JP2003200242A - Immersion nozzle for continuous casting and method for continuously casting molten steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an immersion nozzle with which the sticking of alumina on the inner surface of the nozzle is prevented and development of surface defect is prevented, and a continuous casting method. <P>SOLUTION: In this immersion nozzle for continuous casting, a part of the inner surface of the nozzle contacting with molten steel is constituted of a refractory containing a graphite as one of the main components and having no gas permeability, and one side electrode for supplying the electric power to the refractory is disposed. This continuous casting method uses this immersion nozzle and other side electrodes disposed at one or more positions from a tundish to a flow rate control mechanism and supplies electric power between the electrode at the one side and the electrode at the other side. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention effectively makes an immersion nozzle used for continuous casting, and an oxide of Al in molten steel (hereinafter referred to as "alumina") adhere to the inner surface of the immersion nozzle. The present invention relates to a continuous casting method for molten steel which can be prevented.

[0002]

2. Description of the Related Art When continuous casting of molten steel deoxidized with Al is carried out, alumina easily adheres to the inner surface of the immersion nozzle, and the flow of molten steel in the immersion nozzle is hindered. Therefore, when casting is performed using the immersion nozzle having a plurality of discharge holes, the discharge flow is not uniform and the specific discharge flow becomes strong, and the flow of the molten steel in the mold is likely to be a single flow. When a one-sided flow occurs in the molten steel in the mold, the mold powder added to the surface of the molten steel is encouraged to be entrained, and powdery defects and slag defects are likely to occur on the surface of the slab.

These defects on the surface of the slab cause surface defects on the product hot-rolled from the slab. Furthermore, when the amount of alumina deposited on the inner surface of the immersion nozzle becomes significant, so-called nozzle clogging occurs, making it difficult to continue the operation thereafter. At that time, although the nozzle clogging can be eliminated by cleaning the inner surface of the immersion nozzle with oxygen gas, the cleanliness of the cast piece is significantly deteriorated.

In order to prevent alumina from adhering to the inner surface of the immersion nozzle, a method is known in which an inert gas is blown into the molten steel passing through the immersion nozzle (iron and steel, vol. 6).
6, S868), recently, various methods have been proposed as preventive measures applicable to operation. For example, JP-A-4-319055 discloses a method of blowing an inert gas into the molten steel passing through the immersion nozzle, corresponding to the flow rate of the molten steel passing through the immersion nozzle, the inert gas blown into the molten steel. A method of adjusting the amount of is proposed.

Further, in Japanese Patent Laid-Open No. 6-182513, a gas refracting porous refractory provided in an immersion nozzle and a molten steel flow passing through the immersion nozzle are applied with an electric current to the molten steel flow. A method of blowing an inert gas has been proposed. In this method, by blowing an inert gas into the molten steel in the nozzle, alumina is prevented from adhering to the inner surface of the nozzle, and by passing an electric current between the gas refractory porous refractory and the molten steel flow, The bubbles formed therein can be made finer. As a result, the diameter of the bubbles trapped in the solidified shell of the slab becomes smaller, and it is possible to suppress the cellular defects generated on the product surface when the slab is hot rolled.

However, in the methods proposed in these publications, the inert gas is hard to be trapped in the solidified shell of the slab, so that when the gas blowing amount is reduced, alumina or the like is prevented from adhering to the inner surface of the immersion nozzle. It cannot be prevented, and conversely, in order to prevent the adhesion of alumina etc. to the inner surface of the immersion nozzle, the amount of gas blown must be increased, and many inert gas bubbles are trapped in the solidification shell in the mold. There is a problem that the occurrence of bubble defects increases on the surface of the slab.

Further, in JP-A-2001-170742 and JP-A-2001-170761, an oxygen ion conductor is arranged on a part or the whole of a refractory material that comes into contact with molten steel such as an immersion nozzle or an upper nozzle, A method of preventing adhesion of alumina or the like to the surface of the oxygen ion conductor or melting of the oxygen ion conductor by applying a current between the oxygen ion conductor and the counter electrode is disclosed.

In the methods disclosed in these publications,
A predetermined effect can be expected by applying a direct current to the oxygen ion conductor portion and supplying or discharging oxygen ions. However, the solid electrolyte, which is an oxygen ion conductor,
Not only is it extremely expensive, but when preheating before continuous casting or when passing molten steel after the start of continuous casting, the immersion nozzle etc. molded using these solid electrolytes is damaged,
There is a problem that stable operation is hindered.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the method proposed in the related art, and the inner surface of a submerged nozzle used for continuous casting and the submerged nozzle passing through the submerged nozzle are used. By energizing between the molten steel and the inner surface of the nozzle, the adhesion of alumina to the inner surface of the nozzle is prevented, and the mold powder and alumina cause slab surface defects, and the products made from this slab also have surface defects. It is an object of the present invention to provide a dipping nozzle for continuous casting and a method for continuously casting molten steel that can effectively prevent the above.

