Manufacturing method and use method of bricks for vacuum degassing equipment

Abstract

To provide a method for manufacturing and using bricks for a vacuum degasifier having excellent thermal shock resistance and corrosion resistance, even when used as a lining material of the vacuum degasifier in which a slag component contains low basicity (CaO / SiO2 mass ratio) and high Al2O3.SOLUTION: A refractory raw material blend comprises 20 to 80 mass% of spinel, 10 to 70 mass% of alumina having a grain size of 0.3 mm or more, 8 mass% (including 0) or less of alumina having a grain size of less than 0.3 mm, and 3 to 15 mass% of graphite, and has a total amount of spinel and alumina being 84 mass% or more, wherein an organic binder is added to the refractory raw material blend, and then is kneaded, molded, and heat-treated. The obtained brick is used in a vacuum degasifier that operates under conditions where a ladle slag after degasification has a basicity (CaO / SiO2 mass ratio) of 0.5 to 2.0 and an Al2O3 content of 20 to 50 mass%.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing and using bricks for vacuum degassing apparatuses, such as RH and DH, which are used for the purposes of vacuum decarburization and degassing as secondary refining equipment for molten steel. [Background technology]

[0002] In vacuum degassing units such as DH and RH, the load on refractories is large, significantly increasing refractory damage, and therefore the supply of materials with excellent durability has been desired. Traditionally, magnesia-chrome bricks and magnesia-carbon bricks, which have excellent corrosion resistance, have been used as refractories for vacuum degassing units. However, magnesia-chrome bricks have a problem with poor thermal shock resistance. Furthermore, it is believed that the main causes of damage to magnesia-carbon bricks are slag corrosion and thermal shock. Furthermore, at high temperatures, the magnesia-carbon reaction (reaction (1) below) occurs, which affects durability. MgO(solid) + C(solid) → Mg(gas) + CO(gas) (1)

[0003] In order to suppress such magnesia-carbon reactions, for example, Patent Document 1 discloses spinel-carbon bricks containing 75 to 99.5 mass% spinel and 0.5 to 25 mass% carbon. Patent Document 1 shows that using a spinel solid solution to reduce the MgO content is effective in suppressing the magnesia-carbon reaction. It also claims that slag with low basicity (CaO / SiO2 mass ratio) and high Al2O3 content has excellent corrosion resistance. Low basicity refers to a CaO / SiO2 mass ratio of 0.5 to 3.0, and high Al2O3 content refers to an Al2O3 content of 20 to 40 mass%.

[0004] Patent Document 2 also discloses an unfired spinel-magnesia-carbon brick containing 65% to 98% by mass of spinel, 1% to 30% by mass of magnesia, and 0.1% to 15% by mass of graphite. However, when this spinel-magnesia-carbon brick was actually used in a vacuum degassing apparatus, it was found to have better durability than magnesia-carbon bricks, but the main factor in the lifespan of the vacuum degassing apparatus was still the insufficient durability of the bricks, and analysis of the bricks after use revealed that further improvements in thermal shock resistance and corrosion resistance were necessary.

[0005] Patent Document 3 also discloses a method for producing an unfired carbon-containing refractory material, which is characterized by adding a carbon-based binder to a mixture containing, by weight, 30 to 90% of an alumina material, 3 to 30% of a carbon material, 5 to 50% of an Al2O3-MgO spinel material having a particle size of 1 mm or less, and 0.1 to 5% of a glass material, and then kneading, molding, and drying the mixture. 3 Even the bricks disclosed in still had durability issues. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-60651 [Patent Document 2] Patent No. 6600729 [Patent Document 3] Japanese Patent Application Publication No. 9-25160 Summary of the Invention [Problem to be solved by the invention]

[0007] The problem to be solved by the present invention is to provide a method for manufacturing and using a brick for a vacuum degasser that has excellent thermal shock resistance and corrosion resistance even when used as a lining material for a vacuum degasser containing slag components with a low basicity (CaO / SiO2 mass ratio) and a high Al2O3 content. [Means for solving the problem]

[0008] The inventors of the present invention analyzed spinel-magnesia-carbon bricks used in vacuum degassing equipment after use and inferred that the cause of the damage was due to the low basicity and high Al2O3 content of the slag, which caused spalling due to thermal shock. They then discovered that bricks with improved thermal shock resistance and corrosion resistance could be obtained by using alumina with a particle size of 0.3 mm or more in the refractory raw material blend. Alumina has better corrosion resistance to low basicity and high Al2O3 content slag than spinel and magnesia, and also has better thermal shock resistance.

