Carbon-containing refractory and immersion nozzle for continuous casting of steel

Abstract

To provide a carbon-containing refractory having higher oxidation resistance, and to provide an immersion nozzle.SOLUTION: The carbon-containing refractory contains a refractory raw material and a carbon material, and further contains a silicon alloy having calcium and aluminum.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a carbon-containing refractory and a submerged entry nozzle used in continuous casting of steel. [Background technology]

[0002] In continuous casting of steel, a submerged entry nozzle (SEN) containing refractory material is used to introduce molten steel from a tundish into a mold. The SEN plays a major role in preventing oxidation of the molten steel by the atmosphere and in rectifying and controlling the direction of the molten steel flow, and its stable operation is extremely important for ensuring productivity in continuous casting and steel quality.

[0003] The submerged entry nozzle has a complex structure: it is long and thin, and has multiple discharge holes on the side. Generally, the lower part of the submerged entry nozzle (near the discharge port) that is immersed in the molten steel in the mold is called the submerged part, the part above the submerged part that comes into contact with the mold powder slag is called the powder line, and the part above that is called the main body (see Figure 1).

[0004] The material for the submerged entry nozzle must have excellent thermal shock resistance against high temperatures ranging from the preheating temperature (several hundred degrees Celsius to around 1000 degrees Celsius) when molten steel passes through at the start of casting to approximately 1550 degrees Celsius. To meet this requirement, materials with a high carbon content of 8 to 35 mass % are typically used, such as zirconia-carbon for the powder line, and alumina-silica-carbon or alumina-magnesia-carbon for the submerged entry nozzle and main body. This is because carbon has an extremely small coefficient of thermal expansion at high temperatures.

[0005] However, carbon has the property of being easily oxidized by oxygen in the atmosphere at high temperatures. For example, in the manufacture of submerged entry nozzles, when the formed submerged entry nozzle is heated and baked up to approximately 1000°C, the carbon in the submerged entry nozzle may be oxidized by the oxygen in the fuel gas (e.g., a mixed gas of air and natural gas) used for heating in the baking furnace.

[0006] When using an SEN, the SEN is heated (preheated) in the atmosphere to a temperature of several hundred degrees Celsius to around 1000 degrees Celsius (the temperature varies depending on the position on the nozzle) just before casting begins. Even during this time, the carbon inside the SEN can be oxidized. It is possible to prevent carbon oxidation to some extent by applying an antioxidant to the surface of the SEN, but this effect is not necessarily sufficient.

[0007] During casting, the outside of the immersion nozzle body is exposed to the heat of the molten steel, reaching temperatures of around 1500°C, and is also exposed to the atmosphere for long periods of time, making it susceptible to oxidation of carbon within the nozzle. When carbon oxidizes, carbon dioxide gas is produced and volatilizes to the outside. This can cause the immersion nozzle's structure to become porous, reducing its strength and even its corrosion resistance to molten steel and molten slag. Therefore, in order to extend the nozzle's service life, it is important to suppress carbon oxidation.

[0008] In order to suppress the oxidation of carbon in the submerged entry nozzle that occurs during each of the above-mentioned heating processes, a method has been proposed in which metallic silicon and metallic aluminum are used as materials for the submerged entry nozzle.

[0009] This is thought to be because at high temperatures, metallic Si and metallic Al react with oxygen before carbon does, and the oxides (Al2O3, SiO2) produced by this reaction densify the structure of the SEN, thereby inhibiting the diffusion of oxygen into the SEN.

[0010] Japanese Patent Laid-Open Publication No. 6-206759 (Patent Document 1) proposes a refractory for continuous casting, characterized in that it is composed of a refractory composition containing 1 to 8 wt % of metallic Al powder (A) with a particle size of 0.2 mm or more and 1 to 8 wt % of metallic Al powder (B) with a particle size of 0.2 to 0.1 mm in a composition ratio of A / B = 1 to 3, metallic Si powder in an amount in the range of 0.5 to 6 wt % but in an amount of 40 wt % or more of the total of the metallic Al powder and the metallic Si powder, 3 to 20 wt % of ultrafine calcined alumina with a particle size of 10 μm or less, 2 to 12 wt % of a carbon raw material, and the remainder being a refractory raw material, and an outer percentage of 2 to 15 wt % of a binder relative to the refractory composition, and is fired in a non-oxidizing atmosphere.

