JP4054318B2 - Continuous casting method for slabs with excellent quality characteristics - Google Patents

Description

The present invention, when pouring molten steel into a mold, nozzle clogging caused by inclusions attached to the immersion nozzle from the lower nozzle of the continuous casting tundish for producing slabs such as bloom, slab, billet, Excellent quality characteristics that can prevent the casting operation from becoming unstable due to clogging at the discharge port of the immersion nozzle, and the quality of the slab from being disturbed by the drift of the discharge flow of molten steel from the discharge port The present invention relates to a continuous casting method of a slab.

Conventionally, pouring of molten steel into a casting mold in continuous casting has been carried out by using a lower nozzle provided at the lower part of the tundish, a sliding nozzle (also referred to as an SN nozzle) that opens and closes by being fitted to the lower nozzle, and a sliding nozzle. This is performed using a bonded immersion nozzle (hereinafter also referred to as a continuous casting immersion nozzle). However, in the pouring system (nozzle for continuous casting) such as the lower nozzle, SN nozzle, and immersion nozzle, Al ₂ O ₃ which is a deoxidation product mixed in the molten steel and Al in the molten steel are contained in the refractory and molten steel. Reacts with oxygen, etc., and Al ₂ O ₃ is generated and present (hereinafter also referred to as alumina inclusions), so that the alumina inclusions adhere to the pouring system and adhere to the molten steel passage. As a result of clogging or accumulation, clogging or clogging occurs in the pouring system, leading to interruption of the casting operation and deterioration of the quality of the slab due to clogging or clogging.
As a countermeasure, a continuous casting nozzle is proposed in which any one of boron carbide, boron nitride, and boron is added to 40 to 90% by weight of lime and 10 to 60% by weight of carbon, and is fired. In addition to suppressing embrittlement due to digestion reaction and thermal expansion, it has been attempted to prevent clogging of the continuous casting nozzle by reducing the melting point of the alumina inclusions that adhere to the continuous casting nozzle (for example, , See Patent Document 1).
Further, powder 3 of a reaction product of alkali or alkaline earth metal salt and slaked lime with respect to 35 to 85% by weight of aggregate made of lime clinker and / or dolomite clinker and 5 to 50% by weight of graphite-based carbon. Carbon-containing calcareous continuous casting nozzles obtained by adding ˜25% by weight have been proposed, and corrosion resistance, spalling resistance, and low melting point of alumina inclusions formed on the continuous casting nozzles have been achieved. (For example, see Patent Document 2).

In particular, as an invention for preventing clogging of the nozzle for continuous casting by suppressing adhesion and accumulation of alumina inclusions, it is selected from silica, alumina, dolomite, magnesia, and zircon that do not contain graphite on the inner surface through which the molten steel passes. It has been proposed to divide a formed body using one or more kinds of refractory aggregates into a plurality of parts and attach them to the inner surface of the immersion nozzle through joints (for example, see Patent Document 3).
Alternatively, the mixed powder is composed of a powder of 2 to 40% by weight in terms of CaO and one or more kinds selected from alumina clinker, spinel clinker and magnesia clinker having a SiO ₂ content of less than 1% by weight. There has been proposed an inner hole body of a nozzle for continuous casting containing 20 to 70% by weight of carbon and SiO ₂ in the powder each having a particle size of 1% by weight or less and a particle size of 0.21 mm or less ( For example, see Patent Document 4).

Furthermore, as a method of equalizing the amount of molten steel discharged from the discharge port of the immersion nozzle and slowing the flow of molten steel discharged from each discharge port, the inside of the cylindrical portion of the immersion nozzle is reduced in diameter to provide a stepped portion. In addition, a method has been proposed in which molten steel falling from above collides with the stepped portion to attenuate the falling energy of the molten steel in the immersion nozzle, or eliminate the uneven flow of molten steel discharged from each discharge port ( For example, see Patent Documents 5 to 8).
In addition, a method has been proposed in which a swirl vane is disposed inside the cylindrical portion of the immersion nozzle, the molten steel falling from above is brought into contact with the swirl blade, and the falling energy of the molten steel in the immersion nozzle is attenuated (for example, (See Patent Document 9).

JP-A-57-56377 JP 57-38366 A Japanese Utility Model Publication No. 3-68962 JP-A-5-285612 JP-A-6-99256 JP-A-11-320046 Japanese Patent Laid-Open No. 11-123509 JP 2001-239351 A JP 2002-248551 A

However, the above-described continuous casting nozzle has the following problems.
First, as in the inventions described in Patent Documents 1 and 2, in the composition mainly composed of lime and carbon, the lime and carbon constituting the continuous casting nozzle react with the components in the molten steel to have a low melting point. Since the compound is produced, adhesion and deposition of alumina inclusions can be prevented, but the refractory aggregate falls off due to the large erosion loss of the continuous casting nozzle, the nozzle life is reduced, the produced low melting point compound and Refractory aggregates dropped from the continuous casting nozzle are mixed in the molten steel, and carbon is picked up in the molten steel by the carbon constituting the continuous casting nozzle, causing a problem that the quality of the molten steel deteriorates. It was.
Furthermore, the unevenness of the inner surface of the continuous casting nozzle becomes severe with melting, and inclusions are deposited and deposited.
And stable casting due to clogging or blockage of the molten steel passage has been impossible.
In particular, clogging or clogging in the vicinity of the discharge port of the immersion nozzle causes inclusion defects due to peeling of the adhered alumina inclusions, or the discharge flow of molten steel from the discharge port drifts from side to side. There is a problem that the surface defect and internal defect of the slab cannot be eliminated.

The invention described in Patent Document 3 uses a refractory aggregate that does not contain carbon, suppresses deterioration of the smoothness of the inner surface of the immersion nozzle due to oxidation of carbon, and prevents adhesion of alumina inclusions Although based on the idea, in practice, alumina inclusions adhered and deposited on the inner surface of the submerged nozzle, resulting in clogging of the continuous casting nozzle, leading to unstable operation during the casting process.
In addition, the invention described in Patent Document 4 suppresses generation of alumina in the vicinity of the molten steel contact surface (hereinafter also referred to as an operation surface) of the inner hole body of the nozzle for continuous casting, and suppresses melting damage on the operation surface. This is based on the technical idea of ensuring the smoothness of the operating surface and preventing the adhesion of alumina inclusions due to its synergistic effect. A clogging occurs in the inner hole of the nozzle, and the same problem as in Patent Documents 1 and 2 described above occurs.

Moreover, when the fall energy of molten steel is attenuated by providing the level | step difference described in patent documents 5-8, or the drift of the discharge flow of molten steel from a discharge port is suppressed, it falls perpendicularly described in patent document 9 In the case where the fall energy is attenuated by bringing the molten steel into contact with the swirl blade and swirling, the metal and inclusions adhere to and accumulate on the step and swirl blade, and the metal and inclusion adhere to the discharge port. Therefore, it is difficult to avoid the deposition, and as a result, inclusions and the like adhere and accumulate to block the discharge port, so that it is impossible to put it to practical use.

From the above, in the pouring system from the lower nozzle having the lower nozzle and the SN nozzle to the submerged nozzle, the outflow of heat is generated from the working surface side, so that the molten steel passes through the submerged nozzle. Therefore, it cannot be avoided that the temperature of the molten steel is lowered and the metal is likely to adhere to the operating surface. Furthermore, the alumina inclusions coming into contact with the working surface adhere as they are, or adhere along with the adhesion of the metal, and this synergistic action causes the alumina inclusions to accumulate, resulting in clogging of the continuous casting nozzle. To do.
Even when the above-mentioned working surface is a low-carbon and low-silica refractory, although there is a slight difference in the adhesion phenomenon of alumina inclusions, Al ₂ O ₃ in the molten steel is also working. Adhering to the surface causes clogging of the continuous casting nozzle.
In this way, it is impossible to prevent the temperature drop of molten steel, deoxidation products in molten steel, oxidation of Al in molten steel, contact of Al ₂ O ₃ components with refractories, etc. due to the structure of the pouring system. For this reason, it is impossible to substantially prevent clogging or blockage of the immersion nozzle.
And when pouring the molten steel in the mold, the molten steel flow from the discharge port cannot be prevented from drifting, the molten metal surface fluctuations caused by the drift, the entrainment of powder, and further the bubbles and inclusions There is a fact that a common problem that quality defects due to penetration into the deep part of the slab cannot be prevented cannot be solved.