[0010]

In order to solve the above-mentioned problems, the present inventors have proposed a refractory used in continuous casting as a method for preventing the adhesion of alumina to the inner surface of a dipping nozzle by using an electron conductive material at a high temperature. Attention was paid to the fact that it has high conductivity and ionic conductivity. Therefore, using an experimental-scale crucible, immersing a refractory rod and an electrode having electrical conductivity in molten steel, and conducting an experiment to apply a potential difference between the refractory rod and the electrode by energizing both went.

As a result of the above experiment, it was confirmed that the amount of alumina adhering to the surface of the refractory can be reduced even if the potential difference is small. Moreover, as the structure of the immersion nozzle, at least a part of the inner surface that comes into contact with the molten steel passing through is made of a non-breathable refractory having graphite as one of the main components, and the refractory is provided with electrodes. It has been found that the arrangement is effective for reducing the amount of adhered alumina.

The present invention has been completed on the basis of the above-mentioned findings, and has the immersion nozzle for continuous casting shown in (1) below.
The main point is the continuous casting method for molten steel shown in (2) below. (1) An immersion nozzle for supplying molten steel contained in a tundish into a mold, in which at least a part of the inner surface in contact with molten steel is graphite, which is one of the main components, and has no air permeability. An immersion nozzle for continuous casting, characterized in that the refractory is provided with an electrode capable of conducting electricity.

The above-mentioned continuous casting immersion nozzle is preferably made of an alumina graphite refractory material and / or a material whose electric resistivity of the electrode is less than 1 (Ω · cm) at 1300 ° C. . Further, when the electrode is made of non-metal, it is desirable to apply a metal spray coating on a part or all of the surface thereof. (2) A continuous casting method using the immersion nozzle according to any one of the above (1), wherein a pair of electrodes, a power supply unit connected to these electrodes, and a flow rate control mechanism for controlling the flow rate of molten steel supplied to the mold are provided. One of the pair of electrodes is arranged in the immersion nozzle, and the other electrode is arranged in one or more places so as to reach the internal space from the tundish to the flow rate control mechanism and come into contact with the molten steel. A continuous casting method for molten steel, characterized in that an electric current is applied between the electrode and the other electrode.

The refractory material having no air permeability specified in the present invention is a refractory material manufactured without compounding pitch or the like which disappears in the subsequent heat treatment in order to ensure air permeability during molding. Usually, the material has an apparent porosity of less than 20%. For example, as specified in JIS R 2115 "Test method for air permeability of refractory bricks", the air permeability obtained by measuring the amount of gas passing under the condition that pressure difference is applied to the front and back of the refractory is 0.05 cm ³・ cm / (cm ²・ s ・ cm
H ₂ O).

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The contents of the immersion nozzle for continuous casting and the continuous casting method for molten steel of the present invention will be described by dividing the items into the immersion nozzle and the continuous casting method, respectively. 1. Immersion Nozzle for Continuous Casting FIG. 1 is a vertical sectional view schematically showing an example of a continuous casting apparatus provided with the immersion nozzle of the present invention. In the figure, a three-layer sliding gate is shown as a molten steel flow rate control mechanism, but the present invention is not limited to this type, and even if it is a two-layer type, a stopper is further used. It may be in a form of control.

In the continuous casting apparatus shown in FIG. 1, a tundish 1 having an upper nozzle 4 provided at the bottom, a sliding gate 5 provided below the upper nozzle 4, and a dipping provided subsequent to the sliding gate 5. The nozzle 7 is arranged. Further, the sliding gate 5 is covered by the cassette holder 6, and the immersion nozzle 7 is held by the nozzle holder 9. The shape of the tundish 1 containing the molten steel 16 and the refractory lined therein may be those commonly used.

The upper nozzle 4 provided at the bottom of the tundish 1 has a nozzle portion for supplying the molten steel 16 in the tundish 1 to the lower portion, and is made of a refractory material. The sliding gate 5 has a three-layer structure including an upper plate, a lower plate, and a movable plate provided therebetween. The upper plate, the lower plate, and the movable plate are also made of refractory material, and the amount of molten steel 16 supplied to the lower portion is controlled by horizontally moving the movable plate by a drive mechanism (not shown).

