JP4589151B2 - Nozzle for continuous casting and continuous casting method - Google Patents

Description

  The present invention relates to a continuous casting nozzle used for continuous casting of molten steel, and more particularly, to a continuous casting nozzle and a continuous casting method in which an inner hole body in which a dolomite clinker is blended is disposed in an inner hole of a nozzle body.

  In recent years, especially aluminum killed steel cast as high-grade steel such as thin plates has become stricter in steel quality, and alumina adheres to the inner hole of the nozzle for continuous casting used when pouring from the tundish into the mold in continuous casting. Much effort has been made to prevent it.

  Alumina adhering to the nozzle for continuous casting is united into large inclusions, which are taken together with the molten steel flow into the slab and become defects in the slab, reducing the quality.

  As an example of a measure for preventing the alumina from adhering to the inner hole of the continuous casting nozzle, a technique is adopted in which argon gas is blown into the molten steel from the inner surface of the nozzle to physically prevent the alumina from adhering. However, in this method, if the amount of argon gas blown is too large, bubbles are taken into the slab and become pinholes, resulting in slab defects. Therefore, there is a restriction on the amount of gas blown in, so that it cannot be a sufficient measure.

  As another effective means for preventing the adhesion of alumina, application of a refractory containing CaO can be mentioned. CaO reacts with the adhered alumina to form a low-melting substance, and this reaction product is easily flowed by the molten steel flow, so that the alumina adhesion can be suppressed. There is dolomite clinker as a typical refractory aggregate containing CaO, and by arranging a refractory containing dolomite clinker as an inner hole in the inner hole of the nozzle, adhesion of alumina can be suppressed. However, the dolomite clinker has two problems, namely, a problem that the thermal expansion is very large and a low melting point substance produced by reacting with alumina that is generally used as a nozzle for continuous casting.

  On the other hand, Patent Document 1 describes an immersion nozzle in which an inner hole body made of dolomite clinker and graphite is arranged in an inner hole through a mortar. This is a type in which the spalling resistance is improved by using graphite, and the reaction with the nozzle body is suppressed by disposing the inner hole body through mortar. However, a feature of dolomite clinker is that it prevents adhesion of alumina by reacting with alumina in steel to form a low-melting substance and melt in molten steel. For this reason, the inner hole body has a very large melting loss, and the inner hole body may be lost before the casting of one ladle is completed. In particular, a large amount of graphite is used as in Patent Document 1. However, this type has a problem that the melting loss is large.

  In order to prevent melting damage, a type with less graphite has also been proposed. Patent Document 2 describes that a gap is provided between the inner hole body and the nozzle body in order to prevent a decrease in spalling resistance (cracking due to expansion of the inner hole body) due to a decrease in graphite. Has been. However, there is a problem in that a complicated structure for securing the gap is required, and it takes time and effort to manufacture. Patent Document 3 describes that a magnesia raw material is used in combination with a dolomite clinker in order to improve durability.

  On the other hand, a nozzle for continuous casting using a magnesia clinker that does not react with the dolomite clinker in the nozzle body has also been proposed. For example, Patent Document 4 proposes an immersion nozzle in which the nozzle body is made of magnesia clinker and graphite, and the nozzle body and the inner hole body are in direct contact with each other. By integrally molding in this way, practical costs can be achieved. However, in this immersion nozzle, in order to improve the corrosion resistance, when the inner hole body does not contain graphite or the amount of graphite is reduced, the inner hole body expands and the inner hole body is damaged by thermal shock. There is a problem that the nozzle body breaks.

Therefore, Patent Document 5 describes increasing the porosity of the inner hole body in order to improve the spalling resistance of the inner hole body. However, when the porosity is more than 30%, the low melting point substance produced from Al ₂ O ₃ and CaO penetrates through the pores during use, so that there is a problem that the melting loss is increased and the life is shortened. is there.
JP-A-61-53150 Japanese Patent Laid-Open No. 7-232249 WO2004 / 082868A1 JP-A-2004-74242 Japanese Patent Laid-Open No. 10-5944

  An object of the present invention is to apply a refractory material containing magnesia-graphite material as a nozzle body and dolomite clinker as an inner pore, and in a continuous casting nozzle having a structure in which these are in direct contact with each other, the spalling resistance It is another object of the present invention to provide a continuous casting nozzle and a continuous casting method capable of suppressing the adhesion of alumina over a long period of time while achieving both corrosion resistance.