[0011] Furthermore, Japanese Patent Laid-Open Publication No. 8-059337 (Patent Document 2) proposes a refractory for casting characterized by being composed of 40 to 70 weight % alumina, 5 to 20 weight % silica, 5 to 15 weight % carbon, 1 to 3 weight % metallic silicon, 0.5 to 2 weight % metallic aluminum, 0.5 to 3 weight % boron carbide, and 1 to 5 weight % soda glass. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Japanese Patent Application Publication No. 6-206759 [Patent Document 2] Japanese Patent Application Publication No. 8-059337 Summary of the Invention [Problem to be solved by the invention]

[0013] However, submerged entry nozzles manufactured from conventional refractories do not necessarily have sufficient resistance to oxidation due to the carbon during the heating process, and there is a demand for carbon-containing refractories with higher oxidation resistance. [Means for solving the problem]

[0014] The carbon-containing refractory according to the present invention is characterized by the following features: A carbon-containing refractory comprising a refractory raw material and a carbon material, The silicon alloy further contains calcium and aluminum.

[0015] In the carbon-containing refractory according to the present invention, it is preferable that the total content of calcium and aluminum in the silicon alloy is 0.8 mass % or more and 5 mass % or less.

[0016] In the carbon-containing refractory according to the present invention, it is preferable that the mass ratio of calcium to aluminum (Ca / Al) in the silicon alloy is 0.3 or more and 1.2 or less.

[0017] In the carbon-containing refractory according to the present invention, it is preferable that the content of the silicon alloy is 0.3 mass % or more and 7 mass % or less.

[0018] The submerged entry nozzle for continuous casting of steel according to the present invention is characterized in that it contains the above-mentioned carbon-containing refractory.

[0019] The submerged entry nozzle for continuous casting of steel according to the present invention is characterized in that it is made of the above-mentioned carbon-containing refractory. [Effects of the Invention]

[0020] The present inventors have systematically investigated the effects of additives on the high-temperature oxidation resistance of carbon-containing refractories used in submerged entry nozzles. As a result, it was found that carbon-containing refractories containing silicon alloys containing calcium and aluminum (hereinafter referred to as Si-Al-Ca alloys) have extremely high oxidation resistance, leading to the completion of the present invention.

[0021] Carbon oxidation begins at temperatures as low as approximately 450°C. Si-Al-Ca alloys exhibit a high degree of carbon oxidation prevention effectiveness over a wide temperature range, from low to high, including the temperatures during baking, preheating, and use of the submerged entry nozzle mentioned above.

[0022] It is thought that the Si-Al-Ca alloy reacts with oxygen at around 350°C, which is lower than the temperature at which carbon begins to oxidize, as follows, and removes oxygen that is present around the carbon. Si-Al-Ca(s, l)+O2(g)→SiO2-Al2O3-CaO(s, l)(1) (s, l): solid or liquid phase, (g): gas phase

[0023] At low temperatures, aluminum (Al) and calcium (Ca) in the Si-Al-Ca alloy are relatively more involved in the reaction, while at high temperatures silicon (Si) is more involved. Correspondingly, the SiO2-Al2O3-CaO slag produced contains more Al2O3 and CaO derived from Al and Ca, which react more easily with oxygen than Si, at low temperatures, and more SiO2 derived from Si at high temperatures.

