JP7614501B2 - Magnesia-spinel bricks for vacuum degassing equipment and vacuum degassing equipment - Google Patents

Description

The present invention relates to a magnesia-spinel brick suitable for lining a vacuum degassing device such as a DH or RH, particularly an RH that performs a process of blowing oxygen into a degassing tank, and a vacuum degassing device that uses this brick for the side wall of the lower tank.

Vacuum degassing equipment such as DH and RH places a large load on refractories, and so there has been a demand for materials with excellent durability. Conventionally, magnesia-carbon bricks have been used as refractories for vacuum degassing equipment. The main cause of damage to magnesia-carbon bricks is melting due to slag, and it has been thought that damage is promoted by the magnesia-carbon reaction (reaction (1) below) that occurs at high temperatures.
MgO(solid) + C(solid) → Mg(gas) + CO(gas) (1)

The magnesia-carbon reaction is more likely to occur at higher temperatures. In addition, in vacuum degassing equipment that reduces pressure during processing to degas and decarburize, the generated Mg (gas) and CO (gas) are removed from the system by vacuuming, which is thought to promote reaction (1). For this reason, it was thought that the reason the durability of magnesia-carbon bricks has not improved is due to the magnesia-carbon reaction as well as their lack of corrosion resistance to slag.

Several methods have been proposed to simultaneously achieve corrosion resistance against slag and suppression of the magnesia-carbon reaction. Patent Document 1 discloses a spinel-carbon brick containing 88-99.5% by mass of spinel and 0.5-12% by mass of carbon raw material, with graphite having a specified specific surface area used for some or all of the carbon raw material. Patent Document 1 describes that the brick has excellent spalling resistance, can be used stably even under intermittent operation, and can improve durability by suppressing the magnesia-carbon reaction. However, when oxygen is blown in for decarburization in a vacuum degassing device, a large amount of FeO slag is generated, and this brick, which does not contain magnesia, which has high durability against FeO slag, gradually melts away from the operating surface and its durability deteriorates.

Patent Document 2 discloses a spinel-magnesia-carbon brick containing 65% to 98% by mass of spinel, 1% to 30% by mass of magnesia, 0.1% to 15% by mass of graphite, and 0.1% to 2.0% by mass of aluminum and/or aluminum alloy. Patent Document 2 describes that the inclusion of spinel inhibits the magnesia-carbon reaction, and the inclusion of magnesia provides excellent corrosion resistance to FeO slag. However, magnesia and graphite contained in carbon cause magnesia-carbon reaction, which leads to structural deterioration, and durability is reduced depending on slag conditions. In particular, the higher the frequency of OB operation, the more severe the corrosion damage caused by slag infiltration in the structurally deteriorated parts, and durability may be reduced.

Patent Document 3 discloses a magnesia-carbon brick obtained by adding an organic binder to a refractory raw material mixture, kneading and molding the mixture, and then heat-treating the mixture, in which the refractory raw material mixture contains a total of 0.1% by mass or more and 2.0% by mass or less of pitch and/or carbon black, a total of 0.1% by mass or more and 1.0% by mass or less of aluminum and/or aluminum alloy, 3.0% by mass or more and 10.0% by mass or less of magnesia having a particle size of less than 0.075 mm, and 87.0% by mass or more and 96.0% by mass or less of magnesia having a particle size of 0.075 mm or more and less than 5 mm, and the mass ratio of magnesia having a particle size of 1 mm or more and less than 5 mm to magnesia having a particle size of 0.075 mm or more and less than 1 mm is 1.66 to 2.34, the magnesia-carbon brick does not contain graphite, and has an apparent porosity of 8.0% or less after heat treatment for 3 hours in a reducing atmosphere at 1400°C. This brick is characterized by suppressing heat dissipation from molten metal and suppressing carbon pick-up by not using graphite, which has high thermal conductivity. Pitch and/or carbon black are used as a carbon source instead of graphite, and the brick has a low elastic modulus and improved spalling resistance. However, since it contains a large amount of magnesia, which has a large thermal expansion and a high elastic modulus, and does not contain graphite, its thermal conductivity is poor and its heat spalling resistance is inferior to that of graphite-containing bricks. For this reason, when this brick was actually used in an actual machine and the brick was observed after use, cracks were found near the working surface, which was an obstacle to improving its durability.