[0009] That is, according to the present invention, there are provided the following methods for producing and using bricks for a vacuum degasser. 1. 20 to 80 mass% of spinel, 10 to 70 mass% of alumina with a particle size of 0.3 mm or more, Alumina with a particle size of less than 0.3 mm is 8% by mass or less (including 0), Contains 3 to 15 mass% graphite, Moreover, the refractory raw material blend contains 84 mass% or more of spinel and alumina in total, This is a manufacturing method for bricks for vacuum degassing equipment, in which an organic binder is added, the material is kneaded, molded, and then heat-treated. 2. 2. The method for producing bricks for a vacuum degasser according to 1 above, wherein the refractory raw material mixture contains at least one of aluminum, an aluminum alloy, and silicon in a total amount of 0.3 to 2.5 mass %. 3. 3. A method for using bricks for a vacuum degasser obtained by the method for producing bricks for a vacuum degasser according to item 1 or 2 above, in a vacuum degasser operated under conditions in which the ladle slag after degassing has a basicity (CaO / SiO2 mass ratio) of 0.5 to 2.0 and an Al2O3 content of 20 to 50 mass%. [Effects of the Invention]

[0010] The brick for a vacuum degasser of the present invention has excellent thermal shock resistance and corrosion resistance, thereby improving the service life of the vacuum degasser. In particular, when used in a vacuum degasser including an operation in which the slag components have low basicity and a high Al2O3 content, a significant improvement in service life can be achieved. DETAILED DESCRIPTION OF THE INVENTION

[0011] In the refractory raw material composition of the present invention, spinel is used in an amount of 20 to 80 mass% to ensure corrosion resistance. If the spinel content in the refractory raw material composition is less than 20 mass%, the corrosion resistance becomes insufficient, and if it exceeds 80 mass%, the thermal shock resistance decreases. Furthermore, by setting the spinel content to 36 to 76 mass%, the corrosion resistance can be further improved.

[0012] Alumina is used because it has a smaller thermal expansion coefficient than spinel, giving it excellent thermal shock resistance, and also because it has better corrosion resistance against low-basicity, high-Al2O3-containing slag. However, in actual degassing treatment, the basicity of the slag varies depending on the type of steel being treated, so alumina has poor corrosion resistance with slag that is relatively basic. To minimize this effect, alumina with a particle size of 0.3 mm or more is used. That is, in the present invention, 10 to 70 mass% of alumina having a particle size of 0.3 mm or more is used. If the content of alumina having a particle size of 0.3 mm or more in the refractory raw material blend is less than 10 mass%, the thermal shock resistance becomes insufficient, and if it exceeds 70 mass%, the corrosion resistance decreases. Furthermore, by setting the content of alumina having a particle size of 0.3 mm or more to 10 to 50 mass%, the corrosion resistance can be further improved.

[0013] The content of alumina having a particle size of 0.3 mm or more can be selected depending on the operating conditions, such as the basicity and Al2O3 content of the slag in the molten steel treated in the vacuum degassing apparatus. For example, when the slag has a lower basicity or a higher Al2O3 content, the content of alumina having a particle size of 0.3 mm or more in the refractory raw material blend can be increased, and vice versa.

[0014] On the other hand, in the refractory raw material composition of the present invention, it is preferable not to use alumina with a particle size of less than 0.3 mm for the reasons mentioned above, but if it is used because it is mixed with alumina with a particle size of 0.3 mm or more and separation would be costly, it can be used as long as its content in the refractory raw material composition is 8 mass% or less, since the effect of reducing corrosion resistance is negligible. If the content of alumina with a particle size of less than 0.3 mm exceeds 8 mass%, corrosion resistance will decrease.