  The inventors of the present invention are particular about a nozzle in which an inner hole body containing a dolomite clinker, which is very easy to manufacture from a practical aspect, is incorporated in a form directly in contact with a magnesia-graphite nozzle body by integral molding, etc. In order to improve the spalling resistance and corrosion resistance at the same time, intensive studies were conducted. As a result, we found that it is possible to satisfy both spalling resistance and corrosion resistance at the same time by setting a specific porosity within a specific composition, and a continuous casting nozzle that is extremely superior in durability that has never been achieved before. Could get.

That is, the continuous casting nozzle of the present invention is a continuous casting nozzle in which at least a part of a nozzle body mainly composed of magnesia and graphite is in direct contact with an inner hole disposed in an inner hole of the nozzle body. The porous body was manufactured by adding an organic binder to a composition having a maximum particle size of 1 mm or less , in which the magnesia clinker is 5% by mass or more and 50% by mass or less and the balance is dolomite clinker, and is heat-treated after molding. In addition, the apparent porosity is 15% or more and 30% or less.

  Furthermore, the continuous casting method of the present invention is characterized in that, in the continuous casting method using the nozzle for continuous casting, the preheating temperature on the nozzle surface immediately before the start of casting is set to 600 ° C. or more.

  In the present invention, magnesia clinker other than dolomite clinker is required to be 5% by mass or more and 50% by mass or less in order to mainly improve the corrosion resistance. When the amount is less than 5% by mass, the corrosion resistance is lowered. When the amount exceeds 50% by mass, the amount of dolomite clinker is relatively reduced, so that the alumina adhesion preventing effect is reduced.

  In the present invention, the connective structure of the inner pore body is mainly composed of carbon bonds. Since carbon bonds are excellent in spalling resistance, they are suitable for inner pores using dolomite clinker. The carbon bond is formed by using an organic binder such as a phenol resin. Furthermore, in order to improve the spalling resistance, carbon raw materials generally used for refractories such as pitch, carbon black, and graphite are effective, and can be used at an outer covering of 5% by mass or less. . However, it is preferable not to use graphite or to use it as little as possible in order to prevent melting. The content of graphite is more preferably 3% by mass or less. In addition to carbon bonds, other additives generally used for refractories, such as metals, carbides, borides, glass, etc., can be used within 5% by mass on the outside.

  The apparent porosity after firing the blend of inner pores of the present invention is preferably in the range of 15% or more and 30% or less, more preferably 18% or more and less than 27%, from the viewpoint of both spalling resistance and corrosion resistance. If it is less than 15%, the elastic modulus of the inner hole body becomes high, and the possibility of cracking the nozzle body due to expansion at the initial stage of pouring becomes high. On the other hand, if it exceeds 30%, the corrosion resistance is greatly lowered, which is not suitable.

Maximum particle size of the dolomite clinker and magnesia clinker, which is a raw material of the inner bore bodies shall be the 1mm or less. In the present invention, during use, Al ₂ O ₃ in molten steel reacts with CaO in dolomite clinker to form a low melting point material, thereby preventing alumina adhesion. For this reason, when the porosity is increased, the low melting point substance is more likely to enter the structure, and the raw material particles are likely to fall off, resulting in a decrease in corrosion resistance. Therefore, when the maximum particle size of the raw material particles is reduced to 1 mm or less, the corrosion resistance is improved. This is presumed that when the raw material particles are small, even if they are dropped, the damage is small and the influence is small.

  In the present invention, a refractory material mainly composed of magnesia and graphite is used for the nozzle body because it has a thermal shock resistance, hardly reacts with the inner pores, and has a large thermal expansion. The amount of graphite is preferably 10% by mass or more and 40% by mass or less. If it is less than 10% by mass, spalling is likely to occur, and if it exceeds 40% by mass, the melting loss increases. The remainder is mainly magnesia, but it does not matter if it contains spinel, calcia, dolomite, alumina, chromite or the like. Further, a small amount of metal, boride, carbide, nitride or the like may be contained. Furthermore, a refractory material of another material may be partially arranged on the nozzle body. For example, in a submerged nozzle, there is no problem even if a zirconia-graphite refractory is applied to a portion in contact with the mold powder. The inner hole body arranged in contact with the nozzle body can be obtained by using the formulation defined in the present invention containing a dolomite clinker. It may be part or all.

