Method for producing magnesia-carbon brick for vacuum degassing furnace

Abstract

To provide a method for producing a magnesia-carbon brick for a vacuum degassing furnace, capable of improving spalling resistance while maintaining resistance to oxidation by iron in the vacuum degassing furnace.SOLUTION: A method for producing a magnesia-carbon brick for a vacuum degassing furnace includes a step of adding an organic binder to a refractory raw material mixture, kneading, molding, and then heat-treating, wherein the refractory raw material mixture: has a total magnesia content of 86 mass% or more and 99 mass% or less; contains 45 mass% or more and 65 mass% or less of magnesia having a particle size of 1 mm or more but less than 5 mm, 1 mass% or more and 12 mass% or less of magnesia having a particle size of less than 0.075 mm, 0.3 mass% or more and 5 mass% or less of alumina having a particle size of less than 0.075 mm, and 0.3 mass% or more and 2 mass% or less of pitch powder having a particle size of less than 0.5 mm; and further has a graphite content of 5 mass% or less (including 0).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing magnesia carbon bricks used in vacuum degassing furnaces such as RH, DH, and VOD. [Background technology]

[0002] Magnesia-carbon bricks generally contain graphite as a carbon source. However, because of the high thermal conductivity of graphite, problems such as heat loss due to heat dissipation from molten metal and carbon pickup are common. In particular, vacuum degassing furnaces for processing ultra-low carbon steels require magnesia-carbon bricks containing no or very little graphite to prevent carbon pickup. From these perspectives, it is desirable for magnesia-carbon bricks to contain no or as little graphite as possible. However, a reduced amount of graphite can lead to a problem of reduced spalling resistance. Therefore, various methods have been proposed to prevent the deterioration of spalling resistance that accompanies the absence or reduction of graphite content.

[0003] Patent Document 1 discloses that graphite-free magnesia-carbon bricks contain 0.1% by mass or more and 2.0% by mass or less of pitch and / or carbon black to reduce the modulus of elasticity of the bricks and improve their spalling resistance. The magnesia-carbon bricks of Patent Document 1 have significantly improved spalling resistance and have been used successfully in actual furnaces. However, when the present inventors analyzed the bricks after use, they found that further improvement in spalling resistance was necessary.

[0004] Patent Document 2 shows that combining alumina with a refractory raw material mainly composed of magnesia results in gentler sintering at high temperatures than when magnesia is used alone, and is particularly effective in preventing oversintering of low-carbon materials and preventing them from becoming too high in elastic modulus. Example 4 discloses a magnesia-carbon brick containing 5% by mass of electrofused alumina with a particle size of 1 mm or less and 1% by mass of carbon black. However, the magnesia-carbon brick of Patent Document 2 has the problem of insufficient iron oxide resistance when used in a vacuum degassing furnace.

[0005] Patent Document 3 also shows that by adding 2 to 10 mass% alumina powder to a compounding composition mainly containing magnesia and 5 to 25 mass% carbon, the spinel formed by heat exposure prevents slag infiltration, improving slag erosion resistance. It also shows that, preferably, the average particle size of the alumina powder is 10 μm or less, making it possible to prevent structural destruction due to expansion during spinel formation. However, the magnesia-carbon brick of Patent Document 3 has the problem of insufficient durability when used in a vacuum degassing furnace. Furthermore, the graphite content is too high, which causes the problem of carbon pickup. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2018-70406 [Patent Document 2] Japanese Patent Application Laid-Open No. 2004-107124 [Patent Document 3] Japanese Patent Application Publication No. 4-119962 Summary of the Invention [Problem to be solved by the invention]