[0024] The slag is first generated on the surface of the Si-Al-Ca alloy particles, and then diffuses into the intergranular spaces of the other refractory raw materials (e.g., oxide aggregates such as alumina, silica, magnesia, and zirconia) while reacting with them in the carbon-containing refractory. As a result, the Si-Al-Ca alloy particles are not covered with the slag, and the oxidation-preventing effect of the carbon can be maintained for a long period of time.

[0025] At the same time, the slag diffuses between the grains of the refractory raw materials, causing the pores between the grains to close at low temperatures. Furthermore, the areas of the refractory raw materials where the slag is generated expand, closing many of the intergranular pores in the refractory raw materials. This closure of the pores blocks oxygen diffusion into the interior of the immersion nozzle, which also contributes to the excellent oxidation resistance of carbon-containing refractories containing Si-Al-Ca alloys. [Brief explanation of the drawings]

[0026] [Figure 1] FIG. 2 is a schematic diagram of a vertical cross section of the submerged nozzle. DETAILED DESCRIPTION OF THE INVENTION

[0027] [Embodiment] Hereinafter, an embodiment of the present invention will be described. (carbon-containing refractories) The carbon-containing refractory according to the present invention comprises a refractory raw material, a carbon material, and a silicon alloy containing calcium and aluminum (hereinafter referred to as Si-Al-Ca alloy).

[0028] Applicable refractory raw materials include, for example, oxide aggregates such as alumina, silica, magnesia, and zirconia, and composite oxides such as mullite and spinel. One of these may be used alone, or a combination of two or more of these may be used.

[0029] The total content of calcium and aluminum in the Si-Al-Ca alloy is preferably 0.8% by mass or more and 5% by mass or less, and more preferably 0.9% by mass or more and 4% by mass or less.

[0030] When the total calcium and aluminum content is 0.8 mass% or more, the reaction temperature of the Si-Al-Ca alloy with oxygen becomes lower than the carbon oxidation temperature. In addition, the slag formed on the surface of the Si-Al-Ca alloy grains contains less SiO2 and is highly reactive with the oxide aggregate, which improves the durability of the reaction of the alloy grains with oxygen and enhances the sealing effect of closing the pores.

[0031] If the total content of calcium and aluminum is 5% by mass or less, carbides are unlikely to be generated by reaction between some of the aluminum and calcium in the Si-Al-Ca alloy and carbon, and these carbides (which have low reactivity with slag) are unlikely to inhibit the diffusion of SiO2-Al2O3-CaO slag between oxide aggregate particles, improving the durability of the reaction of Si-Al-Ca alloy particles with oxygen and enhancing the sealing effect.

[0032] The mass ratio of calcium to aluminum (Ca / Al) in the Si-Al-Ca alloy is preferably 0.3 or more and 1.2 or less, and more preferably 0.4 or more and 1.0 or less.

[0033] When the mass ratio of calcium to aluminum in the Si-Al-Ca alloy is between 0.3 and 1.2, the reaction initiation temperature between the Si-Al-Ca alloy and oxygen is lowered, the reactivity of the resulting slag and oxide aggregate is increased, the reaction of the Si-Al-Ca alloy particles with oxygen is sustained, and the sealing effect is enhanced, resulting in a significant increase in the alloy's ability to prevent carbon oxidation.

[0034] The purity of the Si-Al-Ca alloy is preferably 99 mass % or more in total of silicon, aluminum and calcium, and the particle size of the Si-Al-Ca alloy may be 100 mesh or less.

[0035] The content of the Si-Al-Ca alloy in the carbon-containing refractory is preferably 0.3 mass % or more and 7 mass % or less, and more preferably 0.5 mass % or more and 5.0 mass % or less.

[0036] If the Si-Al-Ca alloy content is 0.3 mass% or more, the amount of Si-Al-Ca alloy added is appropriate and the carbon oxidation prevention effect is enhanced. If the Si-Al-Ca alloy content is 7 mass% or less, silicon, aluminum, and calcium carbides are less likely to be generated, the SiO2-Al2O3-CaO slag formed on the surface of the Si-Al-Ca alloy particles is less likely to be inhibited from diffusing between the oxide aggregate particles, the reaction of the Si-Al-Ca alloy particles with oxygen is more sustained, the sealing effect is enhanced, and the carbon oxidation prevention effect is also enhanced.