  The dolomite clinker used in the present invention is a refractory raw material mainly composed of CaO and MgO, and can be used without any problem as long as it is a raw material generally used as a refractory raw material such as dolomite brick. be able to. For example, in addition to natural dolomite clinker obtained by heat-treating natural dolomite mainly composed of about 60% by mass of CaO and about 40% by mass of MgO, synthetic dolomite clinker prepared in an arbitrary composition using artificial raw materials can be used. .

  Examples of magnesia clinker usable in the present invention include clinker such as electrofused magnesia clinker and sintered magnesia clinker which are generally used for refractories. The purity of the magnesia clinker is not particularly limited, but is preferably 95% by mass or more, more preferably 98% by mass or more.

  The continuous casting nozzle of the present invention can be produced by a known method for producing a continuous casting nozzle. For example, in the case of an immersion nozzle, the composition of the nozzle body and the composition of the inner hole are kneaded separately. When kneading, an organic binder such as an organic synthetic polymer resin such as a phenol resin is used. Then, a cylindrical partition is put into the molding frame of the immersion nozzle to divide it into an inner hole part and a main body part. The kneaded product is filled, and then the partition is removed and pressure molding is performed with CIP. After molding, an immersion nozzle in which the inner hole body is disposed in the inner hole is obtained by heat treatment. The porosity can be controlled by the particle size configuration of the raw material used, the amount of the binder, or the volatile matter in the binder. The inner hole body may be disposed on the entire inner hole surface, or may be divided and disposed at a plurality of locations. The thickness is more preferably 5 to 20 mm.

  In addition, when the continuous casting nozzle of the present invention is applied to continuous casting as an immersion nozzle or the like, it is preferable that the preheating temperature on the nozzle surface immediately before the start of casting is 600 ° C. or higher. This is because, when the temperature is lower than 600 ° C., spalling may occur depending on the shape and material conditions.

  According to the continuous casting nozzle and the continuous casting method of the present invention, it is possible to prevent spalling at the initial stage of casting and to prevent alumina adhesion for a long time. Therefore, the quality of the slab can be improved and the life of the continuous casting nozzle itself can be significantly extended.

  Embodiments of the present invention will be described by way of examples.

[Example A]
In this example, the influence of the content of magnesia clinker in the composition for an inner pore (refractory material) was examined.

Table 1 shows the blending ratio of the blends as the test materials. The blend of the dolomite clinker having a CaO content of 60% by mass and the MgO content of 40% by mass and the magnesia clinker having a purity of 98% by mass. Each ratio is changed.

  Carbon black and an appropriate amount of phenol resin are added to each compound as an outer shell, and the uniformly kneaded soil is press-molded. The resulting molded body is heat-treated at 1000 ° C., and these samples are used as specimens in JIS. Based on R2205, the apparent porosity was measured, and the corrosion resistance and the poor alumina adhesion were evaluated.

  On the other hand, 75% by mass of magnesia clinker having a purity of 98% by mass, 25% by mass of graphite having a purity of 98% by mass, and further adding an appropriate amount of phenol resin as an outer shell and uniformly kneading the soil is used as the nozzle body. Samples shown in Table 1 were used as inner pores, and two-layer samples having an outer diameter of 130 mm, an inner diameter of 70 mm, a height of 200 mm, and an inner pore thickness of 10 mm were integrally molded and heat-treated at 1000 ° C. Was evaluated for spalling resistance.

  The apparent porosity was almost constant at around 22% for all samples. Corrosion resistance was evaluated by melting low-carbon aluminum killed steel at 1550 ° C in a high-frequency furnace, immersing the specimen in molten steel while rotating at a peripheral speed of 1.5 m / sec, and measuring the rate of erosion by pulling up after 4 hours. It was done by doing. The rate of erosion was expressed as an index with Comparative Example 1 taken as 100. The smaller the index, the better the corrosion resistance.

  As a result of evaluating the corrosion resistance, it is clear that the corrosion resistance is improved by increasing the content of magnesia clinker. In particular, a remarkable improvement effect is exhibited when the content is 5% by mass or more.

  In addition, the evaluation of adhesion to difficult alumina was conducted by melting low-carbon aluminum killed steel at 1550 ° C. in a high-frequency furnace, immersing the specimen in the molten steel while still, and 0.5% by mass of aluminum in the molten steel every 30 minutes. The addition was performed by suspending alumina in the molten steel and measuring the rate of alumina adhesion to the specimen by pulling up after 4 hours. The alumina deposition rate was expressed as an index with Comparative Example 1 taken as 100. The smaller the index, the better the adhesion of difficult alumina. In the difficulty-alumina adhesion, it decreases as the content of magnesia clinker increases. In particular, when the content exceeds 50% by mass, alumina adhesion increases rapidly, which is not practical.