[0007] For example, in RH furnaces, especially under operating conditions with a high oxygen-blowing ratio from the top lance, the working surface temperature of bricks in the furnace becomes extremely high. This heat, combined with the oxygen blown into the furnace, oxidizes the molten steel, generating iron oxide, which can easily corrode the molten steel. Because this iron oxide has low viscosity at high temperatures, it penetrates the brick more easily than conventional ladle slag, which is primarily composed of CaO and SiO2. Furthermore, iron oxide dissolves in magnesia, lowering its melting point and significantly increasing brick corrosion. Furthermore, the presence of a large amount of alumina in the brick structure further lowers the melting point when infiltrated by iron oxide. Therefore, the addition of alumina tends to reduce iron oxide resistance. While it is well known that magnesia-carbon bricks contain alumina to improve spalling resistance, the simple addition of alumina alone reduces corrosion resistance to high-temperature iron oxide during oxygen-blowing operation in a vacuum degassing furnace, resulting in poor durability in actual operation.

[0008] In view of the above, an object of the present invention is to provide a method for producing a magnesia-carbon brick for a vacuum degassing furnace, which can improve spalling resistance while maintaining iron oxidation resistance in the vacuum degassing furnace. [Means for solving the problem]

[0009] The present inventors have discovered that in a method for producing low-carbon magnesia-carbon bricks, adding a specific amount of alumina having a particle size of less than 0.075 mm to a refractory raw material mixture improves the filling property during molding and densifies the structure, thereby improving spalling resistance while maintaining iron oxidation resistance, and further that adding a specific amount of pitch powder having a particle size of less than 0.5 mm to the refractory raw material mixture significantly reduces the elastic modulus of the brick after heat treatment and improves spalling resistance.

[0010] That is, according to one aspect of the present invention, there is provided the following method for producing magnesia carbon bricks for a vacuum degassing furnace. A method for producing magnesia carbon bricks for a vacuum degassing furnace, comprising the steps of adding an organic binder to a refractory raw material mixture, kneading the mixture, forming the mixture, and then heat treating the mixture, A method for producing magnesia carbon bricks for a vacuum degassing furnace, characterized in that the refractory raw material mixture has a magnesia content of 86% by mass or more and 99% by mass or less, contains 45% by mass or more and 65% by mass or less of magnesia with a particle size of 1 mm or more and less than 5 mm, 1% by mass or more and 12% by mass or less of magnesia with a particle size of less than 0.075 mm, 0.3% by mass or more and 5% by mass or less of alumina with a particle size of less than 0.075 mm, and 0.3% by mass or more and 2% by mass or less of pitch powder with a particle size of less than 0.5 mm, and has a graphite content of 5% by mass or less (including 0). [Effects of the Invention]

[0011] The bricks obtained by the manufacturing method of the present invention have excellent resistance to iron oxidation and spalling, and therefore, when used as lining materials, it is possible to significantly extend the life of vacuum degassing furnaces such as RH furnaces. DETAILED DESCRIPTION OF THE INVENTION

[0012] As mentioned above, adding a specific amount of fine alumina to the refractory raw material mixture of magnesia-carbon bricks improves the packing properties during molding, resulting in bricks with a dense structure. Specifically, the alumina located in the fine voids (pores) forms spinel upon exposure to heat, and the expansion of this spinel generates microcracks, absorbing thermal stress and improving spalling resistance. Furthermore, because the alumina and spinel fill the fine voids (pores), they cut off the infiltration pathways of iron oxide, suppressing infiltration and preventing a decrease in iron oxide resistance at high temperatures. It is also known in the art that the inclusion of alumina improves corrosion resistance in slag primarily composed of CaO and SiO2. Alumina reacts with surrounding magnesia particles and the MgO component in the slag to form a spinel, which prevents slag infiltration. Furthermore, the Al2O3 component melts into the slag, increasing the slag's viscosity and preventing slag infiltration into the brick structure. However, as mentioned above, simply including alumina can result in a decrease in iron oxidation resistance. In the present invention, the inclusion of a specific amount of alumina as fine powder with a particle size of less than 0.075 mm improves the density of the brick structure, prevents iron oxide infiltration, and suppresses the decrease in iron oxidation resistance. In other words, a specific amount of alumina fine powder with a particle size of less than 0.075 mm is required to improve the packing property during molding. Alumina with a particle size of 0.075 mm or greater does not improve the packing property during molding. Furthermore, ultrafine alumina powder with a particle size of less than 0.045 mm can be used to further improve packing property during molding.