The present invention has been made in view of such circumstances, and prevents clogging or clogging of a pouring nozzle for pouring molten steel into a mold, thereby stabilizing the pouring operation, and the molten steel of the immersion nozzle. It aims at providing the continuous casting method of the slab excellent in the quality characteristic which can manufacture the slab provided with favorable quality by high speed casting by suppressing the uneven flow of a discharge flow.

In the continuous casting method of a slab excellent in quality characteristics according to the first invention in accordance with the object, dolomite clinker is blended in at least a part of the aggregate, and the content W1 of the CaO component and the content W2 of the MgO component The mass ratio W1 / W2 is 0.46 to 3 and the refractory containing the MgO component is contained in an amount of 30 to 70% by mass. The molten steel is applied from the lower nozzle of the tundish that stores the molten steel to the immersion nozzle. It arrange | positions to the area of 1 to 48% of the inner side surface which touches, Molten steel is poured into a casting_mold | template using this, and it draws out from the said casting_mold | template at a casting speed of 0.6 m / min or more, solidifying molten steel.

In this way, the refractory having the above-described composition is disposed in a portion corresponding to an area of 1 to 48% of the inner surface in contact with the molten steel from the lower nozzle of the tundish that stores the molten steel to the immersion nozzle (for example, the portion thereof) to use, or by using those lining, etc.) was, molten steel as Al ₂ O ₃ and a deoxidation product, alumina type inclusions made of Al ₂ O ₃ produced by oxidation of Al in the molten steel Further, it is possible to prevent clogging or clogging of the passage caused by adhering or accumulating on the pouring system such as the lower nozzle and the immersion nozzle. Thereby, since the drift of the molten steel from the discharge port of the immersion nozzle can be suppressed, it is possible to cast a good quality slab by suppressing the entrainment of powder and inclusions and bubbles in the deep part of the slab.
In particular, when nozzle clogging or clogging occurs, the uneven flow of the molten steel discharge was suppressed by lowering the casting speed. By using the nozzle described above, nozzle clogging or clogging was eliminated. The For this reason, even when high speed casting is performed, the discharge flow from the immersion nozzle can be made, for example, left and right, so that the upward flow and the downward flow that collide with the solidified shell in the mold and reverse can be slowed down, and powder entrainment and casting can be performed. It is possible to realize high-speed casting in which inclusions and bubbles are prevented from entering into the deep part.

Here, clogging and clogging in the pouring system can be suppressed even if Al ₂ O ₃ composed of oxidation or deoxidation products of Al in molten steel adheres to the molten steel contact surface, that is, the working surface of the refractory. reacts this Al ₂ O ₃ and CaO components in the refractory, Al ₂ O ₃ -CaO based liquid phase is formed on the operating surface, CaO component is dissolved in the Al ₂ O ₃ -CaO based liquid phase At the same time, it is considered that the MgO particles gradually aggregate while moving gradually away from the working surface side.
By repeating such movement and aggregation of the MgO particles in the Al ₂ O ₃ —CaO-based liquid phase, an MgO-rich layer is formed on the working surface side with the coarsening of the MgO particles.
Then, CaO components present in the MgO-rich layer is dissolved by continuously until saturation concentration determined by the temperature during the Al ₂ O ₃ -CaO based liquid phase, the melting point of Al ₂ O ₃ -CaO based liquid phase Gradually decreases, it becomes easy to flow, alumina inclusions and bullion flow out together with the liquid phase of the working surface, nozzle clogging and clogging caused by alumina inclusions and bullion can be suppressed, and Since the outlet can be made normal, the drift of the discharge flow of molten steel can be greatly suppressed, and the degree of freedom of the inclination angle of the discharge port can be increased.

By setting the casting speed to 0.6 m / min or more, it is possible to produce a slab without a surface layer or internal defects of the slab, but the productivity is further increased, and the slab is subjected to a subsequent process such as a heating furnace at a high temperature. In order to effectively use the heat energy by supplying, the casting speed is preferably 0.8 m / min or more, and more preferably 1.0 m / min or more. On the other hand, although an upper limit is not specified, it is preferable to cast at a casting speed of 2.3 m / min or less, for example, considering the cooling capacity of a continuous casting facility that solidifies molten steel.

Here, when the mass ratio W1 / W2 between the CaO component content W1 and the MgO component content W2 is less than 0.46, the amount of CaO supplied to the working surface side is insufficient and sufficient Al ₂ O _3. A CaO-based liquid phase is not formed. For this reason, alumina inclusions easily adhere to the working surface side. Further, the MgO content in the composition of the refractory is excessively increased, and spalling and cracking are likely to occur.

On the other hand, when the mass ratio W1 / W2 exceeds 3, the amount of CaO supplied to the working surface becomes excessive, an excessive Al ₂ O ₃ —CaO-based liquid phase is formed, and an MgO-rich layer that can serve as a protective layer Since the formation of is hindered, the melting loss becomes severe. In addition, liquid phase components and aggregates in the refractory that have fallen off due to melting damage are mixed into the molten steel, thereby reducing the quality of the slab. And if CaO content increases, digestion (weathering) of a refractory will progress remarkably, and the quality of a refractory will be inhibited remarkably.

Moreover, when the content rate of MgO component exceeds 70 mass%, MgO component of a refractory increases, the spalling resistance of a refractory deteriorates, a crack and peeling arise in a refractory, Al adhered to an operation surface _The amount of CaO that reacts with ₂ O ₃ to form a low melting point compound is insufficient, and an Al ₂ O ₃ —CaO-based liquid phase cannot be formed on the working surface.
On the other hand, when the content of the MgO component is less than 30% by mass, it reacts with Al ₂ O ₃ adhering to the working surface, resulting in a large melting loss, and the refractory dolomite clinker falls off and the life of the refractory increases Reduced or dropped aggregate is mixed into the molten steel, impairing the quality of the slab.
From the above, in order to form a liquid phase on the operating surface and further improve the quality of the slab, it is preferable to set the content of MgO component of the refractory to 35 to 65% by mass, and further to 40%. It is preferable to make it -60 mass%. Thereby, an above-described effect | action can be expressed more notably.

Further, in this continuous casting method, since the refractory is disposed in an area of 1 to 48% or less of the inner surface in contact with the molten steel from the lower nozzle of the tundish that stores the molten steel to the immersion nozzle, the alumina system Adhering and depositing of inclusions is prevented and stable casting can be performed. In addition, if stable casting can be performed, it is preferable that the arrangement area of the refractory is 45% or less, more preferably 40% or less of the inner surface in contact with the molten steel.
On the other hand, if it is possible to place refractory in the part where the alumina inclusions are most likely to adhere and deposit on the inner surface in contact with the molten steel, and to prevent the alumina inclusions from adhering and depositing, and always allow the molten steel to pass through The area is preferably 5% or more, more preferably 10% or more.

The continuous casting method of a slab excellent in quality characteristics according to the second invention in accordance with the above object is a continuous casting method of a slab excellent in quality characteristics according to the first invention, wherein the refractory is the immersion nozzle. Provided as an inner body on at least a part of the inner surface of the cylindrical portion, the tilt angle of the molten steel discharge port of the immersion nozzle is in the range of 10 degrees upward to 35 degrees downward with respect to the horizontal position, The discharge port is immersed at a depth of 150 to 350 mm from the meniscus position, and molten steel is poured into the mold.
In this way, since the refractory compounded with dolomite clinker is provided as an interior body on at least a part of the inner surface of the cylindrical portion of the immersion nozzle, it prevents clogging of the immersion nozzle and captures and floats up inclusions in the molten steel. Promotion can be aimed at, and the flow can be made uniform by suppressing the drift of the discharge flow of molten steel.
In addition, by regulating the inclination angle of the discharge port of the immersion nozzle and the immersion depth of the immersion nozzle in the molten steel in the mold, the upward and downward flow rates of the molten steel discharged from the discharge port can be suppressed. In addition, it is possible to suppress defects due to fluctuations in the molten metal surface caused by the upward flow and powder entrainment, and intrusion of bubbles and inclusions due to the downward flow into the deep part of the slab. In addition, since there is no drift of the discharge flow of molten steel, it is possible to perform casting at a wide range of discharge port angles, and at the same time, the immersion depth is set in the range of 150 to 350 mm from the meniscus position, and stable high speed casting is possible. .