  In the evaluation of the spalling resistance, a heat insulating material was wound around the outer periphery of the specimen, immersed in aluminum killed steel at 1550 ° C. to a depth of 150 mm for 5 minutes, and the inner hole body was heated. Table 1 shows the presence or absence of a crack from the cut surface of the sample after the test. Table 1 shows a case where there is no crack, a case where a crack that does not penetrate is △ as a microcrack, and a case where a crack penetrates as x. . In all of the examples and comparative examples shown in Table 1, no cracks were generated.

  From these evaluation results, it is understood that the total amount of magnesia clinker added is preferably 5% by mass or more and 50% by mass or less.

[Example B]
In this example, the influence of the apparent porosity in the inner pore body was examined.

Table 2 shows the blending ratios of the blends as the test materials. For the soil in which carbon black and an appropriate amount of phenol resin were added to each blend and kneaded uniformly, The plasticity of the soil was changed by adjusting the amount. As a result, the bulk specific gravity of the molded body changes, so that the apparent porosity after firing can be changed.

  Samples were prepared in the same manner as in Example A, and the apparent porosity, corrosion resistance, poor alumina adhesion, and spalling resistance were evaluated.

  As the apparent porosity increases, the corrosion resistance decreases. In particular, when it exceeds 30%, the corrosion resistance is significantly decreased. Further, when the apparent porosity was less than 15%, a large crack occurred in the spalling resistance test.

  From these evaluation results, it can be seen that the overall porosity is preferably 15% or more and 30% or less.

[Example C]
In this example, the influence of the maximum particle size of the composition for the inner pores was examined.

Table 3 shows the blending ratio of the blend as the test material, in which the maximum particle size of the dolomite clinker is changed.

  An appropriate amount of phenolic resin was added to each formulation in a pitch and an outer shell and the kneaded clay was press-molded. The resulting molded body was heat-treated at 1000 ° C., and these samples were used as test samples. In the same manner, the apparent porosity, corrosion resistance, poor alumina adhesion, and spalling resistance were evaluated.

  When the maximum particle size was 3 mm, the melting rate increased. There was no significant difference in adhesion to hard alumina and spalling resistance.

  From these evaluation results, it can be seen that the maximum particle size of the compound for inner pores is preferably 1 mm or less comprehensively.

[Example D]
As the inner pores, the clay of Comparative Example 1 and Example 2 shown in Table 1, the magnesia-graphite material described in Example A as the nozzle body, and the zirconia-graphite material in the powder line portion, A submerged nozzle having an inner hole was integrally formed, heat-treated at 1000 ° C. and processed into a predetermined shape, and an antioxidant was applied to obtain a specimen.

  These immersion nozzles were applied to cast aluminum killed steel. The preheating temperature was about 600 ° C. just before the start of casting, and the casting conditions were a pan capacity of 250 tons, a tundish capacity of 45 tons, a slab drawing speed of 1.0 to 1.3 m / min, and a casting time of about 320 minutes. It was.

  After the casting was finished, the immersion nozzle was cut and the cross section was observed. As a result, the adhesion of alumina to both nozzles was very slight and good. Regarding the erosion, the amount of erosion of the inner hole body of the nozzle using the soil of Example 2 was less than 3 mm, whereas the inner hole body of the nozzle using the soil of Comparative Example 1 was partially Clearly, the nozzle to which the inner hole of Example 2 was applied was better, such as about 5 mm melting.

  The present invention can be applied to immersion nozzles, long nozzles, lower nozzles, upper nozzles, sliding nozzle plates, open nozzles and the like used for continuous casting of steel.

Claims (3)

In a continuous casting nozzle in which at least part of an inner hole body arranged in an inner hole of the nozzle body and a nozzle body mainly composed of magnesia and graphite are in direct contact, the inner hole body has a magnesia clinker of 5% by mass or more. It is manufactured by adding an organic binder to a compound having a maximum particle size of 1 mm or less, the balance of which is composed of dolomite clinker and the balance is 50% by mass or less, kneading and heat-treating after molding, and has an apparent porosity of 15% or more Continuous casting nozzle that is 30% or less. The nozzle for continuous casting according to claim 1 , wherein the composition of the inner pore body contains 3% by mass or less of graphite .   The continuous casting method using the nozzle for continuous casting according to claim 1 or 2, wherein the preheating temperature on the nozzle surface immediately before the start of casting is 600 ° C or higher.