[0013] In the present invention, alumina having a particle size of less than 0.075 mm is used in a content of 0.3% by mass or more and 5% by mass or less in the refractory raw material blend. If the content of alumina having a particle size of less than 0.075 mm is less than 0.3% by mass, the effect of improving spalling resistance cannot be obtained. On the other hand, if the content of alumina having a particle size of less than 0.075 mm exceeds 5% by mass, the Al2O3 component in the brick structure increases, increasing the amount of liquid phase generated when iron oxide infiltrates, and as a result, iron oxidation resistance at high temperatures decreases. From the viewpoint of further improving iron oxidation resistance and spalling resistance, the content of alumina having a particle size of less than 0.075 mm can be set to 1% by mass or more and 3% by mass or less.

[0014] The alumina having a particle size of less than 0.075 mm that can be used in the present invention may be one that is commonly used as a raw material for refractories, such as one or more of fused alumina, sintered alumina, calcined alumina, and bauxite. Calcined alumina can be used in particular from the viewpoint of improving packing properties during molding and suppressing a decrease in iron oxidation resistance. Calcined alumina is an ultrafine powder with an average particle size of approximately 10 μm or less, and is necessarily a type of alumina having a particle size of less than 0.075 mm or alumina having a particle size of less than 0.045 mm.

[0015] Here, the particle size referred to in the present invention refers to the size of the sieve openings when the refractory raw material particles are sieved and separated. For example, alumina having a particle size of less than 0.075 mm means alumina that passes through a sieve with 0.075 mm openings, and alumina having a particle size of 0.075 mm or more means alumina that does not pass through a sieve with 0.075 mm openings. The average particle size in the present invention refers to the particle size at which the cumulative mass ratio reaches 50% when the relationship between particle size measured with a laser diffraction / scattering particle size distribution analyzer and mass ratio is plotted on a graph.

[0016] In the present invention, pitch powder is used in a content of 0.3% by mass or more and 2% by mass or less in the refractory raw material blend for the purposes of lowering the elastic modulus of the brick and improving spalling resistance, and also as a binder. If the pitch powder content is less than 0.3% by mass, the effect of lowering the elastic modulus is insufficient, while if it exceeds 2% by mass, the volatile content causes too many pores, increasing the porosity and reducing iron oxidation resistance. From the perspective of prioritizing iron oxidation resistance, the pitch powder content can be 0.3% by mass or more and 1% by mass or less. By using the pitch in powder form, after the kneaded refractory raw material mixture is molded into a brick shape, it remains in the molded body in a state adhering to the surface of the magnesia particles. When the molded body is heat-treated, the pitch reduces in volume or liquefies, and voids are generated in the areas where the pitch was present after the heat treatment. As a result, it is believed that the effect of lowering the elastic modulus of the brick is significant. This effect is more pronounced in the manufacturing method of magnesia-carbon bricks that contain no or very little graphite, particularly when the graphite content in the refractory raw material mixture is 5% by mass or less (including 0).

[0017] The pitch powder that can be used in the present invention can be any that is used in ordinary refractories as long as it is powdery at room temperature, but preferably has a softening point of 70 to 370°C. Pitch powder with a softening point of 70 to 370°C softens during heat treatment and is absorbed in part or in whole into the brick structure, forming voids that tend to reduce the elastic modulus and also to form a uniform carbon bond. The particle size of the pitch powder should be less than 0.5 mm to ensure uniform dispersion in the brick structure.