Patent Document 4 discloses a lining refractory material for secondary refining equipment with reduced pressure, characterized by being made of spinel-carbonaceous bricks containing 75-99.5% by mass of spinel and 0.5-25% by mass of carbon. According to Patent Document 4, magnesia-carbon bricks are superior in corrosion resistance to high-basicity, low-alumina-containing slag similar to converter slag, but spinel-carbon bricks are superior to magnesia-carbon bricks in corrosion resistance to low-basicity, high-alumina-containing slag. It also describes that the slag after secondary refining is considered to be substantially the same as the slag remaining in the ladle in the continuous casting operation, and that the basicity and alumina content of the slag can be determined by analyzing the ladle slag remaining after continuous casting, and that low-basicity, high-alumina-containing slag is slag with a basicity of 0.5-3.0 and an alumina content in the range of 20-40% by mass in ladle slag after secondary refining with reduced pressure collected by the above method. However, when the inventors used this spinel-carbon brick in a vacuum degassing apparatus operating within the above range, they found that the side wall of the lower vessel had inferior durability to conventional magnesia-carbon bricks. In Patent Document 4, the components of the slag generated in the vacuum degassing apparatus (hereinafter, vessel slag) are estimated by analyzing the ladle slag remaining after continuous casting, but the estimation accuracy is very low because the ladle slag after continuous casting is affected by the suction of the ladle slag into the vacuum degassing apparatus. In fact, in a vacuum degassing apparatus operated within the above range, the durability of the spinel-carbon brick was inferior to that of the magnesia-carbon brick. When the working surface of the spinel-carbon brick was observed, the erosion of spinel particles by the high calcia content slag was confirmed.

JP 2019-137565 A JP 2020-55726 A Patent No. 6279052 JP 2016-60651 A

The object of the present invention is to provide a magnesia-spinel brick that combines corrosion resistance to slag, suppression of magnesia-carbon reaction, and heat spalling resistance by selecting a material for the side wall of the lower vessel that is suitable for the operating conditions of the vacuum degassing equipment, and a vacuum degassing equipment that uses this for the side wall of the lower vessel.

The present inventors have discovered that magnesia-spinel bricks containing no graphite, 1.0% by mass to 69.7% by mass of spinel, 30.1% by mass to 96.0% by mass of magnesia, 0.1% by mass to 2.0% by mass of aluminum and/or aluminum alloy, and 0.1% by mass to 2.0% by mass of pitch and/or carbon black have excellent durability when used as a lining material for the side wall of the lower tank of a vacuum degassing apparatus that satisfies the following (2) to (7).

0.04≦Average tank slag index (hereinafter average RSI) [-]≦0.30 (2)

Average RSI [-]
= Σ Slag index in each process [-] / number of processes per furnace [ch / furnace] (3)

Slag index for each treatment [-]
= (amount of calcia added [kmol/ch] + amount of alumina added [kmol/ch] × 0.2) / (amount of calcia added [kmol/ch] + amount of silica added [kmol/ch] + amount of alumina added [kmol/ch]) (4)

Amount of calcia fed [kmol/ch] = amount of Ca-containing material fed [kg/ch] x calcium content of Ca-containing material [mass%] ÷ 40.08 (5)

Amount of silica fed [kmol/ch] = Amount of silicon-containing material fed [kg/ch] × Silicon content of silicon-containing material [mass%] ÷ 28.09 (6)

Amount of alumina fed [kmol/ch] = Amount of aluminum-containing material fed [kg/ch] x Aluminum content of aluminum-containing material [mass%] ÷ 53.96 (7)

In a vacuum degassing apparatus that operates to satisfy the above (2)-(7), by applying the magnesia-spinel brick of the present invention as the lining material for the side wall of the lower vessel, it is possible to simultaneously achieve corrosion resistance against slag, suppression of the magnesia-carbon reaction, and heat spalling resistance, thereby significantly improving the durability of the brick. Furthermore, a vacuum degassing apparatus that uses this magnesia-spinel brick will significantly improve the life of the side wall of the lower vessel.