[0015] Here, the particle size 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 with a particle size of 0.3 mm or more refers to alumina that does not pass through a sieve with 0.3 mm openings, and alumina with a particle size of less than 0.3 mm refers to alumina that passes through a sieve with 0.3 mm openings.

[0016] In the refractory raw material composition of the present invention, the total amount of spinel and alumina is set to 84 mass % or more from the viewpoint of thermal shock resistance and corrosion resistance. The spinel and alumina can be electrofused or sintered products that are generally commercially available as raw materials for refractories. Common spinel (Al2O3: 71.7% by mass, MgO: 28.3% by mass) can be used as the spinel, as well as alumina-rich spinel with a high Al2O3 content and magnesia-rich spinel with a high MgO content. The alumina can be electrofused alumina or sintered alumina with an Al2O3 purity of 90% by mass or more.

[0017] The graphite content in the refractory raw material composition of the present invention is set to 3 to 15% by mass. If the graphite content is less than 3% by mass, it is difficult to obtain thermal shock resistance, and if it exceeds 15% by mass, the structure deteriorates due to oxidation, making it prone to wear. To improve wear resistance and suppress carbon pickup, the graphite content can be set to 7 to 13% by mass. As graphite, flaky graphite, expanded graphite, electrode powder, etc. can be used, with particle sizes of less than 0.1 mm being preferred. Expanded graphite is flaky graphite that has been rapidly heated with sulfuric acid or the like contained between its structures, causing it to expand several tens or even a hundred times. In the present invention, this expanded graphite can be crushed and made thin. Even with the same graphite content, the number of expanded graphite particles increases, resulting in uniform distribution within the refractory structure. This improves thermal shock resistance without increasing the carbon content that causes carbon pickup. At the same time, wear resistance and corrosion resistance can also be improved.

[0018] The refractory raw material composition of the present invention can contain one or more metals selected from aluminum, aluminum alloys, and silicon for the purposes of improving the strength of the brick and preventing oxidation. To fully function as an antioxidant, the total content of these metals can be 0.3% by mass or more. On the other hand, if the total content of these metals is too high, the metals may turn into oxides, carbides, or nitrides during use, resulting in an overly dense structure and high elasticity, which reduces thermal shock resistance. Therefore, if a lower elastic modulus is desired for the brick, the total content of the metals can be 2.5% by mass or less. For aluminum, aluminum alloy, and silicon, fine powders with a particle size of less than 0.1 mm can be suitably used.

[0019] In addition to the above-mentioned refractory raw materials, the refractory raw material composition of the present invention may also incorporate magnesia, pitch, carbon black, boron carbide, and silicon carbide as appropriate to improve the oxidation resistance, residual expansion resistance, and thermal shock resistance of the brick, as known in the art. In this case, the respective contents may be determined by reference to known techniques, and even if the total amount of these ingredients is up to 5 mass %, no adverse effects can be ignored. Pitch and carbon black can be contained in a total amount of 3 mass % or less to strengthen the carbon bond. As pitch and carbon black, powdered ones with a particle size of less than 0.2 mm can be suitably used. Boron carbide and silicon carbide are used as antioxidants, and boron carbide can be used in a range of 2 mass % or less, and silicon carbide can be used in a range of 5 mass % or less.

[0020] The brick for a vacuum degasser of the present invention can be manufactured by a general manufacturing process for unburned carbon-containing bricks. Specifically, the brick for a vacuum degasser of the present invention can be obtained by adding an organic binder to a refractory raw material mixture, kneading and molding the mixture, and then heat-treating the mixture. The heat-treatment temperature can be 200 to 800°C, as with general unburned carbon-containing bricks, and the heat-treatment time can also be 2 to 24 hours. Examples of organic binders that can be used include those used in general unburned carbon-containing bricks, such as furan resins and phenolic resins. The organic binder can be used in the form of powder, a liquid dissolved in an appropriate solvent, or a combination of liquid and powder. The methods and conditions for kneading, molding, and heat-treatment are similar to those used in the manufacturing process for general unburned carbon-containing bricks.