The submerged nozzle 7 has two discharge holes 8 in the lower part, and the portion including these discharge holes 8 is inserted into the mold 14. A mold powder 15 is added to the upper surface of the molten steel 16 supplied into the mold 14, and a solidified shell 17 is formed in contact with the mold 14.

One electrode 11 of the pair of electrodes is installed at one location of the immersion nozzle 7, and the other electrode 12 is arranged so as to reach the internal space of the tundish 1 and be immersed in the molten steel. A power source 10 connected to the electrode 11 and the other electrode 12 is provided.

In FIG. 1, a three-layer type sliding gate 5 is shown as a molten steel flow rate control mechanism, but as another example, even a two-layer type sliding gate is installed above the upper nozzle 4. It may be a stopper for. Furthermore, when a control stopper is provided, this may be used as the other electrode 12.

When the molten steel 16 is not housed in the tundish 1, it is necessary to electrically insulate between the one electrode 11 and the other electrode 12. Although FIG. 1 shows a structure in which an insulator 13 is provided to insulate the other electrode 12 and the tundish 1, regardless of this, any part between the immersion nozzle 7 and the tundish 1 is shown. Insulate properly.

Further, when disposing the electrodes 11 and 12, one of the electrodes may be grounded. In this case, it is difficult to insulate the energizing circuit that flows from the immersion nozzle to the outside through the molten steel inside, the mold, etc., so it is desirable to ground one electrode 11 side.

The immersion nozzle of the present invention is characterized in that at least a part of the inner surface which comes into contact with the molten steel is made of a refractory having graphite as one of the main components and having no air permeability. In the present invention, it is essential that the composition relating to the main component and the air permeability is required at the same time, even if the refractory containing graphite as one of the main components is energized when the contact portion with the molten steel has the air permeability. This is because melting loss will progress at the contact portion.

Since a refractory material having air permeability has a high porosity, its strength is lower than that of an ordinary dense refractory material. For example, in a refractory having air permeability, which has pores formed by blending pitch and the like during molding of the refractory and eliminating it in a subsequent heat treatment step, the apparent porosity is 20% or more. When such a refractory material is applied to the inner surface of the immersion nozzle and electric current is applied for casting, melting damage on the inner surface of the immersion nozzle becomes significant.

FIG. 2 shows the immersion nozzle of the present invention with one electrode.
FIG. 11 is a cross-sectional view showing another example in which 11 is arranged. In order to pass an electric current from the inner surface of the immersion nozzle to the molten steel flowing inside, at least a part of the inner surface of the nozzle is made of a conductive material, and it is necessary to dispose one electrode 11 capable of conducting electricity.

FIG. 2A shows the simplest structure in which the entire immersion nozzle is made of a conductive refractory material containing graphite as one of the main components. In the immersion nozzle of the present invention, it is not necessary to form the entire inner surface of the nozzle with a conductive refractory material.
As shown in (b), the inner surface of the immersion nozzle may be partially lined with another material having an insulating property or a low electric conductivity. By using the dipping nozzle having the structure shown in (b), it becomes possible to intensively apply a current to a portion where it is desired to prevent the adhesion of alumina or the like, and the current density per unit area can be increased.

FIG. 2 (c) shows an example in which one electrode 11 and a conductive refractory material facing the inner surface of the nozzle are connected and insulated from the other part of the immersion nozzle 7. In addition to the example shown in (c),
The slag line portion of the outer layer may be structured by combining it with a material having excellent melting resistance. The shape of the immersion nozzle discharge hole 8 is not limited to the illustrated shape.

FIG. 3 shows one electrode of the immersion nozzle of the present invention.
FIG. 11 is a front view showing another example in which 11 is arranged. Figure 3 (a)
In (b) and (b), an electrode made of a wire rod or an annular plate is attached to the outer surface of the immersion nozzle 7. This allows
It is possible to energize the portion of the inner surface of the immersion nozzle that comes into contact with the molten steel via the main body of the immersion nozzle 7.

The electrode may be attached to the main body of the immersion nozzle by any method such as adhesion with a heat-resistant and conductive adhesive, mechanical joining such as screwing, and contact with a holding metal fitting, It may be attached so as to make electrical contact.

The refractory of the current-carrying portion of the inner surface of the immersion nozzle of the present invention is electrically conductive and can be molded into a predetermined immersion nozzle shape, withstands thermal shock during preheating or during passing of molten steel, and melts during casting. It must be a material that does not generate.