FIG. 2 is a diagram illustrating a cross-sectional configuration of an RH vacuum chamber. FIG. 1 is a diagram showing the relationship between the average slag index (RSI) in the vessel at an actual RH and the damage rate compared using the indexes of various bricks.

1. Magnesia-Spinel Brick The configuration of the magnesia-spinel brick according to the present invention will be described below. The magnesia-spinel brick according to the present invention does not contain graphite, and contains 30.1% by mass to 96.0% by mass of magnesia, 1.0% by mass to 69.7% by mass of spinel, 0.1% by mass to 2.0% by mass of aluminum and/or aluminum alloy, and 0.1% by mass to 2.0% by mass of pitch and/or carbon black, and is used as a lining for the side wall of the lower vessel of a vacuum degassing apparatus which operates to satisfy the above-mentioned (2) to (7).

Magnesia is used because it suppresses the formation of low melting point compounds with high calcia-containing slag and has excellent durability against FeO slag. The magnesia content is 30.1% by mass or more. If the magnesia content is too low, not only will the durability against high calcia-containing slag decrease, but the durability will also decrease due to the infiltration of FeO slag into the refractory. In addition, the magnesia content is 96.0% by mass or less. If the magnesia content is too high, due to its large thermal expansion coefficient, it will compete with the surrounding refractory when used hot in the degassing tank, causing cracks and leading to spalling damage and reduced durability. The magnesia content may be 95.0% by mass or less, 90.0% by mass or less, 85.0% by mass or less, 80.0% by mass or less, or 75.0% by mass or less. The magnesia may be any type commonly used in refractories, such as fused 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 purity may be used, such as magnesia with an MgO purity of 96.0% by mass or more, or even 98.0% by mass or more.

Spinel is used to suppress the magnesia-carbon reaction and to reduce the thermal expansion of the brick. The spinel content is 1.0% by mass or more. If the spinel content is too low, the amount of magnesia becomes relatively high, resulting in a decrease in durability due to the magnesia-carbon reaction, as well as an increase in the thermal expansion coefficient, which causes cracks due to competition with the surrounding refractory. The spinel content may be 2.0% by mass or more, 5.0% by mass or more, 10.0% by mass or more, 15.0% by mass or more, or 20.0% by mass or more. The spinel content is 69.7% by mass or less. If the spinel content is too high, not only will the effect of a decrease in durability due to the formation of low-melting point compounds with high-calcia slag increase, but the decrease in durability due to the formation of low-melting point compounds with FeO slag will occur, causing greater damage. The spinel is mainly composed of alumina and magnesia, and contains a total of 95% by mass or more of alumina and magnesia. The theoretical composition of spinel is alumina:magnesia = 71.7:28.3 by mass, but there are various compositions, and those containing more alumina than the theoretical composition are called alumina-rich spinel, and those containing more magnesia than the theoretical composition are called magnesia-rich spinel. In the present invention, any spinel can be used, and they may be used in combination. The spinel may be produced by sintering or electrofusion, and these may be used in combination. In order to obtain higher corrosion resistance, it is preferable that the amount of impurities such as calcia and silica is small, and for example, the amount of impurities may be 5 mass% or less, or even 2 mass% or less. The magnesia in the spinel and the magnesia alone mentioned above have clearly different crystal structures, and can be easily distinguished by X-ray diffraction measurement, etc. In addition, the spinel content in the brick and the magnesia content alone mentioned above can be easily measured independently by known measurement methods such as EPMA (probe microanalyzer) and XRF (X-ray fluorescence analysis).