Here, when the inclination angle of the discharge port of the immersion nozzle exceeds 10 degrees upward with respect to the horizontal position, fluctuations in the molten metal surface and entrainment of powder due to upward flow occur.
On the other hand, when the inclination angle of the discharge port of the immersion nozzle exceeds 35 degrees downward with respect to the horizontal position, the downward flow becomes strong, and inclusions and bubbles accompanying the downward flow penetrate into the deep part of the slab, and the slab High quality slabs cannot be produced.
From the above, in order to manufacture a high quality slab, the inclination angle of the discharge port of the immersion nozzle is preferably in the range of 5 degrees upward to 20 degrees downward relative to the horizontal position, and further horizontal. It is preferable that the range is 5 degrees upward to 15 degrees downward with respect to the position.

For example, when the immersion depth of the upper end of the discharge port of the immersion nozzle becomes shallower than 150 mm, the upward flow of the molten steel discharged from the discharge port acts on the molten metal surface, causing the molten metal surface fluctuation and powder entrainment. .
On the other hand, when the immersion depth exceeds 350 mm, the downward flow of the molten steel becomes strong, and bubbles and generated Al ₂ O ₃ —CaO-based low melting point compounds and other inclusions accompany the deep part of the slab and float And the quality inside the slab deteriorates.
From the above, in order to manufacture a high quality slab, it is preferable that the immersion depth of the upper end portion of the discharge port of the immersion nozzle is 200 to 300 mm from the meniscus position.

The continuous casting method of the slab excellent in quality characteristics according to the third invention in accordance with the object is the continuous casting method of slab excellent in quality characteristics according to the second invention, wherein the inner diameter of the interior body is the above-mentioned A stepped portion is formed between the inner side surface of the interior body and the inner side surface of the cylindrical part, which is smaller than the inner diameter of the cylindrical part of the immersion nozzle.
In this way, a step is formed between the inner surface of the interior body and the inner surface of the immersion nozzle, preventing adhesion and accumulation of Al ₂ O _{3 on} the working surface, and at the same time attenuating the drop energy of molten steel and discharging Since the drift of the molten steel can be suppressed, the discharge flow of the molten steel can be made a slow flow and the flow can be made uniform. In addition, it can relieve the upward and downward flow of molten steel that collides with the inner surface of the solidified shell that is growing due to cooling of the mold and reverses it, causing fluctuations in the molten metal surface (meniscus) due to the upward flow, entrainment of powder, and downward flow It is possible to prevent internal defects caused by intrusion of bubbles and inclusions into the deep part of the slab.

The continuous casting method of a slab excellent in quality characteristics according to the fourth invention according to the above object is the continuous casting method of a slab excellent in quality characteristics according to the first to third inventions, in the immersion nozzle. The amount of argon gas to be blown is set to 0 or 0.2 to 10 NL / min.
Here, with respect to argon gas (hereinafter also referred to as Ar gas), it is possible to prevent Al ₂ O ₃ from becoming a low melting point compound and depositing on the inner surface of the immersion nozzle without blowing argon gas into the immersion nozzle. Therefore, casting without using any Ar gas is possible, and defects caused by air bubbles can be surely prevented in the surface layer and inside of the slab, and the quality of the steel type with strict air bubble defects can be significantly improved. .

In particular, when Al ₂ O ₃ inclusions tend to adhere, such as aluminum killed steel, Ar gas is blown to lower the melting point of the alumina inclusions that have become turbid in the molten steel in the mold. It is possible to raise the cleanliness of the slab by separating and floating from the molten steel by the bubble floating action, and to prevent defects due to inclusions in the slab.
Here, if the amount of Ar gas blown is less than 0.2 NL / min, the effect of promoting the floating of inclusions by Ar gas bubbles decreases.
On the other hand, if the blowing rate is higher than 10 NL / min, the effect of preventing the closing of the immersion nozzle can be improved, but the fluctuation of the molten metal surface due to the increase of Ar gas bubbles, the entrainment of powder, the trapping of bubbles in the solidified shell, the inside of the slab This may cause problems such as the intrusion of bubbles into the slab, leading to deterioration of the quality of the slab.
From the above, better results can be obtained by blowing Ar gas. When Ar gas is blown, the blow amount is less than the conventional blow amount (for example, 20 NL / min). The low melting point generated by reacting with the CaO component, which is a component contained in the immersion nozzle, by utilizing the buoyancy of the bubbles of Ar gas by making it 2-10 NL / min, more preferably 0.2-5 NL / min. The levitation of the Al ₂ O ₃ —CaO-based product is promoted to produce a slab having a high cleanliness.

The continuous casting method of the slab excellent in quality characteristics according to the fifth invention according to the above object is the continuous casting method of the slab excellent in quality characteristics according to the first to third inventions, in the immersion nozzle. The argon gas to be blown is blown from one or more of the upper nozzle made of dolomite attached to the tundish, the sliding nozzle of the lower nozzle, and the immersion nozzle.
In particular, the working surface of the refractory is protected with argon gas by using a dolomite refractory using dolomite clinker for the upper nozzle of the tundish, and by blowing argon gas into the molten steel also from this upper nozzle. It is possible to suppress the adhesion of Al ₂ O ₃ inclusions and to easily form a rich layer of MgO on the working surface of the dolomite refractory, thereby obtaining the effect of suppressing the melting damage of the working surface of the refractory. .
In addition, by blowing argon gas from one or both of the upper nozzle and the sliding nozzle, the effect of suppressing the adhesion of Al ₂ O ₃ inclusions to the sliding nozzle becomes significant, and fluctuations in the pouring amount are suppressed. And stable casting becomes possible.
The argon gas can be blown from any one of the upper nozzle, the sliding nozzle, and the immersion nozzle, but is preferably carried out from two or more places.

The continuous casting method of a slab excellent in quality characteristics according to the sixth invention that meets the above object is the continuous casting method of slab excellent in quality characteristics according to the first to fifth inventions, wherein the CaO component and the The mass ratio W1 / W3 of the content W1 of the CaO component to the content W3 of the remaining component excluding the MgO component is 2 to 30.
This remaining component refers to a commonly used refractory composition such as ZrO ₂ , Al ₂ O ₃ , SiC, SiO ₂ , Fe ₂ O _{3, and the} like.
Thus, when a predetermined amount of the remaining component is present in the refractory, the Al ₂ O ₃ —CaO-based liquid phase formed from the reaction between Al ₂ O ₃ adhering to the working surface side and CaO contained in the refractory is formed. The generation and the lowering of the melting point of the generated Al ₂ O ₃ —CaO-based liquid phase can be promoted, corrosion resistance as a refractory can be maintained, and the working surface can be maintained in a substantially smooth state throughout the casting process.

Here, when the mass ratio W1 / W3 is less than 2, the refractory itself has a low melting point or the production of Al ₂ O ₃ due to the oxidation of Al in the molten steel is promoted, and alumina inclusions are present on the working surface side. It becomes easy to adhere.
On the other hand, when the mass ratio W1 / W3 exceeds 30, the activation of CaO in the refractory decreases, the supply of CaO through the MgO-rich layer decreases, and alumina inclusions adhere to the working surface side. It becomes easy to do.
From the above, the mass ratio W1 / W3 is preferably 4 to 27, more preferably 5 to 25, in order to further suppress the adhesion of alumina inclusions on the working surface side.

A slab continuous casting method with excellent quality characteristics according to the seventh aspect of the present invention is a continuous slab casting method with excellent quality characteristics according to the sixth invention, wherein the remaining component contains SiO ₂ . The content is 3% by mass or less.
The continuous casting method of the slab excellent in quality characteristics according to the eighth invention according to the above object is the continuous casting method of slab excellent in quality characteristics according to the sixth and seventh inventions, wherein the remaining component is Fe. ₂ O ₃ is contained, and the content is 3% by mass or less.
In particular, SiO ₂ and Fe ₂ O _{3 in} the remaining components of the refractory are Al ₂ O ₃ —CaO-based liquids formed from the reaction between Al ₂ O ₃ adhering to the working surface side and CaO contained in the refractory. It promotes the formation of phases and suppresses the extremely low melting point of the refractory itself.