Solid electrolytes such as CaO-stabilized ZrO ₂ and TiB
A boride such as ₂ or the like, a nitride such as AlN or a carbide such as SiC is also conductive and can be molded into a nozzle shape.
However, as will be described later, it is desirable that the resistance between one electrode and the other electrode is made as small as possible in order to prevent the risk of electric shock at the time of electric leakage and malfunction of the device. Therefore, it is desirable that the electrical resistivity at the temperature during energization is less than 1 Ω · cm.

From the above viewpoint, solid electrolytes such as CaO-stabilized ZrO ₂ and nitrides such as AlN are not suitable as the material of the current-carrying portion on the inner surface of the immersion nozzle. Further, these materials and borides such as TiB ₂ are vulnerable to thermal shock, and it is difficult to stably perform continuous casting of steel. It is possible to use it for a part of the inner surface of the nozzle, but in any case, it is very expensive because the molding process is difficult.

All of the refractory materials containing graphite as a main component have high electrical conductivity, exhibit relatively high thermal shock resistance, and can be manufactured at low cost. Therefore, a refractory containing graphite as one of the main components was selected as the conductive material for the inner surface of the nozzle. However, although pure graphite satisfies such conditions, it easily elutes in molten steel, so a refractory material containing graphite containing other oxides as an aggregate was selected as an aggregate.

As the refractory material containing graphite as one of its main components, various refractory materials can be mentioned as exemplified below. Among them, the alumina graphite refractory material is the cheapest and has excellent thermal shock resistance. Therefore, it is a desirable refractory material.

Alumina graphite based refractory material content%, carbon content 15-35%, Al ₂ O ₃ 35-70%.
%, And in addition, it is preferable to contain SiO ₂ , SiC, ZrO ₂ and the like. Carbon is an element that plays a role in the conductivity of refractories, and is also an element that is required to prevent damage due to thermal shock when passing molten steel, so it is necessary to add at least 15%. But,
If the content exceeds 35%, the strength decreases and the melt damage property against molten steel deteriorates, so 35% was made the upper limit. Si
O ₂ , SiC, ZrO _{2 and the} like are components added for the purpose of improving melting resistance and thermal shock resistance, but if necessary, 30
There is no problem even if it is added up to about%.

Zirconia Graphite-based refractory material content%, carbon content 10-20%, ZrO ₂ 50-80%
contains. Carbon is necessary for imparting conductivity to the refractory material, and if it is less than 10%, the conductivity is remarkably reduced. Also,
When the carbon content exceeds 20%, the strength decreases, so the upper limit is made 20%. Besides, it contributes to the stabilization of ZrO _2.
It can contain 10 to 25% of CaO. In addition, SiO ₂
Or SiC may be contained in less than 10%.

% Magnesia graphite refractory material, carbon content 10 to 20%, other than MgO and Al ₂ O
₃ , containing SiO _{2 and the} like. Carbon must be added in an amount of 10% or more in order to impart conductivity to refractory, while 20%
If it exceeds%, the strength will decrease.

The electrode arranged in the immersion nozzle of the present invention is preferably made of a material having an electric resistivity of less than 1 Ω · cm at 1300 ° C. When energizing between the refractory of the immersion nozzle and the molten steel, if the electrical resistance between them becomes large, the applied voltage also becomes large, the power supply becomes large, and there is a concern that heat will be generated in the electrode part, and electric shock may occur. This is because

In order to avoid such a situation, the resistance of the electrode itself is preferably less than 1 Ω · cm at 1300 ° C., and more preferably less than 0.1 Ω · cm at 1300 ° C. In continuous casting of molten steel, the immersion nozzle reaches a temperature of about 1300 ° C, so the electrical resistance value at that temperature was specified.

The one electrode 11 arranged in the immersion nozzle 7 in FIG. 1 does not directly contact the molten steel 16, but is exposed to a high temperature of about 1300 ° C. in the immersion nozzle 7. Therefore, the one electrode 11 needs to be made of a metal or a conductive refractory material that has heat resistance up to 1300 ° C. Specifically, steels such as carbon steel and stainless steel, refractory metals such as molybdenum, graphite, refractory materials containing carbon as a main component described below, SiC, and boride-based materials such as ZrB ₂ and TiB ₂ are used. Refractory can be used.

Steel such as carbon steel and stainless steel and molybdenum are deteriorated and become brittle when exposed to high temperatures. Therefore, the material is selected in consideration of the ultimate temperature of the portion where one electrode 11 is arranged during operation. There is a need to.