The aluminum and/or aluminum alloy improves the oxidation resistance of the brick. The content of aluminum and/or aluminum alloy (the total content of aluminum and aluminum alloy when both are included) is 0.1 mass% or more. If the content of aluminum and/or aluminum alloy is too small, the oxidation resistance decreases, carbon in the brick is oxidized, and durability decreases. The content of aluminum and/or aluminum alloy is 2.0 mass% or less. If the content of aluminum and/or aluminum alloy is too high, magnesia reacts with aluminum and/or aluminum alloy to generate a large amount of spinel in the structure, causing oversintering and significantly decreasing thermal shock resistance. The aluminum alloy may be any one that contains aluminum. The aluminum alloy may contain, for example, 49 mass% or more of aluminum. Specific examples of aluminum alloys include an alloy of aluminum and silicon, an alloy of aluminum and magnesium, etc.

Pitch and/or carbon black are used to improve the heat spalling resistance by reducing the elastic modulus. The content of pitch and/or carbon black (the total content of pitch and carbon black when both are included) is 0.1 mass% or more. If the content of pitch and/or carbon black is too small, the effect of improving spalling resistance becomes insufficient and durability decreases. In addition, the content of pitch and/or carbon black is 2.0 mass% or less. If the content of pitch and/or carbon black is too large, the porosity increases due to the magnesia-carbon reaction, and the slag infiltrates, resulting in a decrease in corrosion resistance. In addition, the liquid phase oxidation by FeO slag (reaction (8) below) causes structural deterioration, resulting in a decrease in durability. The content of pitch and/or carbon black is preferably 1.0 mass% or more and 2.0 mass% or less.
FeO(liquid) + C(solid) → Fe(liquid) + CO(gas) (8)

The brick of the present invention may further contain other components such as carbonaceous raw materials, antioxidants, and/or metals, which are contained in normal magnesia-carbon bricks, in addition to spinel, magnesia, aluminum and/or aluminum alloys, and pitch and/or carbon black. The content of the other components may be, for example, 3 mass% or less. Specific examples of the other components include SiC, B4C, and Si. In addition, amorphous carbon derived from an organic binder to form a binding structure may also be contained.

The magnesia-spinel brick of the present invention does not need to contain graphite. By not containing graphite, the magnesia-carbon reaction is less likely to occur at high temperatures than when graphite is contained. Furthermore, by not containing graphite, it is expected that the heat dissipation from the molten metal and carbon pickup can be suppressed during operation of the vacuum degassing device. In this application, "not containing graphite" does not mean that the brick is limited to a form in which the graphite content is completely zero, but means that the brick does not substantially contain graphite. Specifically, if the graphite content in the brick is 0.1 mass% or less, the brick is considered to be "not containing graphite".

The magnesia-spinel brick of the present invention can be manufactured by the same method as that for manufacturing general magnesia-carbon bricks. That is, the magnesia-spinel brick of the present invention can be obtained, for example, by adding an organic binder to a refractory raw material mixture, kneading and molding the mixture, and then heat treating it. The heat treatment temperature can be, for example, in the range of 200°C to 800°C.

As the organic binder, the organic binders used in ordinary magnesia-carbon bricks can be used, such as furan resin or phenol resin. The organic binder can be used in any form, such as powder, 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 for manufacturing ordinary magnesia-carbon bricks.

The magnesia-spinel bricks of the present invention are used as a lining for the side wall of the lower vessel of a vacuum degassing apparatus. Figure 1 shows a cross-sectional view of an RH vacuum vessel. As shown in the figure, the lower vessel side wall in this application refers to the side wall portion (hatched portion in Figure 1) made of refractory material from the bottom portion up to the flange portion that joins with the upper vessel.

In the present invention, the magnesia-spinel bricks are used as a lining material, particularly for the side wall of the lower vessel, in a vacuum degassing apparatus that operates to satisfy the following (2)-(7), thereby preventing damage to the side wall of the lower vessel and extending the service life of the lower vessel. In the following (3), "furnace life" refers to one cycle from when refractory material is installed in the vacuum degassing apparatus and use is started until the refractory material wears out and it becomes necessary to install new refractory material.

