Slag line bricks for stainless steel ladles

Abstract

To provide a slag line brick for stainless steel ladles that has sufficient residual expansion and excellent thermal shock resistance. [Solution] A slag line brick for a stainless steel ladle is obtained by adding an organic binder to a refractory raw material mixture, kneading and molding it, and then heat-treating it, wherein the refractory raw material mixture contains 30% to 70% by mass of spinel with a particle size of 1 mm or more and less than 5 mm, and 1% to 67% by mass of magnesia, and the content of spinel with a particle size of less than 1 mm is 23% by mass or less (including 0) and the content of graphite is 3% by mass or less (including 0) in proportion to 100% by mass of the refractory raw material mixture.

Description

Technical Field

[0001] The present invention relates to the slag line bricks of the ladle used in the manufacturing process of stainless steel. The slag line of the ladle refers to the part where the slag existing in the ladle contacts the side wall surface of the ladle.

Background Art

[0002] Stainless steel has a low content rate of carbon component contained in molten steel, and bricks used for the slag line of a ladle for stainless steel are also magnesia-chrome bricks or low-carbon magnesia-carbon bricks from the viewpoint of suppressing carbon pickup. However, when magnesia-chrome bricks are applied to the slag line of a ladle for stainless steel, there is a problem that slag infiltration occurs, structural spoilage occurs, and it leads to large peeling. Further, when low-carbon magnesia-carbon bricks are applied to the slag line of a ladle for stainless steel, the use temperature may be 1700 °C or higher, so there is a problem that joint opening due to high thermal expansion and cracks due to a decrease in thermal shock resistance are likely to occur. In particular, when joint opening occurs, the vicinity of the joints of the bricks centered on the joint mortar is preferentially melted, so the operating surface becomes uneven, and the protruding bricks lose the restraining force from the surroundings, so there is a problem that the operating surface is greatly peeled off due to crack generation and the wear becomes extremely large. Furthermore, when low-carbon magnesia-carbon bricks with a graphite content of 3 mass% or less are used, the expansion rate of the bricks becomes extremely large, so the damage to the bricks due to joint opening becomes the bottleneck rather than the melting loss due to slag.

[0003] Generally, it is known that increasing the residual expansion of bricks is necessary to suppress joint opening. For example, to increase the residual expansion of magnesia carbon bricks, adding a small amount of alumina can result in magnesia carbon bricks with high residual expansion due to spinel expansion during use. For example, Patent Document 1 discloses that by using ultrafine powder with a particle size of 74 μm or less as part of the alumina used as a refractory raw material, and by using it in combination with aluminum, aluminum alloy, or silicon, a dense structure with excellent corrosion resistance can be obtained, and a magnesia alumina carbon brick with sufficient residual expansion can be obtained. However, alumina has significantly lower slag resistance than magnesia, and the spinelization reaction with magnesia progresses over a long period of time, leading to high porosity and reduced corrosion resistance. Furthermore, since Patent Document 1 contains 8 to 18 mass% graphite, it has the problem of not being usable as a slag line brick for stainless steel ladles from the viewpoint of carbon pickup.

[0004] Patent Document 2 proposes a magnesia-spinel-carbon brick for use in stainless steel ladle slag lines, using 65-97% by mass of magnesia raw material, 1-30% by mass of spinel raw material, and 2-4% by mass of carbon raw material. However, with a spinel raw material content of 1-30% by mass, the effect of reducing the coefficient of thermal expansion is small, and it is insufficient from the viewpoint of suppressing joint opening and crack generation due to thermal shock.

[0005] Patent Document 3 proposes a magnesia-spinel-carbon brick containing 3% to 17% by mass of carbon raw material, and where, when the sum of spinel and magnesia raw materials is 100 parts by mass, the spinel raw material content is 50% to 95% by mass. However, even with this brick, there was still a problem of insufficient durability.