[0021] The brick for a vacuum degasser of the present invention has improved corrosion resistance and thermal shock resistance when used as a lining material for a vacuum degasser in which the slag components have low basicity and a high Al2O3 content. In particular, by using the brick for a vacuum degasser of the present invention as a lining material for a vacuum degasser operated under conditions in which the ladle slag after degassing has a basicity (CaO / SiO2 mass ratio) of 0.5 to 2.0 and an Al2O3 content of 20 to 50 mass%, the life of the vacuum degasser can be further extended. [Example]

[0022] The compositions of the refractory raw material blends and the physical properties of the resulting bricks in the examples and comparative examples of the present invention are shown in Tables 1 and 2. The refractory raw materials used were: spinel, which was an electrofused spinel containing 72 mass% Al2O3 and 28 mass% MgO; alumina, which was an electrofused alumina containing 98 mass% Al2O3; and magnesia, which was an electrofused magnesia containing 98 mass% MgO.

[0023] [Table 1]

[0024] [Table 2]

[0025] These bricks were manufactured by adding an appropriate amount of phenolic resin as an organic binder to the refractory raw material compositions in Tables 1 and 2, kneading them, molding them into shapes of 230 mm x 114 mm x 100 mm using an oil press, and then heat treating them at a maximum temperature of 250°C for 5 hours. Samples for measuring physical properties were cut out from these samples, and the apparent porosity was measured, as well as the corrosion resistance and thermal shock 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] Corrosion resistance was evaluated by a rotary erosion test. In the rotary erosion test, the inner surface of a drum with a horizontal rotating shaft was lined with test bricks, and slag was added and heated to erode the test brick surfaces. Two rotary erosion tests were conducted, one using slag with a CaO / SiO2 mass ratio (hereinafter referred to as "C / S") of 0.5 and the other using slag with a C / S of 2.0. Furthermore, slag containing 13 mass% CaO, 26 mass% SiO2, and 50 mass% Al2O3 was used as slag with a C / S ratio of 0.5, and 40 mass% CaO, 20 mass% SiO2, and 30 mass% Al2O3 was used as slag with a C / S ratio of 2.0. The heating source was an oxygen-propane burner, the test temperature was 1700°C, and the slag was discharged and added every 30 minutes, repeating this procedure 10 times. After the test, the size (mm) of the maximum corrosion damage of each test brick was measured and expressed as a corrosion index, with the size of the corrosion damage of the brick of "Comparative Example 1" listed in Table 1 being set at 100. A smaller index value indicates better corrosion resistance. Since the basicity of the slag varies depending on the steel type and operating conditions under actual conditions of use, the overall evaluation of corrosion resistance was performed using the average corrosion index calculated from the corrosion index 1 of the slag with C / S=0.5 and the corrosion index 2 of the slag with C / S=2.0. In other words, the average corrosion index = (corrosion index 1 + corrosion index 2) / 2.

[0028] Thermal shock resistance was evaluated by using 40 x 40 x 190 mm specimens that had been fired at 1400°C for five hours in a reducing atmosphere. These specimens were then immersed in molten iron heated to 1600°C for 90 seconds, followed by 30 seconds of water cooling, a cycle repeated five times. After the test, the specimens were cut and the cross sections were inspected for evaluation. In Table 1, ◎ indicates specimens with no visible cracks, ○ indicates specimens with minor cracks that were not problematic for use, and × indicates specimens with visible cracks that were deemed unsuitable for use in actual equipment.