Here, when both SiO ₂ and Fe ₂ O ₃ exceed 3% by mass, they react with Al in the molten steel to produce Al ₂ O ₃ on the operating surface, and adhere and deposit as alumina inclusions on the operating surface. Is likely to occur. For this reason, Al ₂ O ₃ and SiO ₂ in the refractory, Fe ₂ O ₃ react to form a low melting point compound and promote melting, so the refractory aggregate is exposed and falls off the operating surface, There is a risk that the life of the refractory may be shortened or molten steel contamination due to inclusions may be caused.

On the other hand, the lower limit is not specified, but when SiO ₂ is less than 0.2% by mass and Fe ₂ O ₃ is less than 0.1% by mass, activation of CaO in the refractory becomes insufficient, The supply of CaO through the MgO rich layer cannot be secured sufficiently. As a result, there is a possibility that adhesion and deposition of alumina inclusions and further adhesion of the metal base may proceed on the molten steel contact surface.
From the above, the CaO in the refractory is activated to secure a low melting point and a liquid phase amount of the Al ₂ O ₃ —CaO liquid phase, and to prevent the adhesion, deposition, and metal adhesion of alumina inclusions. In order to suppress it, it is preferable that the content of SiO ₂ is 0.2 to 3% by mass, more preferably 0.8 to 3% by mass, and the content of Fe ₂ O ₃ is 0.1 to 3% by mass. %, More preferably 0.2 to 3% by mass.

The continuous casting method of the slab excellent in quality characteristics according to the ninth invention in accordance with the object is the continuous casting method of slab excellent in quality characteristics according to the first to eighth inventions, wherein the refractory is not used. It is baking, Comprising: 1-10 mass% of carbon components are contained in this refractory.
Here, the carbon component does not contain graphite, for example, from various carbon powders to be added, from residual carbon such as tar, pitch, phenol resin, tar impregnation, and other resins used as a binder. Obtainable. In particular, when digestion is considered, impregnation with tar is more effective for imparting carbon content and blocking pores.
Thus, the refractory is non-fired, and since the refractory contains a carbon component, the presence of this carbon component can absorb and mitigate the thermal expansion strain of the refractory, and the structure The stability as a body can be improved.

Here, when the content of the carbon component is less than 1% by mass, the ability to absorb and relax the thermal expansion strain of the refractory is small, and the stability as a structure cannot be improved.
On the other hand, when the content of the carbon component exceeds 10% by mass, oxidation due to oxygen in the molten steel of the carbon component and direct dissolution due to melting loss increase and the melting loss becomes remarkable. Moreover, the life as a refractory is extremely reduced, for example, the refractory aggregate is exposed and dropped on the operating surface side.
From the above, the content of the carbon component is set to 1 to 10% by mass, preferably 1 to 7% by mass, and more preferably more than 1% by mass and 5% by mass or less.

In claims 1-9 continuous casting method of the slab with excellent quality characteristics described, consisting of Al ₂ O ₃ or the like produced by the oxidation of Al as Al ₂ O ₃ or in the molten steel is deoxidized product of the molten steel It is possible to prevent clogging or clogging of the passage caused by the alumina inclusions adhering to and depositing on the pouring system such as the lower nozzle and the immersion nozzle. As a result, the drift of molten steel from the discharge port of the immersion nozzle can be suppressed, so that fluctuations in the molten metal surface can be avoided and defects such as entrainment of powder and heat supply to the vicinity of the molten metal surface can be prevented to prevent dekel formation. Stable casting becomes possible. In addition, the washing (cleaning) effect of the inner surface of the solidified shell of the slab can be positively expressed, and the bubbles and inclusions trapped in the solidified shell can be quickly lifted to reduce the surface layer defects. . And since the shape of each discharge outlet is stabilized and melt-resistant property can be expressed, the quality slab with few surface layers and internal defects resulting from a bubble and an inclusion can be manufactured.
Therefore, it is possible to stably cast a high quality slab in which powder entrainment, inclusions in the deep part of the slab, and intrusion of bubbles are suppressed.
In particular, even when performing high-speed casting, the discharge flow from the immersion nozzle can be made, for example, left and right, so that the upward and downward flow that collides with the solidified shell in the mold and reverses can be slowed down, and powder entrainment and slab It is possible to realize high-speed casting in which inclusions and bubbles in the deep part are suppressed.

In particular, in the continuous casting method of a slab excellent in quality characteristics according to claim 2, a refractory material containing dolomite clinker is provided as an interior body on at least a part of the inner surface of the cylindrical portion of the immersion nozzle. In addition, it is possible to prevent clogging of the immersion nozzle, capture inclusions in the molten steel, and promote floating, and to suppress the uneven flow of the discharge flow of the molten steel and make the flow uniform. Moreover, by regulating the inclination angle of the discharge port of the immersion nozzle and the immersion depth of the immersion nozzle in the molten steel in the mold, it is possible to suppress the upward and downward flow rates of the molten steel discharged from the discharge port. .
This makes it possible to slow down the upward and downward flow that collides with the solidified shell in the mold and reverses it, and realizes high-speed casting that suppresses powder entrainment, inclusions in the deep part of the slab and intrusion of bubbles, and increases productivity. It can be further increased.

In the continuous casting method of a slab excellent in quality characteristics according to claim ₃ , the adhesion and deposition of Al ₂ O _{3 on} the working surface is prevented, and the fall energy of the molten steel is attenuated at the same time, and the drift of the molten steel to be discharged is prevented. Since it can suppress, the discharge flow of molten steel can be made a slow flow, and the flow can be made uniform. In addition, it is possible to prevent internal defects caused by fluctuations in the molten metal surface and powder entrainment caused by the upward flow, and by bubbles and inclusions penetrating into the cast slab deep due to the downward flow. Thereby, the slab which raised the cleanliness further can be manufactured.

In the continuous casting method of a slab excellent in quality characteristics according to claim 4, casting without using argon gas is possible, and defects caused by bubbles can be reliably prevented in the surface layer and inside of the slab, The quality of steel types with strict cell defects can be significantly improved. In addition, when argon gas is blown, the alumina inclusions that have become low melting point and become turbid in the molten steel in the mold are separated and floated from the molten steel by the floating action of argon bubbles, thereby increasing the cleanliness of the slab. And defects caused by inclusions in the slab can be prevented.
Thereby, a high quality slab can be manufactured economically.

In the continuous casting method of a slab excellent in quality characteristics according to claim 5, the argon gas blown into the immersion nozzle is any one of an upper nozzle, a sliding nozzle, and an immersion nozzle made of dolomite attached to a tundish. Since it blows in from a location or two or more locations, the working surface of a refractory can be protected with argon gas. As a result, it is possible to suppress the adhesion of Al ₂ O ₃ inclusions and to suppress the melting of the working surface of the refractory, and to easily form a rich layer of MgO on the working surface of the dolomite refractory, Suppression of adhesion and resistance to erosion can be further improved.
Furthermore, the floating action of the blown-in argon gas can promote the floating of inclusions, which are low melting point compounds mixed in the molten steel in the mold, thereby cleaning the molten steel.

In the continuous casting method of a slab excellent in quality characteristics according to claim 6, since a predetermined amount of the remaining component is present in the refractory, Al ₂ O ₃ adhering to the working surface side and CaO contained in the refractory The formation of the Al ₂ O ₃ —CaO liquid phase formed from the reaction of the above and the lowering of the melting point of the generated Al ₂ O ₃ —CaO liquid phase can be promoted, and the corrosion resistance as a refractory can be maintained, and the casting process Since the working surface can be maintained in a substantially smooth state, stable casting can be performed.

In the continuous casting method of a slab excellent in quality characteristics according to claim 7, SiO ₂ is contained in the balance component, and in the continuous casting method of slab excellent in quality characteristics according to claim 8, the balance component Contains Fe ₂ O _3, which promotes the formation of Al ₂ O ₃ —CaO-based liquid phase on the operating surface side, and also suppresses the extremely low melting point of the refractory itself. It becomes possible to perform casting with the adhesion of system inclusions suppressed.

In the continuous casting method of a slab excellent in quality characteristics according to claim 9, since the content of the carbon component is 1 to 10% by mass, it absorbs and relaxes the thermal expansion strain of the refractory and serves as a structure. Stability can be enhanced, and carbon pickup of molten steel can be suppressed to a minimum while preventing adhesion, deposition, and adhesion of metal inclusions. As a result, it becomes possible to manufacture a cast of ultra-low carbon steel.