Table 1 shows examples of electrode materials that can be used in the present invention, together with the electrical resistivity ρ and the resistance value R. The electric resistivity ρ in Table 1 is a value at 1300 ° C., and the resistance value R is a value when the electrode is a round bar having an outer diameter of 1 cm and a length of 10 cm.

[0043]

[Table 1]

Among the electrode materials shown in Table 1, CaO-stabilized Zr
O ₂ has an electric resistivity ρ of 5 Ω · cm, which exceeds the range that is desirable in the present invention, and the resistance R is as large as 63 Ω. Further, the ion-conductive oxide generally has a smaller resistance value as the temperature rises, and when it is used as an electrode material, the resistance value further increases. For this reason, the resistance value of the entire path increases when energized, and the required applied voltage also increases, which is not suitable as an electrode material.

Since the resistance value R of SiC is 1.3 Ω, which is slightly above the range desired in the present invention, in order to use it as an electrode, it is necessary to devise a means for reducing the resistance value such as increasing the cross-sectional area. Becomes

The other materials shown in Table 1, namely, graphite, alumina graphite, borides such as ZrB ₂ and TiB ₂ , and steel all have an electrical resistivity ρ of less than 1 Ω · cm and a resistance value of It is desirable to use R as a sufficiently small electrode. Among them, graphite and alumina graphite have a characteristic that the electric resistance value R hardly changes even when the temperature changes.

Other borides such as ZrB ₂ and TiB ₂ and steels have an electric resistance which increases as the temperature rises. Therefore, the temperature is about 1300 ° C. in the dipping nozzle, and the temperature at the connection with the conductor is low. It is desirable to use it as an environmental electrode.

As described above, a material that can withstand use in an environment where the electrical resistivity is less than 1 Ω · cm, one end is about 1300 ° C., and the other end is less than 200 ° C. is used as the electrode. Is desirable. A material having an electric resistivity ρ of less than 0.1 Ω · cm is more preferable as an electrode material. The reason why the other end of the electrode is less than 200 ° C is that a lead wire made of a metal such as copper, aluminum, or nickel is connected to the other end of the electrode, but if the temperature of the contact is high, the durability of the contact This is because problems such as decrease in resistance, increase in resistance, and deterioration in operability are likely to occur.

When the electrode arranged in the immersion nozzle is made of non-metal, it is desirable to apply a sprayed coating of metal on a part or all of the surface. If the electrode is made of non-metal, contact resistance is generated at the connection portion with the conductive wire, and the resistance of the entire path increases. In this case, the contact resistance easily becomes 1Ω or more, so it is necessary to increase the applied voltage.
In the case of graphite and other carbon-containing materials, there is a problem that the carbon content burns when exposed to high temperature in the atmosphere. In such a situation, not only the resistance increases and the energization is hindered, but also the immersion nozzle is damaged and the operation is hindered.

Therefore, when the electrode material is a non-metallic material, if the surface of the electrode is sprayed with a metallic material, the conductivity can be secured and the wear of the electrode can be prevented. Regarding the material to be sprayed, it is sufficient that the conductivity can be maintained in the temperature atmosphere to which the electrode is exposed, and a commonly used spray material such as aluminum or nickel can be used. Further, the range for spraying may be determined according to the purpose such as ensuring electrical continuity with the conducting wire and preventing electrode wear. 2. Continuous casting method of molten steel In the casting method of the present invention, as the counter electrode of one electrode arranged in the immersion nozzle, the other electrode reaches the internal space from the tundish to the sliding gate for controlling the flow rate of molten steel, so as to contact with the molten steel. Place 1 or 2 or more in

On the other hand, for example, it is possible to install the other electrode in a slab or a mold and to conduct electricity between the other electrode arranged in the dipping nozzle. The rate of energization from the outer surface rather than the inner surface of the nozzle increases, and it becomes impossible to effectively prevent alumina from adhering to the inner surface of the immersion nozzle.

As shown in FIG. 1, the other electrode 12 must be made of a material that withstands molten steel 16 for a long time in a state of being in contact therewith and has electrical conductivity. Therefore, refractory materials containing steel or carbon as one of the main components, graphite, high melting point metal Si.
A refractory material containing C as a main component, a boride such as ZrB ₂ or TiB ₂ , and a composite material containing these materials can be used. However, since graphite and steel undergo remarkable melting loss in molten steel, there are restrictions on the types of steel that can be applied, and it is necessary to devise a construction method.