[0029] The overall evaluation was made on a three-point scale: ◎: very excellent, ○: excellent, ×: poor. Specifically, ◎ was evaluated as follows: an average corrosion index of less than 95 and a thermal shock resistance evaluation of ◎ or ○; an average corrosion index of 95 or more but less than 100 and a thermal shock resistance evaluation of ◎; ○ was evaluated as an average corrosion index of 95 or more but less than 100 and a thermal shock resistance evaluation of ○; and × was evaluated as an average corrosion index of 100 or more or a thermal shock resistance evaluation of ×. ◎ and ○ were evaluated as pass.

[0030] Examples 1 to 5 are examples in which the content of alumina with a particle size of 0.3 mm or more in the refractory raw material blend varies within the range of the present invention. It can be seen that the use of alumina with a particle size of 0.3 mm or more improves thermal shock resistance. Regarding corrosion resistance, for slag with a C / S ratio of 0.5, the corrosion resistance improved as the content of alumina with a particle size of 0.3 mm or more increased, but for slag with a C / S ratio of 2, the corrosion resistance tended to decrease as the content of alumina with a particle size of 0.3 mm or more increased. However, in the overall evaluation of corrosion resistance, Examples 1 to 5 showed excellent results. In contrast, Comparative Example 1 is an example that does not contain alumina with a particle size of 0.3 mm or more, and resulted in poor corrosion resistance and thermal shock resistance. Comparative Example 2 is an example that contains 5 mass% alumina with a particle size of 0.3 mm or more, but this is below the lower limit of the present invention and therefore the thermal shock resistance and corrosion resistance were insufficient. Comparative Example 3 is an example that increases the content of alumina with a particle size of 0.3 mm or more to 80 mass%, which exceeds the upper limit of the present invention and therefore resulted in insufficient corrosion resistance.

[0031] Although Examples 6 to 8 are examples in which the content of alumina having a particle size of less than 0.3 mm in the refractory raw material composition was different, it was within the range of the present invention and resulted in excellent corrosion resistance and thermal shock resistance. In contrast, Comparative Example 4 had a content of alumina having a particle size of less than 0.3 mm that exceeded the upper limit of the present invention, resulting in insufficient corrosion resistance. Examples 9 and 10 are examples in which the graphite content in the refractory raw material composition is different, but is within the range of the present invention and produced good results. In contrast, Comparative Example 5 had a graphite content below the lower limit of the present invention, resulting in insufficient thermal shock resistance. Furthermore, Comparative Example 6 had a graphite content above the upper limit of the present invention, resulting in reduced corrosion resistance. Examples 11 and 12 are examples in which expanded graphite was used, and the corrosion resistance and thermal shock resistance were excellent. Examples 13 to 16 are examples in which the aluminum content in the refractory raw material blend was different, but were within the range of the present invention and provided good results.

[0032] When the bricks of Comparative Example 1 and Example 3 were used as linings in the lower layer of an actual RH vacuum degassing apparatus and a test was conducted, the service life of the lower layer using Example 3 was improved by 16%. Note that the ladle slag after the degassing treatment had a basicity (CaO / SiO2 mass ratio) of 0.5 to 2.0 and an Al2O3 content in the range of 20 to 50 mass%.

Claims

1. 20 to 80 mass% of spinel, 10 to 70 mass% of alumina with a particle size of 0.3 mm or more, Alumina having a particle size of less than 0.3 mm is 8% by mass or less (including 0), Contains 3 to 15 mass% graphite, Furthermore, the refractory raw material blend contains 84 mass% or more of spinel and alumina in total, This is a manufacturing method for bricks for vacuum degassing equipment, in which an organic binder is added, the material is kneaded, molded, and then heat-treated.

2. 2. The method for producing bricks for a vacuum degasser according to claim 1, wherein the refractory raw material mixture contains at least one of aluminum, an aluminum alloy, and silicon in a total amount of 0.3 to 2.5 mass %.

3. The brick for a vacuum degasser obtained by the method for producing a brick for a vacuum degasser according to claim 1 or 2 is used to produce a ladle slag having a basicity (CaO / SiO ２ mass ratio) is 0.5 to 2.0, and Al ２ O ３ A method for using a brick for a vacuum degasser in a vacuum degasser operated under conditions where the content is 20 to 50 mass %.

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