0.04≦Average tank slag index (hereinafter average RSI) [-]≦0.30 (2)

Average RSI [-] = Σ Slag index for each process [-] / number of processes per furnace [ch/furnace cost] (3)

Slag index [-] for each process = (amount of calcia input [kmol/ch] + amount of alumina input [kmol/ch] x 0.2) / (amount of calcia input [kmol/ch] + amount of silica input [kmol/ch] + amount of alumina input [kmol/ch]) (4)

Amount of calcia fed [kmol/ch] = amount of Ca-containing material fed [kg/ch] x calcium content of Ca-containing material [mass%] ÷ 40.08 (5)

Amount of silica fed [kmol/ch] = Amount of silicon-containing material fed [kg/ch] × Silicon content of silicon-containing material [mass%] ÷ 28.09 (6)

Amount of alumina fed [kmol/ch] = Amount of aluminum-containing material fed [kg/ch] x Aluminum content of aluminum-containing material [mass%] ÷ 53.96 (7)

The durability of the lower vessel bricks of the vacuum degassing device is thought to be directly affected by the slag generated within the vacuum degassing device (hereinafter referred to as vessel slag), but it is very difficult to extract it. Therefore, in Patent Document 4, it is replaced with ladle slag remaining after continuous casting.

The source of slag in the vessel is thought to be the absorption of some of the ladle slag into the vacuum degassing equipment, the oxidation of the molten steel components, and the oxidation of auxiliary materials added during processing. However, since the slag is discharged from the vacuum degassing equipment into the ladle at the same time as it is generated, the slag components fluctuate over time. The ladle slag remaining after continuous casting, which is substituted in Patent Document 4, is presumed to have a composition significantly different from that of the slag in the vessel. When the inventors actually performed a vacuum degassing process, they found that spinel carbon bricks, which were considered to have excellent durability, were inferior in durability to magnesia carbon bricks within the range of operating conditions shown in Patent Document 4.

After extensive research, the inventors discovered that the rate at which the bricks used in the side walls of the lower tank of the vacuum degassing equipment were damaged was positively correlated with the implementation rate of aluminum-silicon deoxidizing steel processing, which uses aluminum and silicon as secondary materials, and therefore that attention should be paid to the amount and type of secondary materials added as slag components in the tank that affect durability.

In this invention, the average vessel slag index (average RSI) is used as an index, taking into account calcium, aluminum, and silicon, which are typical inputs, to enable the selection of a material suitable for the side wall material of the lower vessel of the vacuum degassing apparatus. Calcia, alumina, and silica, which are oxides of calcium, aluminum, and silicon, are all components that have a significant effect on the amount of wear of magnesia and spinel. Laboratory experiments by the inventors have revealed that in an environment where magnesia-carbon reactions are very likely to occur, such as inside a vacuum degassing apparatus, magnesia is more difficult to wear and is more advantageous than spinel for slag with high calcia, low alumina, and low silica content. This is because spinel is more easily dissolved in slag with low alumina content. In other words, it was found that the optimal blend of magnesia and spinel cannot be determined unless the alumina content in the slag, as well as the slag basicity determined by the mass ratio of calcia and silica, is also taken into consideration as a factor that determines the slag corrosion resistance of the refractory used in the side wall of the lower vessel of the vacuum degassing apparatus.

After using the actual RH, the slag components attached to the lower tank side wall material were analyzed, and when the average RSI was less than 0.04, the slag tended to have low calcia, low alumina, and high silica content, and when the average RSI was more than 0.30, the slag tended to have high calcia, low alumina, and low silica content. That is, when the average RSI is 0.04 or more and 0.30 or less as in (2) above, the brick of the present invention is considered to have excellent durability, as it is suppressed from dissolving the spinel due to calcia, dissolving the spinel due to low-alumina slag, and dissolving the magnesia due to silica. The RSI becomes low when the amount of Si-containing material is large, and becomes high when the amount of Ca-containing material is large. In the present invention, the average RSI may be 0.07 or more, or 0.20 or less.