In FIG. 1, the other electrode 12 is attached by immersing it in the surface portion of the molten steel 16 housed in the tundish 1 from above, but the method is not limited to this method and the tundish is not limited to this. It may be provided so as to penetrate through the iron skin 2 of the dish and the refractory lining 3, and at least a part of the upper nozzle and the sliding gate may be made of a conductive refractory, and this may be used as the other electrode.

In the casting method of the present invention, it is desirable that the one electrode 11 and the other electrode 12 be in an insulating state before the start of casting. The insulator installed between the one electrode 11 and the other electrode 12 has a reached temperature that differs depending on the insulating location,
An appropriate material that can maintain insulation may be selected according to the ultimate temperature. Usually, fibers, sheets, coating materials, etc. made of an insulating refractory material containing Al ₂ O ₃ or SiO ₂ as a main component are used.

Generally, these insulating refractories have high electric conductivity at high temperatures, and if the refractory such as the tundish or nozzle is preheated before the start of casting, the insulation deteriorates. Therefore, at the end of preheating before supplying molten steel from the tundish to the mold, one electrode 11 and the other electrode are
It is desirable that the electrical resistance with 12 be 500Ω or more. Furthermore, considering that not only the loss of applied electric power and loss of applied power, but also the cause of electric shock and erroneous operation of instruments, it is more desirable to set the resistance to 5000Ω or more when insulation deteriorates.

In the casting method of the present invention, the voltage applied between one electrode and the other electrode arranged in the immersion nozzle is 1
It is desirable to set the current to -100V and the current to 10-300A. In this case, the current may be alternating current, direct current, or intermittent current. In addition, it may not be possible to temporarily energize when, for example, replacing a ladle for continuous casting.

When current is applied between the inner surface of the immersion nozzle and the molten steel under such conditions, the action of alumina adhering to the surface of the refractory is reduced, and it becomes difficult for alumina to adhere to the inner surface of the immersion nozzle.

The upper limit of the voltage is limited to 100V in consideration of safety in the case of short circuit, and the more desirable upper limit is 60V. On the other hand, the upper limit of the current is set to 300A, because it is necessary to increase the cross-sectional area of the electric wiring in order to increase the current, and the equipment becomes large in order to withstand the large current exceeding 300A. The upper limit of the current is limited. The current that prevents the adhesion of alumina, etc. to the inner surface of the nozzle is the area where the refractory material having conductivity on the inner surface of the nozzle contacts molten steel, that is, it depends on the area to be energized, and the current density per unit area is 0.001. ~ 0.5 A · cm ²
Is appropriate.

The circuit resistance between the one electrode and the other electrode not only reduces the power loss, but also prevents the risk of electric shock or malfunction of the device at the time of leakage.
It is desirable to make it as small as possible. The preferred circuit resistance condition is less than 3Ω, more preferably less than 0.5Ω.

According to the casting method of the present invention, alumina adhesion to the inner surface of the nozzle can be reduced or prevented. Therefore, for this reason, it is possible to reduce or stop the blowing of the inert gas from the upper nozzle, the sliding gate, and the immersion nozzle, which has been conventionally performed. By making the amount of the inert gas blown into zero, it becomes possible to prevent the bubble defect.

However, when the blowing amount of the inert gas is set to zero, the molten steel temperature in the vicinity of the meniscus in the mold may be lowered, which may hinder the operation. In this case, a small amount of blowing gas may be required. It is desirable to carry out. In the casting method of the present invention, there is no problem even if it is carried out while blowing the inert gas, and in that case, it is desirable to set the flow rate of the inert gas to 2 to 10 Nl / min.

When the current-carrying portion to the molten steel has a porous structure and an electric current is passed while blowing an inert gas from this portion, the porous portion is significantly worn, and the gas-blowing portion and the current-carrying portion to the molten steel are clearly separated. There is a need to.

As described above, the casting method of the present invention is optimally used in the continuous casting method of molten steel deoxidized with Al. Furthermore, the application effect is great in steel types such as ultra-low carbon steel where the adhesion of alumina to the inner surface of the nozzle affects the product quality. Further, in addition to Al deoxidized steel species, it is also effective in preventing and suppressing adhesion to the inner surface of the nozzle in continuous casting of steel containing zirconium, titanium, calcium, rare earth metals, etc., which causes nozzle clogging.

[0064]

Example An ultra-low carbon steel having the chemical composition shown in Table 2 was continuously cast using a vertical bending type continuous casting machine. Casting is 270 tons
6 times of molten steel continuously, thickness 270mm, width 1600
A slab of ~ 1200 mm was continuously cast at a casting speed of 1.5-1.7 m / min.

