the same amount as in Example 1. As a result, the bubble diameter expansion index increases and the back pressure reduction index decreases, and a stable bubbling state cannot be obtained.

In Comparative Example 2, compared with Example 1, the same amount of fine-grained fused silica raw material was used. As a result, the structure deterioration index is increased, and the bubbling state is good, but good durability cannot be obtained.

Comparative Example 3 is different from Examples 1, 2, 3, and 4
The amount of the fused silica raw material is small. As a result, there was no problem in the bubbling property and the abrasion resistance, but the thermal shock resistance was lacking, so that it could not be put to practical use at all.

Comparative Example 4 is different from Examples 1, 2, 3, and 4
The amount of the fused silica raw material is large. As a result, an increase in the bubble diameter expansion index, a decrease in the back pressure reduction index, and an increase in the tissue deterioration index are also caused, and it is difficult to obtain a stable bubbling state and good durability.

It is desirable that substantially all of the fused silica used has a particle size of 0.1 to 0.6 mm, but considering the practical range, it is sufficient that 80% or more of the particle size falls within this range.

Example 5 has no problem, but Comparative Examples 5 and 6
Is out of this range, there is a problem in the bubble diameter expansion index and the back pressure drop index, or the tissue deterioration index, and it is difficult to obtain a stable bubbling state and good durability.

[0046]

The effects of the present invention are described as follows.

1. It has good back pressure stability during ventilation and excellent thermal shock resistance, so there is no problem such as cracking due to thermal shock, and it should be used stably as a gas permeable material for continuous casting gas bubbling of steel. Can be.

2. Having good wear resistance of molten steel and relatively small gas bubble diameter expansion index, stable bubbling is possible even for long-time casting, preventing adhesion of nonmetallic inclusions and stabilizing slab quality. .

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a blow-in type casting nozzle.

[Explanation of symbols]

 1 Base material 2 Slit 3 Ventilation material for inner hole 4 Powder line material 5 Discharge port

Claims (2)

[Claims] 1. 80% of particles having a particle size of 0.1 to 0.6 mm
An immersion nozzle for continuous casting having a gas blowing function in which a permeable material composed of 5 to 25% by mass of the fused silica raw material occupied as described above and the balance of alumina and graphite aggregate is disposed on the inner side of the slit. 2. As the gas permeability of the gas-permeable material, when the bubble diameter expansion index in the bubbling mechanism is a, the average gas bubble diameter after use is r, and the average gas bubble diameter after firing is r ′, Satisfies the condition of the following formula (1), a = r / r '= 1.0 to 2.1 (1) The back pressure drop index is b and the back pressure after hot metal heating in the state after use is p. When the back pressure after firing is defined as p ′, each satisfies the condition of the following formula (2), b = p / p ′ = 0.3 to 1 (2) and a structural deterioration index indicating abrasion of molten steel Is c, BS after use
When the wear value is m and the BS wear value after firing is m ',
C = m / m ′ = 1 to 1.7, each satisfying the following condition (3): (3) The continuous casting immersion nozzle having a gas blowing function according to claim 1.