Next, embodiments of the present invention will be described with reference to the accompanying drawings for understanding of the present invention.
Here, FIG. 1 is an explanatory view of a continuous casting facility to which a continuous casting method of a slab excellent in quality characteristics according to an embodiment of the present invention is applied, and FIGS. 2 (A) and 2 (B) are the same continuous casting. Explanatory drawing of immersion nozzle of equipment, and explanatory drawing of immersion nozzle according to modification, FIG. 3 and FIG. 4 are explanatory views of the mechanism by which an MgO-rich layer is formed on the interior of the immersion nozzle of the continuous casting equipment, FIG. 5 is an explanatory diagram of quality determination of the refractory when the mass ratio of CaO / MgO is changed, and FIG. 6 is a quality determination of the quality of the refractory when the mass ratio of CaO / (remaining component) is changed. FIG. 7 is a graph showing the erosion rate index and the deposition rate index of alumina inclusions in the immersion nozzle according to Example 1, and FIGS. 8A and 8B are the Example 1 and the conventional example, respectively. The level of the molten metal during casting of the slab and the finger of the immersion nozzle opening FIG. 9 is a graph showing the slab inclusion index existing in the slab cast using the immersion nozzle according to Example 1 and Example 2, and FIG. 10 is a graph showing the quality of the slab produced. FIG. 11 is a graph showing the relationship between the yield index and the heating furnace calorie index (productivity index) according to the slab temperature and the casting speed. FIG. 11 shows the defect index of powder entrainment, internal bubbles, and inclusions, and the inclination angle of the discharge nozzle of the immersion nozzle. 12A and 12B are explanatory diagrams showing the replacement frequency index and the drift index of the immersion nozzle according to Example 1, respectively.

As shown in FIG. 1, a continuous casting facility 10 to which a continuous casting method of a slab excellent in quality characteristics according to an embodiment of the present invention is applied includes a tundish 12 that stores molten steel 11, and a tundish 12. It has a casting mold 15 into which molten steel 11 is poured from the tundish 12 through an immersion nozzle 14 communicating with the lower nozzle 13. Further, on the downstream side of the mold 15, a support segment 16 having a large number of cooling water nozzles (not shown) and a plurality of pressing rolls (not shown) are arranged on the downstream side of the support segment 16. A lightly pressed segment 17 and a pinch roll 19 for pulling out a slab 18 produced by solidifying the molten steel 11 at a predetermined speed are provided.

As shown in FIGS. 1 and 2A, the immersion nozzle 14 communicates with a lower nozzle 13 provided below an upper nozzle (not shown) whose main component is a dolomite clinker attached to the tundish 12. The inner surface of the bottomed cylindrical portion 20 and the molten steel 11 from the lower nozzle 13 to the immersion nozzle 14 (here, the molten steel 11 in the cylindrical portion 20 and its molten metal surface is called a secondary meniscus). 1 to 48% of the area 25, that is, the cylindrical interior body 21 disposed in a part of the inner side surface 25 of the cylindrical portion 20. A space portion 22 for mounting the interior body 21 is provided inside the tubular portion 20, and the space portion 22 is arranged so that the axial center of the tubular portion 20 and the axis center of the interior body 21 coincide with each other. The interior body 21 is disposed, and a joint 23 formed between the tubular portion 20 and the interior body 21 is filled with magnesia mortar so that the tubular portion 20 and the interior body 21 are integrated. The inner surface of the interior body 21 becomes the operation surface 24 (hereinafter also referred to as a molten steel contact surface), and the inner diameter D1 of the cylindrical portion 20 and the inner diameter D2 of the interior body 21 are substantially the same. 24 is arranged on the same curved surface as the inner side surface 25 of the cylindrical portion 20.

The interior body 21 has a mass ratio W1 / W2 between the CaO component content W1 and the MgO component content W2 of 0.46 to 3.0, and the MgO component is contained in an amount of 30 to 70% by mass. It is based on things. This refractory is non-fired, and the refractory contains 1 to 10% by mass of a carbon component.
Further, in this refractory, the mass ratio W1 / W3 of the content W1 of the CaO component to the content W3 of the remaining component excluding the CaO component and the MgO component is 2 to 30, in particular, SiO ₂ in the remaining component and each content of Fe ₂ O ₃ have been adjusted to respectively below 3 wt%.

This refractory uses dolomite clinker as a part of the aggregate so as to satisfy the above composition, and, for example, 3 to 30% by mass of MgO particles having a particle size of 0.5 mm or less (MgO particles) are added thereto. Furthermore, as a binder, for example, a phenol resin can be added and adjusted.
And the interior body 21 can be formed by shape | molding the above-mentioned refractory into a cylinder shape and hardening-processing a phenol resin.
Moreover, the cylindrical part 20 can be formed using the refractory for immersion nozzles conventionally used, for example, an alumina graphite refractory.

Moreover, since the interior body 21 is attached to the cylindrical portion 20 via the joint 23, the thermal expansion that occurs when the interior body 21 is heated by the molten steel 11 can be absorbed by the joint 23. The thermal expansion of the inner body 21 is not constrained, and the inner body 21 can be prevented from being damaged and the inner body 21 from falling off the cylindrical portion 20.
And the lower end part of the cylindrical part 20 is provided with the discharge port 26 of the molten steel 11 each opened to the both sides centering on the cylindrical part 20, The inclination | tilt angle (theta) of this each discharge port 26 is in a horizontal position. On the other hand, it is set in the range of upward 10 degrees to downward 35 degrees.
Furthermore, the cross-sectional shape of the discharge port 26 can be, for example, a circle, an ellipse, a rectangle, or the like, and a member having a through-hole having a circular cross section can be attached.

2B, the inner diameter D3 of the inner body 30 is made smaller than the inner diameter D1 of the cylindrical portion 20 (for example, 1/2 × D1 ≦ D3 <D1), and the inner surface of the inner body 30 An immersion nozzle 34 in which a stepped portion 33 is formed by the (operational surface) 31 and the inner side surface 32 of the cylindrical portion 30 can also be used.
Thereby, the molten steel 11 falling from the tundish 12 is made to collide with the level | step-difference part 33, and the fall energy of the molten steel 11 can be attenuated.

Then, the continuous casting method of the slab excellent in the quality characteristic which concerns on one embodiment of this invention is demonstrated, referring FIG. 1, FIG. 2 (A).
Molten steel 11 was put in a tundish 12 and further poured into a mold 15 from an immersion nozzle 14 provided below the tundish 12. In addition, the casting_mold | template 15 is a cross-sectional rectangular shape of 250 mm x 1000-1800 mm, for example. The solidified shell (solidified shell) 35 is generated by cooling with the casting mold 15 and water spraying from the cooling water nozzle provided in the support segment 16, and the growth of the solidified shell 35 is promoted. The slab 18 was cast by drawing from the mold 15 at a casting speed of 6 m / min or more.

The immersion nozzle 14 is arranged so that the upper end portion of each discharge port 26 of the immersion nozzle 14 is immersed in the molten steel 11 in the mold 15 at a depth D in the range of 150 to 350 mm from the meniscus (molten metal surface) position. It is fixed. At this time, there are both a case where argon gas is not blown into the immersion nozzle 14 and a case where argon gas is blown because adhesion of Al ₂ O ₃ inclusions becomes significant. Argon gas is blown into the immersion nozzle 14 by any one of the upper nozzle made of dolomite clinker of the tundish 12, the sliding nozzle (SN) plate (an example of the sliding nozzle) of the lower nozzle 13, and the immersion nozzle 14. This can be done through two or more locations. The argon gas is blown into the immersion nozzle 14 through, for example, a porous refractory provided in each nozzle, a slit, and the like, and the total blow amount is 0.2 to 10 NL / min. The range has been adjusted.

Here, the operation surface 24 of the interior body 21 has a property of being difficult to adhere to the metal and alumina inclusions, and the phenomenon that an MgO rich layer is formed on the operation surface 24 side of the interior body 21. This will be described with reference to FIGS.
As shown in FIG. 3, when the molten steel 11 is passed inside the interior body 21, Al ₂ O ₃ generated from Al in the molten steel 11 adheres to the working surface 24 of the interior body 21.
Here, the composition of dolomite clinker, for example, since the mass ratio W1 / W2 of the content W2 content W1 and MgO components of CaO component is set to 0.46 or more, the Al ₂ O ₃ deposited in dolomite clinker A low melting point Al ₂ O ₃ —CaO-based liquid phase can be formed by reacting with CaO.
Furthermore, since the mass ratio W1 / W2 is 3 or less, formation of an excessive Al ₂ O ₃ —CaO-based liquid phase can be suppressed, and digestion of the refractory can also be suppressed.