2. Vacuum degassing apparatus The present invention also has an aspect of a vacuum degassing apparatus. The vacuum degassing apparatus according to the present invention is characterized in that the side wall of the lower vessel is lined with the magnesia-spinel brick according to the present invention. The composition of the brick and the operating conditions of the vacuum degassing apparatus are as described above.

3. Method for Operating a Vacuum Degassing Apparatus The present invention also has an aspect of a method for operating a vacuum degassing apparatus. The method for operating a vacuum degassing apparatus according to the present invention is characterized in that the magnesia-spinel bricks according to the present invention are used as a lining on the side wall of the lower vessel of the vacuum degassing apparatus, and the vacuum degassing apparatus is operated so as to satisfy the above-mentioned (2) to (7).

In particular, when the frequency of oxygen injection into the vacuum degassing tank exceeds 30% of the furnace life cycle (when oxygen injection into the vacuum degassing tank is performed more than 30% of the time during one furnace operation (for example, when oxygen injection is performed more than 30 times, assuming that the total number of operations for one furnace operation is 100)), the brick of the present invention is effective as a brick for the side wall of the lower tank of the vacuum degassing apparatus. That is, when oxygen injection is performed in the vacuum degassing apparatus for decarburization, etc., a large amount of FeO-containing slag is generated, but the brick of the present invention contains magnesia, which is highly corrosion-resistant to FeO-containing slag, in an amount of 30.1% by mass to 96.0% by mass, and therefore excellent corrosion resistance can be ensured even during operations in which oxygen is injected. Magnesia has a large thermal expansion and poor thermal shock resistance, so graphite is generally added, but in the present invention, graphite is not added to avoid the magnesia-carbon reaction. Therefore, in the present invention, in order to improve thermal shock resistance, pitch and/or carbon black is blended in an amount of 0.1% by mass to 2.0% by mass, and spinel is blended in an amount of 1.0% by mass to 69.7% by mass. This makes it possible to operate the furnace while suppressing a decrease in furnace life, even when the frequency of operation in which oxygen is blown in is high.

1. Manufacturing conditions of bricks according to the examples and comparative examples The compositions and physical properties of the magnesia-spinel bricks according to the examples and comparative examples are shown in Tables 1 to 3. The bricks in Tables 1 to 3 were manufactured by adding an appropriate amount of phenolic resin as an organic binder to the refractory raw material mixture, kneading it, forming it into a shape of 230 mm x 114 mm x 110 mm using an oil press, and then carrying out a heat treatment at a maximum temperature of 250°C for 5 hours.

2. An example of the in-vessel slag index Table 4 shows an example of the in-vessel slag index (RSI) and the amount of auxiliary material charged in each process. In the aluminum-silicon deoxidization steel process, the auxiliary materials are charged in the configuration shown in Table 4 as an example, but the amount of each auxiliary material charged varies depending on the process conditions. In this example, the average RSI (above (2) to (7)) obtained by averaging the RSI, which varies slightly for each channel, over one furnace is used.

3. Evaluation of bricks according to examples and comparative examples 3.1 Corrosion resistance (wear rate)
The corrosion resistance was evaluated from the wear rate when various bricks were applied as the side wall material of the lower tank of the actual RH. The average slag index in the tank was in the range of 0.02 to 0.35 when applied. The wear rate was calculated by dividing the change in the brick dimensions before and after operation by the total number of treatments. The wear rate of Comparative Example 2 at each average slag index in the tank was set to 100. Figure 2 shows the relationship between the wear rate and the average RSI when various bricks were applied to the actual RH. When the average RSI was 0.04 or more and 0.30 or less, the wear rate of the bricks according to the examples was lower than that of the bricks according to the comparative examples that were lined at the same time. Figure 2 shows only the results of Examples 1 to 4 and Comparative Examples 1 and 2 as representative examples, but when the average RSI was in the range of 0.04 or more and 0.30 or less, the wear rate index of each of the bricks according to Examples 1 to 13 was smaller than that of the bricks according to Comparative Examples 1 to 6, and the corrosion resistance was superior. The operation frequency at which oxygen was blown in when the representative example shown in Fig. 2 was applied was 30% or more of the furnace life cycle. Although not shown in the figure, when the average RSI was 0.07 and the operation frequency at which oxygen was blown in was 10%, the damage rate of Example 1 was 99 and the damage rate of Comparative Example 1 was 103, assuming that the damage rate of Comparative Example 2 was 100. The higher the operation frequency at which oxygen was blown in, the higher the durability of the bricks of Examples 1 to 13 was, and the lower the durability of the conventional brick was.