[0006] Patent Document 4 proposes a brick for a vacuum degassing apparatus in which the refractory raw material mixture contains 5% to 15% by mass of graphite, 40% to 70% by mass of spinel with a particle size of 1 mm to less than 5 mm, and 20% to 50% by mass of magnesia with a particle size of less than 1 mm, and in proportion to 100% by mass of the refractory raw material mixture, the content of spinel with a particle size of less than 1 mm is 10% by mass or less (including 0), and the content of magnesia with a particle size of 1 mm to less than 5 mm is 5% by mass or less (including 0). However, because it contains 5% by mass or more of graphite, carbon pickup becomes a problem. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2019-123635 [Patent Document 2] Japanese Patent Publication No. 2022-87681 [Patent Document 3] Japanese Patent Publication No. 2017-7901 [Patent Document 4] Japanese Patent Publication No. 2020-200530 [Overview of the project] [Problems that the invention aims to solve]

[0008] The problem that this invention aims to solve is to provide a slag line brick for stainless steel ladles that has sufficient residual expansion and excellent thermal shock resistance. [Means for solving the problem]

[0009] The inventors have found that in magnesia spinel bricks obtained from a refractory raw material mixture with a graphite content of 3% by mass or less, the occurrence of joint separation can be suppressed when applied to the slag line of a stainless steel ladle by setting the particle size composition and content of magnesia and spinel in the refractory raw material mixture to specific ranges. Specifically, the thermal expansion coefficient of magnesia is 13.7 × 10⁻⁶. -6 / ℃, in contrast, the thermal expansion coefficient of spinel is 8.86 × 10⁻⁶. -6 The thermal expansion coefficient is small at / °C. Furthermore, since stainless steel ladles reach high temperatures during use, it is thought that spaces are created around the spinel particles, which have a small expansion coefficient, in the brick structure. Subsequently, when the molten steel is discharged and the ladle is rapidly cooled, the brick structure cannot shrink uniformly at this time, leaving spaces around some spinel particles, which is presumed to cause residual expansion. In addition, spinel is suitable as a raw material for slag line bricks in ladles used in the manufacturing process of stainless steel because it does not react with magnesia during use, has a stable composition, has relatively high corrosion resistance, and its thermal expansion coefficient is almost constant at all temperatures.

[0010] In other words, according to one aspect of the present invention, the following stainless steel ladle slag line brick is provided. A slag line brick for stainless steel ladles, obtained by adding an organic binder to a refractory raw material mixture, kneading, shaping, and then heat-treating it. The refractory raw material composition contains 30% to 70% by mass of spinel with a particle size of 1 mm or more and less than 5 mm, and 1% to 67% by mass of magnesia. Furthermore, a slag line brick for stainless steel ladles, wherein the spinel content (including 0) of particles with a particle size of less than 1 mm is 23% by mass or less (inclusive) and the graphite content is 3% by mass or less (inclusive) in 100% by mass of the refractory raw material composition. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a slag line brick for stainless steel ladles that has sufficient residual expansion and excellent thermal shock resistance. [Modes for carrying out the invention]

[0012] In this invention, spinel is used to improve residual expansion and thermal shock resistance. Specifically, spinel with a particle size of 1 mm or more and less than 5 mm is used in a proportion of 30% to 70% by mass in 100% by mass of the refractory raw material composition. If the proportion is less than 30% by mass, the improvement in thermal shock resistance and residual expansion rate is insufficient, and if it exceeds 70% by mass, there is an excess of particles with a particle size of 1 mm or more, resulting in poor filling during molding and a brick structure with high porosity, thus reducing corrosion resistance. If it is desired to further improve residual expansion, thermal shock resistance, and corrosion resistance, the proportion of spinel with a particle size of 1 mm or more and less than 5 mm can be set to 40% to 60% by mass.

[0013] On the other hand, spinel particles with a particle size of less than 1 mm expand less than spinel particles with a particle size of 1 mm or more and less than 5 mm, making it easier to create an expansion difference with the surrounding magnesia, which is effective from the viewpoint of improving residual expansion. However, using a large amount of spinel particles with a particle size of less than 1 mm reduces corrosion resistance. Therefore, in this invention, the amount of spinel particles with a particle size of less than 1 mm used is set to 23% by mass or less (including 0) in 100% by mass of the refractory raw material mixture, and can be set to 10% by mass or less (including 0) if further improvement of corrosion resistance is desired.

[0014] In this invention, magnesia is used to ensure corrosion resistance. Specifically, it is used in a content of 1% to 67% by mass in 100% by mass of the refractory raw material composition. If the magnesia content is less than 1% by mass, the corrosion resistance will be insufficient, and if it exceeds 67% by mass, the thermal shock resistance will decrease. If further improvement in corrosion resistance is desired, the magnesia content can be set to 15% to 58% by mass.