[0065]

[Table 2]

One electrode was connected to the immersion nozzle of the continuous casting machine, and a structure was adopted in which a refractory material having conductivity was brought into contact with the molten steel on the inner surface of the nozzle. Further, the other electrode was immersed in the molten steel in the tundish and cast while applying an electric current. The main body material of the immersion nozzle used was alumina graphite, its inner diameter was 85 mm, and it had two discharge holes of 30 ° downward.

The nozzle shape is shown in FIG.
As a seed, zirconia graphite (A), alumina graphite (B), refractory material that comes into contact with molten steel on the inner surface of the nozzle,
As four kinds of fused silica (C) and CaO-stabilized ZrO ₂ (D), an immersion nozzle was constructed by combining these shapes and materials. However, whichever dipping nozzle
The powder line portion on the outer surface of the nozzle was made of zirconia graphite to prevent melting damage.

Table 3 shows the composition of the refractory material which comes into contact with the molten steel on the inner surface of the nozzle.

[0069]

[Table 3]

During casting, Ar was blown from the upper part of the sliding gate at 2 to 5 Nl / min. Also,
The effect was investigated by changing the applied current and voltage during casting.

After the completion of casting, the immersion nozzle was cut, and the thickness of alumina adhering to the inner surface of the nozzle was measured. When an electric current was applied, the thickness of the deposit on the inner surface of the nozzle made of a conductive material was measured. Furthermore, the circuit resistance between the one electrode and the other electrode during energization was measured.

The slab obtained by continuous casting was hot-rolled and cold-rolled, and the occurrence of surface flaws in the cold-rolled steel sheet was investigated. Specifically, the slab is hot-rolled to a thickness of 4 to 6 mm,
After pickling, a cold rolled steel sheet was manufactured by cold rolling to a thickness of 0.6 to 1.2 mm. The cold rolled coil thus obtained was visually inspected, and the length of the flawed portion at this time was divided by the total length of the coil to obtain the flaw occurrence rate.

Further, the immersion nozzle (shape, shape,
Regarding refractory material), the nozzle manufacturing cost was calculated assuming that it will be used in large quantities on a daily basis.

Table 4 shows the casting conditions, casting results and trial calculation results adopted in the examples.

[0075]

[Table 4]

In Test Nos. 1 and 2, the entire immersion nozzle was composed of zirconia graphite or graphite containing alumina as one of the main components, and was cast from one electrode while energizing the molten steel. As a result, the thickness of the deposit on the inner surface of the nozzle was thin and the occurrence rate of surface defects was small. Test No. 1 uses zirconia graphite, so the nozzle manufacturing cost is slightly higher, the preheating of the immersion nozzle before the start of casting is insufficient, and the time until the start of casting after preheating is long and the nozzle temperature drops. If it does, it may be damaged by thermal shock. Therefore, from the viewpoint of stabilizing the operation and reducing the cost, it is desirable to configure the immersion nozzle with alumina graphite.

In Test No. 3, since the entire immersion nozzle was made of fused (fused) silica, the circuit resistance during energization was large, and no current flowed despite the application of a voltage of 100V. Therefore, the thickness of the deposit on the inner surface of the nozzle is large,
The occurrence rate of surface defects was also high.

In Test No. 4, alumina graphite containing carbon as one of the main components was used as the material of the immersion nozzle, but since no current was applied, the deposits on the inner surface of the nozzle became thicker and the surface defects Incidence also increased.

In the test No. 5, the inner surface of the nozzle has a length of about 70 mm.
Since the current was applied only to the annular part, the thickness of the deposit on the energized part was 0.8 mm, which was good, but on the other non-energized part, there was a deposit of 7 to 10 mm. The incidence was high. However, the effect of preventing adhesion at the energized portion is clear, and it can be seen that the defect generation rate can also be improved by devising the shape of the energized portion that is effective for improving the quality of the cold rolled coil.

Test No. 6 shows the shape of the energized portion in Test No. 5
Same as in the case of, but with an annular shape of about 70 mm,
With the CaO-stabilized ZrO ₂ solid electrolyte, no alumina adhesion was observed in this part. However, CaO
Stabilized ZrO ₂ solid electrolyte part has a large current resistance and voltage
With the application of 90 V, the current was kept at 20 A.

In Test No. 6, it is necessary to reduce the wall thickness in order to reduce the circuit resistance during energization, but it is difficult to increase the current density because it may be weak against thermal shock. . In addition, the other non-current-carrying parts have adhesion, and the occurrence rate of surface defects is high. In addition, the nozzle manufacturing cost
It was extremely high at 250 thousand yen, and this solid electrolyte part was sometimes damaged by thermal shock.