Here, FIG. 5 shows the quality determination result of the refractory when the mass ratio of CaO / MgO is changed. ◯ in FIG. 5 indicates a refractory with good quality, × indicates a refractory with poor quality, and a solid line indicates the effect when the carbon component in the refractory is adjusted to the above-described amount. In addition, the solid line which shows the influence of a carbon component shows that the quality of a refractory falls, so that the line rises.
As shown in FIG. 5, when the mass ratio W1 / W2 between the CaO component content W1 and the MgO component content W2 is less than 0.46, the amount of CaO supplied to the working surface is insufficient. Since an Al ₂ O ₃ —CaO-based liquid phase is not formed, alumina inclusions easily adhere to the working surface side. Further, the MgO content in the composition of the refractory is excessively increased, and spalling and cracking are likely to occur.

On the other hand, when the mass ratio W1 / W2 exceeds 3, the amount of CaO supplied to the working surface becomes excessive, an excessive Al ₂ O ₃ —CaO-based liquid phase is formed, and an MgO-rich layer that can serve as a protective layer Since the formation of is hindered, the melting loss becomes severe. In addition, the digestion (weathering) of the refractory is much easier to proceed, and the quality of the refractory is significantly impaired. And the liquid phase component and the aggregate in the refractory dropped off due to melting damage are mixed in the molten steel, and inclusion defects are generated in the manufactured slab, thereby degrading the quality of the slab.
From the above, by adjusting the carbon component to the above-mentioned amount and setting the mass ratio W1 / W2 to 0.46 to 3, the oxidation by oxygen in the molten steel and the direct melting by melting loss are reduced, The refractory can be prevented from melting. In addition, when the blending amount of dolomite clinker is 50% by mass or more, preferably 60% by mass or more, the MgO rich layer on the working surface can be stably formed. Thereby, on the operating surface side, exposure and dropping off of the refractory aggregate can be suppressed, and the life as a refractory can be extended as compared with the prior art.
In addition, a refractory with better quality can be obtained by blending 60% by mass or more of dolomite clinker and adjusting the mass ratio W1 / W2 to a range of 1.0 to 2.0.

The dolomite clinker has a mass ratio W1 / W3 of the content W1 of the CaO component with respect to the content W3 of the remaining component excluding the CaO component and the MgO component of 2 to 30, and SiO ₂ and Fe ₂ O ₃ in the remaining component. Are adjusted to be 3% by mass or less.
Here, the quality determination result of the quality of the refractory when the mass ratio of CaO / (remaining component) is changed is shown in FIG.
When the mass ratio W1 / W3 of the content W1 of the CaO component to the content W3 of the remaining component excluding the CaO component and the MgO component is less than 2, the spalling resistance is reduced, or Al due to oxidation of Al in the molten steel Generation of ₂ O ₃ is promoted, and alumina inclusions are likely to adhere to the working surface side.
On the other hand, when the mass ratio W1 / W3 exceeds 30, the activation of CaO in the refractory is reduced, the supply of CaO through the MgO-rich layer is reduced, the melting loss becomes severe, and the operating surface side is reduced. Unevenness is generated, and alumina inclusions are easily attached.

Thus, when there is a remaining component having SiO ₂ and Fe ₂ O ₃ at the grain boundaries of the dolomite clinker crystal particles, it reacts with CaO in the dolomite clinker to form a low melting point compound, which actively moves CaO. And the reactivity of CaO can be improved.
As a result, the reaction between Al ₂ O ₃ adhering to the dolomite clinker surface and CaO in the dolomite clinker is promoted, and the formation of a low melting point Al ₂ O ₃ —CaO-based liquid phase is promoted.
Furthermore, the presence of SiO ₂ and Fe ₂ O _{3 at} the grain boundaries of the dolomite clinker crystal particles can suppress digestion of the dolomite clinker and prevent deterioration of the quality of the interior body 21.

When the Al ₂ O ₃ —CaO-based liquid phase is formed, this liquid phase continuously dissolves the CaO component in the dolomite clinker until the CaO saturation concentration composition is reached.
As a result, an Al ₂ O ₃ —CaO-based liquid phase having a low melting point and improved fluidity is formed on the working surface 24 side of the interior body 21 of the dolomite clinker. _{The 2} O ₃ —CaO-based liquid phase flows out from the working surface 24.
Note that the thickness of the Al ₂ O ₃ —CaO-based liquid phase formed is considered to be governed by the penetration distance of Al ₂ O ₃ into the dolomite clinker.

On the operating surface 24 of the dolomite clinker, the generated Al ₂ O ₃ —CaO-based liquid phase flows out into the molten steel 11, and Al ₂ O ₃ generated from Al in the molten steel 11 frequently adheres. For this reason, an Al ₂ O ₃ —CaO-based liquid phase is formed almost continuously on the operating surface 24 side of the dolomite clinker. Therefore, in this Al ₂ O ₃ —CaO-based liquid phase, undissolved CaO particles in the dolomite clinker are gradually dissolved, and an Al ₂ O ₃ —CaO-based liquid phase exists around the MgO particles. It becomes like this.
In the state in which an Al ₂ O ₃ —CaO-based liquid phase is present around the MgO particles and the CaO particles are dissolved in the Al ₂ O ₃ —CaO-based liquid phase, the MgO particles are separated from the dissolved CaO particles. It gradually moves (diffuses) in a direction away from the operating surface 24 side so as to exchange its position, and gradually aggregates.

Further, in the state where the Al ₂ O ₃ —CaO-based liquid phase is present around the MgO particles having a particle diameter of 0.5 mm or less, the MgO particles having a particle diameter of 0.5 mm or less are also added to the Al ₂ O ₃ — It gradually moves away from the working surface 24 side so as to exchange its position with the CaO particles dissolved in the CaO-based liquid phase, and gradually aggregates.
Then, by repeating the movement and aggregation of the MgO particles in the Al ₂ O ₃ —CaO-based liquid phase, the MgO particles become coarse as shown in FIG. 4, and the MgO-rich layer continues on the working surface 24 side. Formed.

Furthermore, since the Al ₂ O ₃ —CaO-based liquid phase exists in this MgO-rich layer, CaO is continuously dissolved in the Al ₂ O ₃ —CaO-based liquid phase until a saturation concentration determined by temperature is reached. . As a result, the melting point of the Al ₂ O ₃ —CaO-based liquid phase gradually decreases and becomes easy to flow.
For this reason, since CaO in the dolomite clinker existing on the back of the MgO-rich layer is supplied to the working surface side through the MgO-rich layer in the form of an Al ₂ O ₃ —CaO-based liquid phase, Al ₂ O ₃ is prevented from adhering to the working surface 24 side. Further, the corrosion resistance on the working surface 24 side is improved by the MgO-rich layer formed on the working surface 24 side.
Thereby, since the drift of the molten steel from the discharge port 26 of the immersion nozzle 14 can be suppressed, it is possible to cast the high quality slab 18 while suppressing the entrainment of powder and inclusions and bubbles in the deep part of the slab. .

The result of having applied the continuous casting method of the slab excellent in the quality characteristic according to the present invention and performing the test will be described.
The inner body 21 of the immersion nozzle 14 is composed of the above-described composition and has a thickness of 25 mm and a length of 400 mm. The inner body 21 is formed after the secondary meniscus formed inside the cylindrical portion 20. It arrange | positioned so that it might become a void joint inside the cylindrical part 20 so that the molten steel contact surface of this may be covered. The area of the working surface 24 of the interior body 21 corresponds to 20% of the inner side surface in contact with the molten steel 11 from the lower nozzle 13 to the immersion nozzle 14 of the tundish 12. The tubular portion 20 is formed using an alumina graphite refractory.
Further, the inner diameter of each of the two discharge ports 26 is set to 60 mm, the inner diameter D1 of the cylindrical portion 20 is set to 70 mm, and the inner diameter D2 of the inner body 21 is mounted so that 70 mm of the inner diameter D1 of the cylindrical portion 20 can be substantially secured. (See FIG. 2A).