[0018] In the present invention, the graphite content in the refractory raw material blend is 5% by mass or less (including 0). If the graphite content exceeds 5% by mass, the graphite is significantly oxidized by iron oxide generated by the oxygen gas used during smelting in a vacuum degassing furnace, resulting in a decrease in corrosion resistance. Furthermore, in recent years, it has been desirable to keep the graphite content of magnesia-carbon bricks used in vacuum degassing furnaces as low as possible from the perspective of carbon pickup, etc., so graphite is either not contained or, if it is contained, is 5% by mass or less. Furthermore, in applications where the impact of carbon pickup is significant, graphite can be omitted or can be kept at 1.5% by mass or less. The graphite used may be scaly graphite or expanded graphite, which are commonly used in magnesia carbon bricks, but synthetic graphite may also be used. The particle size of the graphite may be less than 0.1 mm.

[0019] In the present invention, to obtain sufficient corrosion resistance and spalling resistance for magnesia-carbon bricks used in vacuum degassing furnaces, the refractory raw material mixture contains 45 to 65% by mass of magnesia with a particle size of 1 to 5 mm and 1 to 12% by mass of magnesia with a particle size of less than 0.075 mm. If the content of magnesia with a particle size of 1 to 5 mm is less than 45% by mass, spalling resistance is insufficient. If the content of magnesia with a particle size of 1 to 5 mm is more than 65% by mass, the refractory raw material mixture will contain a large amount of coarse particles, resulting in poor packing during molding, high porosity, and poor iron oxidation resistance and slag resistance. If the content of magnesia with a particle size of less than 0.075 mm is less than 1% by mass, poor packing during molding will result in high porosity and poor iron oxidation resistance. If the content of magnesia with a particle size of less than 0.075 mm is more than 12% by mass, sintering will occur during use, resulting in a high elastic modulus and poor spalling resistance. The total content of magnesia in the refractory raw material blend is set to 86% by mass or more and 98% by mass or less. If the content is less than 86% by mass, the amount of magnesia is too small, resulting in reduced corrosion resistance, while if the content is more than 98% by mass, the amount of magnesia is too high, resulting in reduced spalling resistance. The magnesia used in the refractory raw material composition of the present invention may be either electrofused magnesia or sintered magnesia, or a combination of these. The composition is not particularly limited, but to obtain higher corrosion resistance, magnesia with a high MgO purity can be used, for example, magnesia with an MgO purity of 96% by mass or more, or even 98% by mass or more.

[0020] Although sufficient effects can be obtained without using aluminum and / or aluminum alloys in the refractory raw material composition of the present invention, they can also be used in a total content of 0.3% by mass to 2.5% by mass in order to densify the brick structure and prevent oxidation. The effects of aluminum and / or aluminum alloys in densifying the brick structure and preventing oxidation are more pronounced when fine aluminum and / or aluminum alloys with a particle size of, for example, less than 0.075 mm are used. Aluminum and aluminum alloys commonly used in magnesia-carbon bricks can be used without any problems. Furthermore, as mentioned above, particle sizes of less than 0.075 mm can be used.

[0021] In addition to the magnesia, alumina, pitch, graphite, aluminum, and aluminum alloys described above, the refractory raw material composition of the present invention may contain, without adverse effects, raw materials generally used as raw materials for magnesia-carbon bricks, in a total amount of about 2 mass % or less, such as one or more of silicon carbide, silicon, carbon black, boron carbide, fiber, and glass.

[0022] The method for producing a magnesia-carbon brick of the present invention is characterized by the composition of the refractory raw material mixture, but otherwise can be a general method for producing unfired magnesia-carbon bricks. That is, the method for producing a magnesia-carbon brick of the present invention includes the steps of adding an organic binder to the above-mentioned refractory raw material mixture, kneading and molding the mixture, and then heat-treating the mixture. The heat-treatment temperature can be, for example, 150 to 600°C, preferably 150 to 400°C. To improve the dispersion of the pitch powder, it is also possible to mix a portion of the refractory raw material mixture with the pitch powder in a mixer, and then add the remaining refractory raw material mixture and the organic binder to the mixer and knead them. [Example]

[0023] Table 1 shows the compositions (mass%) of the refractory raw material blends in the examples and comparative examples of the present invention, as well as the physical properties of the resulting bricks. The refractory raw materials used were electrofused magnesia containing 98 mass% MgO, sintered alumina containing 98 mass% Al2O3 for alumina with particle sizes of less than 1 mm and less than 75 μm, calcined alumina containing 98 mass% Al2O3 for alumina with particle sizes of less than 45 μm, and flaky graphite with a C content (fixed carbon content) of 97 mass%. The pitch powder used had the softening points shown in Table 1.