Test Nos. 2, 7 and 8 were the same in nozzle shape and refractory material, and the applied current was changed.
It can be seen that the thickness of the deposit on the inner surface of the nozzle and the rate of occurrence of surface defects increase as the applied current decreases.

Test Nos. 9 and 10 used a nozzle in which about 1/2 of the upper part of the inner surface of the immersion nozzle was coated with high Al ₂ O ₃ . By thus partially covering with a non-conductive refractory, it becomes possible to energize a specific part. What adversely affects the fluidity in the mold and deteriorates the quality of the thin plate is
Since it is the adhering matter in the vicinity of the discharge hole, by energizing this part to suppress the adhering matter, test Nos. 2, 7 and 8
In comparison with the case where the entire nozzle was energized, the rate of occurrence of surface defects was about the same with a smaller current.

Test Nos. 11 to 13 are Test Nos. 2, 7 and 8
Although the nozzle of the same material and structure was used, this is the result of changing the electrode material. Test No. 11 is the result of using an alumina graphite rod not subjected to thermal spraying as an electrode,
Due to the large contact resistance between the conductors, only about 20 A flowed even when 36 V was applied. Further, the voltage fluctuated greatly, and spark marks remained on the electrode surface. When energized in such a situation, electrical efficiency is poor and it is difficult to perform stable energization.

On the other hand, Test No. 12 is the result of using the alumina graphite rod having the back surface sprayed with Al as the electrode, and it was possible to stably supply 50 A by applying 7 V. .

Test No. 13 is C having an electric resistivity of 2 Ω · cm.
It is the result of using Al ₂ O ₃ impregnated with
Despite the installation of two enlarged electrodes, there was a problem that the circuit resistance when energized was as large as 6.3Ω and the electrode itself generated heat.

[0087]

According to the immersion nozzle for continuous casting of the present invention, during continuous casting of molten steel, it is possible to prevent oxides in molten steel such as alumina from adhering to the inner surface of the nozzle. Furthermore, by applying the continuous casting method using this immersion nozzle, it is possible to prevent the occurrence of defects due to defects in the slab, such as mold powder, alumina, and bubbles, in products made of the obtained slab. You can

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view schematically showing an example of a continuous casting apparatus provided with the immersion nozzle of the present invention.

FIG. 2 is a cross-sectional view showing another example in which one electrode 11 is arranged in the immersion nozzle of the present invention.

FIG. 3 is a front view showing another example in which one electrode 11 is arranged in the immersion nozzle of the present invention.

[Explanation of symbols]

1: Tundish 2: Tundish iron skin 3: Tundish lining refractory 4: Upper nozzle 5: Sliding gate 6: Cassette holder 7: Immersion nozzle 8: Discharge hole 9: Nozzle holder 10: Power supply 11: One Electrode 12: Other electrode 13: Insulating sheet 14: Mold 15: Mold powder 16: Molten steel 17: Solidified shell

Continued front page    (72) Inventor Masayuki Kawamoto             4-53 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture             Sumitomo Metal Industries, Ltd. (72) Inventor Toshihiko Murakami             4-53 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture             Sumitomo Metal Industries, Ltd.

Claims (5)

[Claims] 1. An immersion nozzle for supplying molten steel contained in a tundish into a mold, wherein graphite is one of the main components and at least a part of the inner surface in contact with the molten steel is breathable. An immersion nozzle for continuous casting, characterized in that it is made of a refractory which is not provided, and an electrode that can energize this refractory is arranged. 2. The continuous casting immersion nozzle according to claim 1, wherein the refractory material is an alumina graphite refractory material. 3. The electrical resistivity of the electrode is 1 at 1300 ° C.
The immersion nozzle for continuous casting according to claim 1 or 2, wherein the immersion nozzle is made of a material of less than (Ω · cm). 4. The immersion nozzle for continuous casting according to claim 3, wherein when the electrode is made of non-metal, a part or all of the surface of the electrode is coated with a sprayed metal film. 5. A continuous casting method using the immersion nozzle according to any one of claims 1 to 4, wherein a flow rate control for controlling the flow rate of molten steel supplied to a pair of electrodes, a power source section connected to these electrodes and a mold. A mechanism is provided, and one electrode of the pair of electrodes is arranged in the immersion nozzle, and the other electrode is arranged in one or more places so as to reach the internal space from the tundish to the flow rate control mechanism and come into contact with molten steel. Then, a continuous casting method for molten steel is characterized in that an electric current is applied between one electrode and the other electrode.