[0015] Furthermore, in this invention, from the viewpoint of ensuring a good balance of residual expansion, thermal shock resistance, and corrosion resistance, the total content of spinel and magnesia can be set to 80% by mass or more. As spinel and magnesia, electrofused or sintered products that are generally available on the market as raw materials for refractories can be used. In addition, common spinel (Al2O3: 71.7% by mass, MgO: 28.3% by mass) can be used, as well as alumina-rich spinel with a high Al2O3 content and magnesia-rich spinel with a high MgO content.

[0016] In this invention, at least one of aluminum and aluminum alloys can be used to densify the brick structure and prevent oxidation. Specifically, at least one of aluminum and aluminum alloys can be used in a total content of 0.3% to 2.5% by mass in 100% by mass of the refractory raw material mixture. Here, the effect of densifying the brick structure and preventing oxidation by aluminum and aluminum alloys is even more pronounced when fine aluminum and aluminum alloys with a particle size of less than 75 μm are used. In addition, any aluminum or aluminum alloy commonly used in magnesia carbon bricks, etc., can be used without any problems. Furthermore, as mentioned above, a particle size of less than 75 μm can be used.

[0017] Because graphite has a high fixed carbon content, using large amounts of graphite can lead to carbon pickup problems. Therefore, in this invention, the amount of graphite used is limited to 3% by mass or less (including 0) of the total mass of the refractory raw material mixture. Suitable graphite options include scaly graphite or expanded graphite commonly used in ordinary magnesia-carbon bricks, but synthetic graphite can also be used. Furthermore, the particle size of the graphite can be less than 0.1 mm.

[0018] Here, the particle size referred to in the present invention is the size of the sieve mesh when the refractory raw material particles are separated by sieving. For example, spinel with a particle size of less than 1 mm means spinel that passes through a sieve with a mesh size of 1 mm, and spinel with a particle size of 1 mm or more means spinel that does not pass through a sieve with a mesh size of 1 mm.

[0019] As raw materials for the slag line bricks of the stainless steel ladle of the present invention, in addition to the above-mentioned magnesia, spinel, graphite, aluminum, and aluminum alloy, raw materials generally used as raw materials for magnesia-carbon bricks can be used as long as the total content rate is about 5% by mass or less without causing adverse effects. For example, pitch, alumina, silicon carbide, silicon, carbon black, boron carbide, fiber, glass, etc.

[0020] The slag line bricks of the stainless steel ladle of the present invention can be manufactured by a general method for manufacturing unburned magnesia-carbon bricks. That is, the slag line bricks of the stainless steel ladle of the present invention can be obtained by adding an organic binder to the above-mentioned refractory raw material mixture, kneading, molding, and then performing heat treatment. The heat treatment temperature can be, for example, 150 to 600 °C, preferably 150 to 400 °C.

Examples

[0021] An appropriate amount of phenolic resin as an organic binder was added to the refractory raw material mixtures of each example described in Table 1, kneaded, and molded into a shape of 230 × 114 × 100 mm by an oil press, and then heat-treated at 250 °C for 5 hours to obtain bricks of each example. Note that the spinel was an electrically fused spinel with Al2O3:MgO = 71.7:28.3 and a purity of 98% by mass, the magnesia was an electrically fused magnesia with 98% by mass of MgO, and the pitch was a powdered pitch with a softening point of 120 °C. Samples of a predetermined shape were cut out from these bricks, and the residual expansibility, thermal shock resistance, and corrosion resistance were evaluated.

[0022]

Table 1

[0023] Residual expansion was evaluated by burying a 20×20×80mm 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 to room temperature. This cycle was repeated three times. Specifically, the residual expansion rate was determined by comparing the length of the test specimen (dimension a, 80mm portion) after the test with the dimension a of the test specimen before the test. A residual expansion rate of 0.5% or more was rated ◎ (good), 0.2% to less than 0.5% was rated ○ (acceptable), and less than 0.2% was rated × (unacceptable). A rating of × indicated that the material was unsuitable for use in an actual furnace.

[0024] Thermal shock resistance was tested by firing a 40 x 40 x 190 mm sample at 1400°C for 5 hours in a reducing atmosphere. This sample was then immersed in molten iron heated to 1600°C for 90 seconds, followed by 30 seconds of water cooling. This cycle was repeated three times. After the test, the sample was cut and the cross-section was observed for evaluation. A sample with no cracks was rated ◎ (good), a sample with minor cracks that do not affect usability was rated ○ (acceptable), and a sample with cracks was rated × (unacceptable). Samples rated × were deemed unsuitable for actual furnace use.