[0024] [Table 1]

[0025] Magnesia carbon bricks were manufactured by adding 2% by mass of phenolic resin as an organic binder to the refractory raw material composition shown in Table 1, kneading the mixture, and molding it into a shape of 230 mm x 114 mm x 100 mm using an oil press, followed by heat treatment at a maximum temperature of 250°C for 5 hours. Samples for measuring physical properties were cut out from these bricks, and the apparent porosity was measured, and the spalling resistance and iron oxidation resistance were evaluated.

[0026] The apparent porosity was measured by embedding a 50 x 50 x 50 mm sample in coke breeze, heating it to 1400°C in an electric furnace, holding it for 5 hours, and then allowing it to cool naturally. After that, the apparent porosity was measured in accordance with JIS R 2205 using kerosene as the solvent.

[0027] Spalling resistance was evaluated using a quench spalling test. In the quench spalling test, a 40mm x 40mm x 200mm sample was placed in molten iron heated to 1600°C, removed after 90 seconds, and water-quenched. This cycle was repeated up to three times. If part of the sample had peeled off within the third test, it was rated as × (fail), if large cracks were observed visually after the third test, it was rated as ○ (pass), and if small cracks were observed, it was rated as ◎ (good). ◎ (good) and ○ (fair) were considered to be passing.

[0028] The iron oxidation resistance was evaluated by a rotary corrosion test. In the rotary corrosion test, the iron oxidation resistance against iron oxide generated by heating during oxygen injection in an actual furnace was evaluated. In this rotary corrosion test, a sample was lined inside an iron drum with a horizontal rotating shaft and held at 1750°C for 30 minutes. After that, oxygen was blown into the drum from an iron pipe for 15 minutes while the drum was rotating, so that the molten high-temperature iron oxide was sprayed onto the surface of the sample. After natural cooling, the sample was recovered, and the amount of wear on the cut surface was measured. The results were expressed as an index, with the amount of wear in Comparative Example 1 being 100. The smaller the index, the better the iron oxidation resistance. The evaluation was performed as follows: an index of 95 or more but less than 105 was rated as ⊚ (good); an index of 105 or more but less than 115 was rated as ○ (passable); and an index of 115 or more was rated × (unsatisfactory).

[0029] The overall evaluation was based on the evaluation results of both spalling resistance and iron oxidation resistance. If both evaluation results were ◎, the evaluation was given as ◎ (good); if both evaluation results were 〇, or if one evaluation result was 〇 and the other evaluation result was ◎, the evaluation was given as 〇 (passable); if either evaluation result was ×, the evaluation was given as × (failable); and if ◎ (good) or ○ (passable), the evaluation was considered to have passed.

[0030] Examples 1 to 5 are examples with different alumina contents, but within the range of the present invention. The results of both the quenching spalling test and the rotational erosion test were good, demonstrating excellent spalling resistance and iron oxidation resistance. In contrast, Comparative Example 1 is an example that does not contain alumina, and showed poor spalling resistance. Comparative Example 3 is an example in which the alumina content exceeds the upper limit of the present invention, and showed poor results in the rotational erosion test and poor iron oxidation resistance. Comparative Example 2 is an example in which alumina with a particle size of less than 1 mm was used, and showed poor iron oxidation resistance.