[0037] In the conventional method of adding and mixing raw materials of metallic silicon, metallic aluminum, and high-calcium alloys to a carbon-containing refractory, the slag formed on the surface of each metal particle does not have a suitable composition and therefore has low reactivity with the oxide aggregate. This reduces the durability of the reaction of the metal particles with oxygen and also reduces the sealing effect. In fact, the use of high-calcium alloys may require additional safety measures as necessary in the production of submerged entry nozzles.

[0038] Applicable carbon materials include, for example, known carbon materials such as flaky graphite, carbon black, pitch, and resin charcoal. One of these may be used alone, or two or more may be used in combination. The carbon material may contain carbon (residual carbon) derived from the binder resin. The content of the carbon material in the carbon-containing refractory is preferably 8% by mass or more and 35% by mass or less.

[0039] (Manufacturing method of Si-Al-Ca alloy) The Si-Al-Ca alloy of the present invention can be produced by a conventional alloy production method. For example, it can be produced by a melting method using metals and alloys of silicon, aluminum, and calcium as raw materials and heating them in an electric induction furnace under an argon atmosphere. It can also be produced by a smelting method using silica, alumina, and calcia ores and carbon as a reducing agent in an electric arc furnace.

[0040] (Method of manufacturing carbon-containing refractories) The carbon-containing refractory according to the present invention can be produced by kneading at least the above-mentioned refractory raw material, carbon material, and Si-Al-Ca alloy using a known kneading device and kneading method. Such a kneading device is not particularly limited, but examples that can be used include a Fukai-type Conner mixer, an upper mixer, and an Eirich mixer.

[0041] (Submerged entry nozzle for continuous casting of steel) As shown in FIG. 1 , the submerged entry nozzle for continuous casting of steel according to the present invention (hereinafter referred to as submerged entry nozzle 1) comprises a cylindrical nozzle body 2, a submerged section 3, a molten steel flow passage 5 provided inside the nozzle body 2 so as to extend longitudinally from a molten steel inlet 4, a discharge hole 6 having one end communicating with the molten steel flow passage 5 and the other end opening on the outer peripheral surface of the nozzle body 2, and a powder line section 7 provided so as to surround a portion of the nozzle body 2 in the circumferential direction.

[0042] The submerged nozzle 1 can be made of the above-mentioned carbon-containing refractory for all of its components, including the nozzle body 2, the submerged section 3, and the powder line section 7. Alternatively, only some of these components, for example, the nozzle body 2 and the submerged section 3, may be made of the above-mentioned carbon-containing refractory. Alternatively, carbon-containing refractories having different compositions may be used for different components of the submerged nozzle 1. For example, the nozzle body 2 and the submerged section 3 may use a carbon-containing refractory containing alumina as a refractory raw material, while the powder line section 7 may use a carbon-containing refractory containing zirconia as a refractory raw material.

[0043] The submerged nozzle 1 may contain, as required and in accordance with a conventional method, the above-mentioned refractory raw material, additives such as carbides and borides, binders, etc. For example, the binder may be an organic binder such as a phenol resin, a furan resin, pitch, or tar, or an inorganic binder such as phosphoric acid or water glass.

[0044] (Method for manufacturing submerged entry nozzle for continuous casting of steel) The method for producing the submerged nozzle according to the present invention is not particularly limited, and any conventional nozzle production process can be used, including weighing, kneading, molding, drying, firing and processing the above-mentioned raw materials.