As a result, the melting point of Al ₂ O ₃ deposited and deposited on the inside of the immersion nozzle 14 and the discharge port 26 can be lowered, the accumulation of alumina inclusions can be prevented, and the nozzle clogging can be eliminated. The drift of the discharge flow of the molten steel 11 is prevented, and stable casting becomes possible.
Further, since the uneven flow of the discharge flow of the molten steel 11 is eliminated, the upward flow and the downward flow that collide with the inner surface of the solidified shell 35 and are reversed are alleviated and become a uniform flow. It is possible to eliminate quality problems such as entrainment.
And since a strong downward flow can be suppressed, it is possible to prevent bubbles and inclusions accompanying the descending molten steel flow from entering the deep portion of the slab 18 and prevent defects caused by the bubbles and inclusions inside the slab 18. can do.

Furthermore, since the Al in the molten steel 11 can be prevented from generating the Al ₂ O ₃ reacts with components of the refractory, to reduce the absolute amount of Al ₂ O ₃ contained in molten steel 11, simultaneously Al ₂ O ₃ and CaO of the interior body 21 can be generated and can easily float in the mold 15.
As described above, the clogging of the discharge port 26 can be prevented, so that the uneven flow of the discharge flow of the molten steel 11 is prevented, and high-speed casting with a large amount of pouring can be performed, which is caused by the upward flow and the downward flow during the high-speed casting. Thus, the quality problems can be solved, and the productivity of high quality slabs can be increased.

Next, the content of the carbon component and SiO _{2 in} the refractory constituting the immersion nozzle 14 was set to 1 mass% or less, and the molten steel 11 was poured into the mold 15 using the immersion nozzle 14. Here, as a result of investigating the nozzle clogging condition and the carbon pickup, the nozzle clogging was minor, and there were few defects of bubbles and inclusions due to the nozzle clogging. There was no carbon pickup.
Thus, compared with the conventional immersion nozzle, the quality of the immersion nozzle 14 was able to be improved.

Further, as Example 1, an inner body 30 having an inner diameter D3 of 50 mm is attached to a cylindrical portion 20 made of alumina graphite refractory and having an inner diameter D1 of 70 mm, and a stepped portion 33 is formed inside the cylindrical portion 20. The immersion steel 34 was used to pour the molten steel 11 into the mold 15 (see FIG. 2B). The investigated items are the following factors: nozzle clogging, slab inclusions, presence or absence of bubble defects, high-speed casting with increased casting speed, nozzle replacement frequency, and drift index. Moreover, the immersion nozzle of a prior art example is substantially the same as the cylindrical part 20 except that the space part is not provided.
FIG. 7 shows a comparison of the melting rate and the deposition rate of the alumina inclusions in the submerged nozzles of Example 1 and the conventional example in an indexed manner. In Example 1, argon gas was blown from one or more of the upper nozzle, sliding nozzle, and immersion nozzle, and the amount of argon gas blown into the immersion nozzle was 0.2 to 10 NL / min. Has been adjusted.
The deposition rate of alumina inclusions was 0 in Example 1 when the conventional example was 1. Moreover, when the conventional example is 1, the melting rate is 0.2 in Example 1.
As a result, in Example 1, it was confirmed that it was possible to satisfy both the melt resistance and the opening property of the immersion nozzle.

Moreover, to FIG. 8 (A) and (B), the fluctuation | variation state of the hot_water | molten_metal surface level in the casting of a slab and an immersion nozzle opening degree index is shown, respectively.
As shown in FIG. 8 (A), in Example 1 in which a normal refractory was used for the upper nozzle of the tundish, the adhesion rate and the erosion rate of the alumina inclusions were small, so the fluctuation of the immersion nozzle opening index It was confirmed that the width was very small and the fluctuation range of the hot water level was very small.
Further, the upper nozzle of the tundish is made a dolomite refractory containing dolomite clinker, and argon gas is blown from the upper nozzle and the immersion nozzle, and the total amount of argon gas blown into the immersion nozzle is 0.2 to 10 NL / min. In the case of (Example 2), the fluctuation range of the opening index of the immersion nozzle can be made extremely small as compared with the upper nozzle using a normal refractory, and the fluctuation range of the molten metal level is accordingly shown with a bold line. It was confirmed that the fluctuation range of the hot water surface level can be much smaller.
On the other hand, as shown in FIG. 8 (B), in the conventional example, the immersion nozzle tends to be closed due to the high deposition rate of alumina inclusions, and the opening of the immersion nozzle is gradually increased to supply a certain amount of molten steel. Therefore, the opening index of the immersion nozzle is gradually increased.

If the immersion nozzle opening increases and the amount of molten steel passing therethrough increases, the adhered alumina inclusions may peel off, and the amount of molten steel supplied increases rapidly. For this reason, it is necessary to reduce the immersion nozzle opening so as to reduce the molten steel supply amount. As a result, the fluctuation range of the hot water level and the immersion nozzle opening index is large.
Therefore, it can be seen from FIGS. 7 and 8 that stable casting operation is possible when the immersion nozzle of Example 1 is used.

In FIG. 9, the amount of slab inclusions present in each slab cast using each of the immersion nozzles of Example 1, Example 2 and the conventional example is shown as an index.
In Example 1, the slab inclusion index is 1/9 compared to the conventional example. This is considered to be a result of a decrease in the amount of contaminants derived from the immersion nozzle mixed in the molten steel because the melt resistance of the immersion nozzle of Example 1 is very excellent.
In particular, in the second embodiment, inclusions are reduced due to suppression of refractory melting, improvement of the floating removal effect in the mold by argon gas of inclusions slightly mixed, and discharge caused by inclusion adhesion Due to synergistic effects such as suppression of flow drift, the inclusion index of the slab could be further improved to 0.2 (1/10 of Example 1).

FIG. 10 shows the relationship between the casting index and the yield index of the quality of the cast slab produced, the furnace heat quantity index (productivity index) according to the slab temperature, and the casting speed. The yield index of slab quality indicates that the higher the index, the more slabs with better quality can be produced. The higher the index of the furnace calorific value (productivity index) by the slab temperature, the higher the index. It shows that the amount of heat required for reheating the manufactured slab can be reduced (productivity is improved).
The yield index of slab quality is better than that of the conventional example (the inclination angle of the discharge port is outside the above range: ×) in the range of the discharge port inclination angle of upward 10 degrees (□) to the downward direction 35 degrees (△). It was found that a numerical value can be obtained, and in particular, a numerical value close to about 100% can be achieved when the inclination angle of the discharge port is in the range of 5 degrees upward (◯) to 15 degrees downward (）). Although the yield index of slab quality has been shown to become worse as the casting speed increases, the yield index is still remarkable when the discharge port inclination angle is in the range of 5 degrees upward to 15 degrees downward. A decrease could not be confirmed.

Also, the heating furnace calorific value index due to the slab temperature indicated by the one-dot chain line increases as the casting speed increases, so the slab temperature drop during the casting process that occurred at low casting speeds as in the past It can be seen that the amount of heat required for reheating can be reduced. Thereby, productivity of a slab can also be improved.

FIG. 11 shows the relationship between the powder entrainment, the internal bubbles, and the defect index of inclusions and the inclination angle of the discharge port of the immersion nozzle. In addition, the defect index | exponent is what set the internal defect of the slab manufactured with the conventional immersion nozzle to 1, and has shown that the slab provided with favorable quality can be manufactured, so that the index | exponent is low.
It can be seen that a slab having a better quality than conventional ones can be produced when the discharge port has an inclination angle of 10 degrees upward to 35 degrees downward. In particular, the inclination angle of the discharge opening is 5 degrees upward to 15 degrees downward. It was confirmed that a higher quality slab can be produced by setting the value within this range.

The immersion depth of the immersion nozzle is set so that the upper end portion of the discharge port is in the range of 150 (□) to 350 (Δ) mm from the meniscus position, so that the casting with better quality than before is provided. It can be seen that a piece can be manufactured, but it is confirmed that a higher quality slab can be manufactured especially by setting the upper end of the discharge port to be in the range of 200 (O) to 250 (A) mm from the meniscus position. did it.
From the above, by setting the inclination angle of the discharge port and the immersion depth of the immersion nozzle in the above-described range, there is no uneven flow of the discharge flow of the molten steel. The downward flow is relaxed and the flow becomes uniform. As a result, quality problems such as fluctuations in the molten metal surface caused by upward flow and entrainment of powder can be eliminated, and bubbles and inclusions accompanying the descending molten steel flow can be prevented from entering the deep part of the slab, Defects caused by bubbles and inclusions inside the slab can be prevented.