[0031] Examples 6 to 11 are examples with different pitch powder contents or softening points, but were within the range of the present invention and produced good results. Comparative Example 4 is an example that did not contain pitch powder and resulted in poor spalling resistance. Comparative Example 6 is an example in which the pitch powder content exceeded the upper limit of the present invention, resulting in reduced iron oxide resistance. Comparative Example 5 is an example in which liquid pitch was used and showed insufficient spalling resistance. Note that the liquid pitch was used in a state dissolved in a solution of the phenolic resin used as a binder, but the liquid pitch content shown in Table 1 is the content of pitch only.

[0032] Examples 12 and 13 are examples in which the content of magnesia particles with a particle size of less than 0.075 mm is different, but the content is within the range of the present invention and showed good results. Comparative Example 7 is an example in which no magnesia particles with a particle size of less than 0.075 mm are contained, resulting in high porosity and reduced iron oxidation resistance. Comparative Example 8 is an example in which the content of magnesia particles with a particle size of less than 0.075 mm exceeds the upper limit of the present invention and therefore reduced spalling resistance.

[0033] Examples 14 and 15 are examples in which the content of magnesia having a particle size of 1 mm or more but less than 5 mm is different, but are within the range of the present invention and showed good results. Comparative Example 9 is an example in which the content of magnesia having a particle size of 1 mm or more but less than 5 mm is below the lower limit of the present invention, and spalling resistance was reduced. Comparative Example 10 is an example in which the content of magnesia having a particle size of 1 mm or more but less than 5 mm is above the upper limit of the present invention, and spalling resistance and iron oxidation resistance were reduced.

[0034] Examples 16 to 18 are examples in which the aluminum content was different, but was within the range of the present invention and produced good results. Examples 19 to 22 are examples in which the graphite content was different, but was within the range of the present invention and produced good results. Comparative Example 11 was an example in which the graphite content exceeded the upper limit of the present invention, resulting in poor iron oxidation resistance. Examples 23 and 24 are examples in which the total content of magnesia was different, but were within the range of the present invention and provided good results.

Claims

1. A method for producing magnesia carbon bricks for a vacuum degassing furnace, comprising the steps of adding an organic binder to a refractory raw material mixture, kneading the mixture, forming the mixture, and then heat treating the mixture, A method for producing magnesia carbon bricks for a vacuum degassing furnace, characterized in that the refractory raw material blend has a magnesia content of 86% by mass or more and 99% by mass or less in total, contains 45% by mass or more and 65% by mass or less of magnesia having a particle size of 1 mm or more and less than 5 mm, 1% by mass or more and 12% by mass or less of magnesia having a particle size of less than 0.075 mm, 0.3% by mass or more and 5% by mass or less of alumina having a particle size of less than 0.075 mm, and 0.3% by mass or more and 2% by mass or less of pitch powder having a particle size of less than 0.5 mm, and has a graphite content of 5% by mass or less (including 0).

2. 2. The method for producing magnesia carbon bricks for use in a vacuum degassing furnace according to claim 1, wherein the content of alumina having a particle size of less than 0.075 mm in the refractory raw material blend is 1% by mass or more and 3% by mass or less.

3. 3. The method for producing magnesia carbon bricks for use in a vacuum degassing furnace according to claim 1, wherein the particle size of the alumina in the refractory raw material mixture is less than 0.045 mm.

4. 4. The method for producing magnesia-carbon bricks for use in a vacuum degassing furnace according to claim 3, wherein the alumina in the refractory raw material blend is calcined alumina.

5. 3. The method for producing magnesia carbon bricks for use in a vacuum degassing furnace according to claim 1, wherein the content of pitch having a particle size of less than 0.5 mm in the refractory raw material blend is 0.3% by mass or more and 1% by mass or less.

6. 3. The method for producing magnesia carbon bricks for use in a vacuum degassing furnace according to claim 1, wherein the content of graphite in the refractory raw material blend is 1.5 mass % or less (including 0).

7. 6. The method for producing magnesia carbon bricks for use in a vacuum degassing furnace according to claim 5, wherein the content of graphite in the refractory raw material blend is 1.5 mass % or less (including 0).

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