[0025] Corrosion resistance was evaluated using a rotary erosion test. In the rotary erosion test, the inner surface of a cylinder with a horizontal rotation axis was lined with the test brick, slag was added and heated to erode the surface of the test brick. An oxygen-propane burner was used as the heat source, the test temperature was 1700°C, and a synthetic slag of CaO / SiO2=3 was used as the slag. The discharge and addition of slag was repeated 10 times every 30 minutes. After the test, the dimensions of the maximum eroded part of each test brick (remaining brick dimensions) were measured and expressed as a corrosion resistance index, with the remaining brick dimensions of "Comparative Example 1" listed in Table 1 set to 100. A higher value on this corrosion resistance index indicates better corrosion resistance. Corrosion resistance was rated as follows: ◎ (Good) if the corrosion resistance index was 110 or higher, ○ (Acceptable) if it was between 100 and 110, and × (Unacceptable) if it was 100 or lower. A × rating indicated that the brick was not suitable for use in an actual furnace.

[0026] The overall evaluation was determined as follows: ◎ (Good) if all evaluation results were ◎, ○ (Acceptable) if at least one evaluation result was ○ and there were no × evaluation results, and × (Unacceptable) if at least one evaluation result was ×. An × result indicated that the furnace was not suitable for actual use.

[0027] Examples 1 to 4 are examples with different spinel content between 1 mm and 5 mm in particle size, but all are within the scope of the present invention, and sufficient residual expansion, thermal shock resistance, and corrosion resistance are obtained. In contrast, Comparative Example 1 has a spinel content of 80% by mass between 1 mm and 5 mm in particle size, which exceeds the upper limit of the present invention, resulting in insufficient corrosion resistance. On the other hand, Comparative Example 2 has a spinel content of 25% by mass between 1 mm and 5 mm in particle size, which falls below the lower limit of the present invention, resulting in insufficient residual expansion and thermal shock resistance.

[0028] Examples 5 to 7 are examples with different spinel content (particle size less than 1 mm), but all are within the scope of the present invention, and sufficient residual expansion, thermal shock resistance, and corrosion resistance are obtained. In contrast, Comparative Example 3 has a spinel content of 28% by mass (particle size less than 1 mm), which exceeds the upper limit of the present invention, and therefore lacks corrosion resistance.

[0029] Examples 8 and 9 are examples with different magnesia content, but they are within the scope of the present invention, and sufficient residual expansion, thermal shock resistance, and corrosion resistance are obtained. In contrast, Comparative Example 4 is an example that does not contain magnesia, and therefore lacks sufficient residual expansion.

[0030] Examples 10 to 12 represent examples with different scaly graphite content, but they are within the scope of the present invention and demonstrate sufficient residual expansion, thermal shock resistance, and corrosion resistance.

[0031] Next, the bricks from Example 2 and Comparative Example 2 were applied to the slag line of a stainless steel ladle and subjected to 10 usage tests. After use, the remaining dimensions of the bricks were measured, and it was confirmed that the bricks from Example 2 had approximately 20% larger remaining dimensions than the bricks from Comparative Example 2, and that cracks and joint opening were suppressed, resulting in superior durability.

Claims

1. A slag line brick for stainless steel ladles, obtained by adding an organic binder to a refractory raw material mixture, kneading, shaping, and then heat-treating it. The refractory raw material formulation contains 30% to 70% by mass of spinel with a particle size of 1 mm or more and less than 5 mm, and 1% to 67% by mass of magnesia. Furthermore, a slag line brick for stainless steel ladles, wherein the spinel content of less than 1 mm in particle size is 23% by mass or less (including 0) and the graphite content is 3% by mass or less (including 0) in 100% by mass of the refractory raw material composition.

2. A slag line brick for a stainless steel ladle according to claim 1, wherein the total content of spinel and magnesia in 100% by mass of the refractory raw material composition is 80% by mass or more.

3. A slag line brick for a stainless steel ladle according to claim 1 or 2, wherein the content of magnesia in the refractory raw material mixture is 15% by mass or more and 58% by mass or less.

4. The slag line brick for a stainless steel ladle according to claim 3, wherein the content of spinel particles with a particle size of 1 mm or more and less than 5 mm is 40% by mass or more and 60% by mass or less in 100% by mass of the refractory raw material composition.

5. A slag line brick for a stainless steel ladle according to claim 1 or 2, wherein the content of spinel particles with a particle size of less than 1 mm is 10% by mass or less (including 0) in 100% by mass of the refractory raw material composition.

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