Further, it can be seen that even if the argon gas blowing rate is 10 NL / min (dotted line), which is smaller than the conventional one, it is possible to produce a slab having better quality than the conventional one. It has been confirmed that a higher quality slab can be produced by setting so as to be in the range of 5 NL / min (solid line).
Conventional blown argon gas, which had been done in order to prevent adhesion of Al ₂ O ₃ to the inner surface of the immersion nozzle, Al ₂ by using the configured inner body with a refractory having a composition described above Since O ₃ is made of a low melting point compound and adhesion and deposition to the inner surface of the immersion nozzle can be prevented, the amount of argon gas blown can be reduced. As a result, it is possible to prevent bubbles from being blown into the surface layer and inside of the slab by the argon gas blown in the conventional manner, and to generate a low melting point Al ₂ O ₃ —CaO system generated by reacting with CaO. It is possible to manufacture a slab having a high cleanliness by utilizing the floating force of bubbles of argon gas to promote the floating of the object.
From the above, there is no nozzle clogging due to the effect of lowering the melting point of Al ₂ O ₃ , and slab inclusions and bubble defects due to the drift of the discharge flow of molten steel can be greatly reduced, and 1.8 m / min. High speed casting was possible.

Next, FIGS. 12A and 12B show the nozzle replacement frequency and drift index of the immersion nozzle.
As shown in FIG. 12A, the frequency of nozzle replacement according to Example 1 was 0.5 when the conventional example was 1. Further, as shown in FIG. 12B, the degree of drift of molten steel discharged from the discharge port was 0.7 in Example 1 when the conventional example was 1.
Thus, by performing casting using the immersion nozzle of the first embodiment, it is more economical than the prior art and the degree of drift can be reduced.
From the above, since the drift of the molten steel from the discharge port of the immersion nozzle can be suppressed, it is possible to cast a good quality slab while suppressing the entrainment of powder and inclusions and bubbles into the deep part of the slab. In particular, even when performing high-speed casting, the discharge flow from the immersion nozzle can be made, for example, left and right, so that the upward and downward flow that collides with the solidified shell in the mold and reverses can be slowed down, and powder entrainment and slab It is possible to realize high-speed casting in which inclusions and bubbles in the deep part are suppressed.

As described above, the present invention has been described with reference to one embodiment. However, the present invention is not limited to the configuration described in the above embodiment, and is described in the claims. Other embodiments and modifications conceivable within the scope of the above are also included. For example, a case where a continuous casting method of a slab excellent in quality characteristics of the present invention is configured by combining some or all of the above-described embodiments and modifications is also included in the scope of the present invention.
Moreover, in the said embodiment, although the case where the refractory was arrange | positioned to the inner surface of the immersion nozzle was demonstrated, 1-48% of the inner surface which contacts the molten steel from the lower nozzle of the tundish which stores molten steel to an immersion nozzle If it arrange | positions to this area, it is also possible to provide the above-mentioned refractory, for example in the inner surface of the lower nozzle of a tundish, or the inner surface from a lower nozzle to an immersion nozzle.

It is explanatory drawing of the continuous casting installation which applies the continuous casting method of the slab excellent in the quality characteristic which concerns on one embodiment of this invention. (A), (B) is explanatory drawing of the immersion nozzle of the same continuous casting installation, respectively, and explanatory drawing of the immersion nozzle which concerns on a modification. It is explanatory drawing of the mechanism in which an MgO rich layer is formed in the interior body of the immersion nozzle of the continuous casting equipment. It is explanatory drawing of the mechanism in which an MgO rich layer is formed in the interior body of the immersion nozzle of the continuous casting equipment. It is explanatory drawing of the quality determination of the quality of a refractory when changing the mass ratio of CaO / MgO. It is explanatory drawing of quality determination of the quality of a refractory when changing the mass ratio of CaO / (remaining component). It is a graph which shows the erosion rate index | exponent in the immersion nozzle which concerns on Example 1, and the adhesion rate index | exponent of an alumina type inclusion. (A), (B) is a graph which shows the fluctuation | variation of the hot_water | molten_metal surface level in the casting of the slab which concerns on Example 1 and a prior art example, respectively, and the immersion nozzle opening degree index, respectively. It is a graph which shows the slab inclusion index which exists in the slab cast using the immersion nozzle which concerns on Example 1 and Example 2. FIG. It is a graph which shows the relationship between the yield index of manufactured slab quality, the heating furnace calorie | heat amount index (productivity index) by slab temperature, and casting speed. It is a graph which shows the relationship between the powder entrainment, an internal bubble, and the defect index of an inclusion, and the inclination angle of the discharge port of an immersion nozzle. (A), (B) is explanatory drawing which shows the exchange frequency index | exponent and drift index of the immersion nozzle which concern on Example 1, respectively.

Explanation of symbols

10: Continuous casting equipment, 11: Molten steel, 12: Tundish, 13: Lower nozzle, 14: Dipping nozzle, 15: Mold, 16: Support segment, 17: Lightly pressed segment, 18: Slab, 19: Pinch roll, 20: cylindrical part, 21: interior body, 22: space part, 23: joint, 24: working surface, 25: inner surface, 26: discharge port, 30: interior body, 31: inner surface, 32: inner surface, 33: Stepped portion, 34: Immersion nozzle, 35: Solidified shell

Claims (9)

Dolomite clinker is blended in at least a part of the aggregate, the mass ratio W1 / W2 between the CaO component content W1 and the MgO component content W2 is 0.46 to 3, and the MgO component is 30 to 30%. The refractory contained in 70% by mass is arranged in an area of 1 to 48% of the inner surface contacting the molten steel from the lower nozzle of the tundish that stores the molten steel to the immersion nozzle, and the molten steel is poured into the mold using this. And a continuous casting method of slabs excellent in quality characteristics, wherein the molten steel is drawn from the mold at a casting speed of 0.6 m / min or more while solidifying the molten steel. The continuous casting method of a slab excellent in quality characteristics according to claim 1, wherein the refractory is provided as an inner body on at least a part of the inner surface of the cylindrical portion of the immersion nozzle, and discharges molten steel from the immersion nozzle. The inclination angle of the outlet is in the range of 10 degrees upward to 35 degrees downward relative to the horizontal position, and the discharge port of the immersion nozzle is immersed to a depth of 150 to 350 mm from the meniscus position, and molten steel is poured into the mold. A continuous casting method for slabs with excellent quality characteristics characterized by hot water. The continuous casting method of a slab excellent in quality characteristics according to claim 2, wherein an inner diameter of the inner body is smaller than an inner diameter of the cylindrical portion of the immersion nozzle, and an inner surface of the inner body and the cylindrical portion A method for continuously casting a slab excellent in quality characteristics, characterized in that a step portion is formed between the inner surface and the inner surface of the slab. The continuous casting method of a slab excellent in quality characteristics according to any one of claims 1 to 3, wherein the amount of argon gas blown into the immersion nozzle is 0 or 0.2 to 10 NL / min. A continuous casting method with excellent quality characteristics. The continuous casting method of a slab excellent in quality characteristics according to any one of claims 1 to 3, wherein the argon gas blown into the immersion nozzle is an upper nozzle made of dolomite attached to the tundish, A continuous casting method of a slab excellent in quality characteristics, characterized by being blown from any one or more of the sliding nozzle of the lower nozzle and the immersion nozzle. In the continuous casting method of the slab excellent in the quality characteristic of any one of Claims 1-5, content W1 of the said CaO component with respect to content W3 of the remainder component except the said CaO component and the said MgO component. A continuous casting method of a slab excellent in quality characteristics, wherein the mass ratio W1 / W3 is 2 to 30. The continuous casting method of a slab excellent in quality characteristics according to claim 6, wherein the remaining component contains SiO ₂ and its content is 3% by mass or less. Continuous casting method. The continuous casting method of a slab excellent in quality characteristics according to any one of claims 6 and 7, wherein the remaining component contains Fe ₂ O ₃ and the content thereof is 3% by mass or less. A continuous casting method with excellent quality characteristics. The continuous casting method of a slab excellent in quality characteristics according to any one of claims 1 to 8, wherein the refractory is non-fired and contains 1 to 10% by mass of a carbon component in the refractory. A continuous casting method for cast slabs with excellent quality characteristics.

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