In addition to the damage rate in the actual equipment, we also evaluated the basic properties required for a refractory material to be used in the side wall of the lower tank of a vacuum degassing device. Specifically, samples for measuring physical properties were cut out from the bricks manufactured as described above, and the thermal shock resistance, magnesia-carbon reactivity resistance, and oxidation resistance were evaluated.

3.2 Thermal shock resistance Thermal shock resistance was evaluated by a hot metal immersion spalling test. In this test, a 40×40×190 mm sample was sintered in a reducing atmosphere at 1500° C. for 10 hours, immersed in hot metal heated to 1650° C. for 10 minutes, and then cooled with water for 1 minute. This cycle was repeated five times. After the test, the sample was cut and the cross section was observed and evaluated. In Tables 1 to 3, ◎ indicates a sample in which no cracks of 10 mm or more were observed, ○ indicates a sample in which three or fewer cracks of 10 mm or more were observed, and × indicates a sample in which more than three cracks of 10 mm or more were observed, and it was determined that the sample was not suitable for use in an actual furnace.

3.3 Magnesia-carbon reactivity Magnesia-carbon reactivity was evaluated by a high-temperature heating test in Ar. This high-temperature heating test was carried out using an electric furnace capable of adjusting the atmosphere. The test temperature was set to 1700°C, and the Ar atmosphere was used. Ar was blown in to lower the partial pressure of Mg gas and the partial pressure of CO gas, and the magnesia-carbon reaction was promoted in the same manner as in the treatment under reduced pressure or vacuum. The magnesia-carbon reaction is a reaction in which solid magnesia and carbon become Mg gas and CO gas, and is accompanied by a mass reduction, so the magnesia-carbon reactivity was evaluated using this mass reduction rate. In other words, the smaller the mass reduction rate, the more the magnesia-carbon reaction is suppressed. In addition, the sample was pre-treated by reducing and firing it in carbon material at a temperature of 1500°C for 10 hours to remove volatile components in the resin. In Tables 1 to 3, ◎ indicates samples with a mass reduction rate of 0% by mass or more and less than 3% by mass, ○ indicates samples with a mass reduction rate of 3% by mass or more and less than 10% by mass, and × indicates samples with a mass reduction rate of 10% by mass or more. The × samples were determined to be unsuitable for use in actual furnaces because slag penetrated into the voids generated by the weight reduction and was dissolved.

3.4 Oxidation Resistance The oxidation resistance was evaluated by a high-temperature heating test in air. This high-temperature heating test was carried out using an electric furnace with an adjustable atmosphere. The test temperature was set to 1700°C, and the air atmosphere was used, and compressed air was blown in to promote the oxidation reaction of carbon. The oxidation reaction of carbon is a reaction in which carbon becomes CO gas or _CO2 gas, and is accompanied by a mass reduction, so the oxidation resistance was evaluated using this mass reduction rate. In other words, the smaller the mass reduction rate, the more the oxidation reaction of carbon is suppressed. In addition, the samples were pre-treated by reducing and baking in carbon material at a temperature of 1500°C for 10 hours to remove volatile components in the resin. In Tables 1 to 3, ◎ indicates a sample with a mass reduction rate of 0% by mass or more and less than 3% by mass, ○ indicates a sample with a mass reduction rate of 3% by mass or more and less than 10% by mass, and × indicates a sample with a mass reduction rate of 10% by mass or more. In the × sample, slag penetrates into the voids generated by the weight reduction and dissolves, so it was determined that the sample was not suitable for use in an actual furnace.