[0045] In the molding step, a known molding method such as cold isostatic pressing (CIP molding) can be used. Furthermore, in the firing step, the atmosphere is not particularly limited and can be appropriately selected from, for example, an air atmosphere, a reducing atmosphere, an inert atmosphere, etc. In addition, the firing temperature is also not particularly limited, but can be, for example, 700 to 1200°C. [Example]

[0046] (Oxidation resistance evaluation test for carbon-containing refractories) Various carbon-containing refractory samples were prepared by the processes of mixing, molding and firing, and oxidation resistance evaluation tests were carried out.

[0047] Cubic carbon-containing refractory samples (Examples 1 to 25 and Comparative Examples 1 to 4) with a side length of 80 mm were placed in an electric furnace and heated to 1500°C at 10°C / min in an air atmosphere, and held for 1 hour. After cooling, the samples were cut along the longitudinal center line. When the carbon in the samples was oxidized, a region where the original black color changed to white or the like was observed on the surface side of the cut sample.

[0048] The area of ​​the discolored area was measured using image processing software, and the area ratio (= area of ​​discolored area / total area of ​​cut surface x 100%) was calculated as the oxidation area ratio. This was used to evaluate the oxidation resistance of the material. The smaller the oxidation area ratio, the higher the oxidation resistance is judged to be.

[0049] The results of the oxidation resistance evaluation test for each sample are summarized in Tables 1 to 4. Table 1 shows a comparison of the results using the Si-Al-Ca alloy with those using metallic silicon and metallic aluminum. Table 2 shows the effect of the total content of aluminum and calcium in the Si-Al-Ca alloy, Table 3 shows the effect of the mass ratio of calcium to aluminum in the Si-Al-Ca alloy, and Table 4 shows the effect of the amount of Si-Al-Ca alloy added.

[0050] [Table 1]

[0051] [Table 2]

[0052] [Table 3]

[0053] [Table 4]

[0054] These results demonstrate that the carbon-containing material of the present invention has excellent oxidation resistance.

[0055] (Oxidation resistance evaluation test for submerged entry nozzles) An immersion nozzle manufactured using the carbon-containing refractory according to this example was used in actual continuous casting of steel. For the immersion nozzle of the present invention, the material of Example 2 was used for both the immersion part and the nozzle body, and the material of Example 4 was used for the powder line part. For the comparative immersion nozzle, the material of Comparative Example 2 was used for both the immersion part and the nozzle body, and the material of Comparative Example 4 was used for the powder line part. During the evaluation test, 200 immersion nozzles of the present invention and 200 comparative immersion nozzles were used in the same continuous casting machine, and approximately 180,000 tons of steel were cast for each.

[0056] As a result, the rate of problems with the SEN of the present invention was reduced by 67% compared to when the comparative SEN was used. Problems refer to events that render the SEN unusable due to breakage, cracking, abnormal melting, etc. When carbon in the SEN is oxidized, the likelihood of these problems occurring increases. [Industrial Applicability]

[0057] The carbon-containing refractory and the submerged entry nozzle for continuous casting of steel according to the present invention are suitably used in the steel industry. [Explanation of symbols]

[0058] 1: Submerged nozzle 2: Nozzle body 3: Immersion section 4:Inlet 5: Molten steel flow path 6:Discharge hole 7: Powder line section

Claims

1. A carbon-containing refractory comprising a refractory raw material and a carbon material, A carbon-containing refractory further comprising a silicon alloy with calcium and aluminum.

2. 2. The carbon-containing refractory according to claim 1, wherein the total content of calcium and aluminum in the silicon alloy is 0.8% by mass or more and 5% by mass or less.

3. 2. The carbon-containing refractory according to claim 1, wherein the mass ratio of calcium to aluminum (Ca / Al) in the silicon alloy is 0.3 or more and 1.2 or less.

4. 2. The carbon-containing refractory according to claim 1, wherein the content of the silicon alloy is 0.3 mass % or more and 7 mass % or less.

5. An immersion nozzle for continuous casting of steel, comprising the carbon-containing refractory according to claim 1.

6. 2. An immersion nozzle for continuous casting of steel, comprising the carbon-containing refractory material according to claim 1.

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