In Examples 1 to 5, the magnesia content was varied within the range of 30.1% by mass to 96.0% by mass, and as shown in Tables 1 to 3, good results were obtained in terms of thermal shock resistance, magnesia-carbon reactivity resistance, and oxidation resistance. As mentioned above, the corrosion resistance shown in Figure 2 was also good. In contrast, Comparative Example 1 had a low magnesia content, and as shown in Figure 2, the corrosion resistance decreased with the average RSI in the range of (2). Comparative Example 2 had a high magnesia content and did not contain spinel, so it had poor thermal shock resistance.

In Examples 3 and 6 to 9, the carbon black content was varied within the range of 0.1% by mass or more and 2.0% by mass or less, and as shown in Tables 1 to 3, the thermal shock resistance, magnesia-carbon reactivity resistance, and oxidation resistance all showed good results. As mentioned above, the corrosion resistance shown in Figure 2 was also good. In contrast, Comparative Example 4 did not contain carbon black, so the thermal shock resistance was reduced. Comparative Example 3 had a high carbon black content, so the magnesia-carbon reactivity resistance and oxidation resistance were reduced.

In Examples 10 to 13, the aluminum content was varied within the range of 0.1% by mass to 2.0% by mass, and as shown in Tables 1 to 3, the thermal shock resistance, magnesia-carbon reactivity resistance, and oxidation resistance all showed good results. As mentioned above, the corrosion resistance shown in Figure 2 was also good. In contrast, Comparative Example 5 did not contain aluminum, so the oxidation resistance was reduced. Comparative Example 6 had a high aluminum content, so the thermal shock resistance was reduced.

In the above examples, carbon black was used, but the same effect can be achieved by using pitch instead of or together with carbon black.

In addition, in the above examples, Al is contained in the bricks to improve their oxidation resistance, but the same effect can be achieved by using an Al alloy instead of Al or together with Al.

In addition, in the above examples and comparative examples, when graphite was used instead of carbon black, the resistance to magnesia-carbon reactivity tended to be slightly lower than when carbon black or pitch was used. Also, there was a tendency for heat dissipation and carbon pickup from the molten metal to occur easily during operation of the vacuum degassing device.

Claims (2)

does not contain graphite, and contains 1.0% by mass or more and 69.7% by mass or less of spinel, 30.1% by mass or more and 96.0% by mass or less of magnesia, 0.1% by mass or more and 2.0% by mass or less of aluminum and/or aluminum alloy, and 0.1% by mass or more and 2.0% by mass or less of pitch and/or carbon black;
It is used as a lining on the side wall of the lower vessel of a vacuum degassing apparatus that satisfies the following (2)-(7):
Magnesia-spinel brick.
0.04≦Average vessel slag index (hereinafter referred to as average RSI) [-]≦0.30 (2)
Average RSI [-] = Σ Slag index [-] in each process / number of processes for one furnace [ch / furnace cost] (3)
Slag index [-] in each treatment = (amount of calcia input [kmol/ch] + amount of alumina input [kmol/ch] × 0.2) / (amount of calcia input [kmol/ch] + amount of silica input [kmol/ch] + amount of alumina input [kmol/ch]) (4)
Amount of calcia fed [kmol/ch] = Amount of Ca-containing material fed [kg/ch] × Calcium content of Ca-containing material [mass%] ÷ 40.08 (5)
Input amount of silica [kmol / ch] = input amount of Si-containing material [kg / ch] × silicon content of Si-containing material [mass%] ÷ 28.09 (6)
Amount of alumina fed [kmol/ch] = Amount of Al-containing material fed [kg/ch] × Aluminum content of Al-containing material [mass%] ÷ 53.96 (7) The magnesia-spinel brick according to claim 1 is used to line the side wall of a lower tank.
Vacuum degassing equipment.

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