JP2019006638A - METHOD OF CHOOSING MgO-C BRICK, METHOD OF OPERATING MOLTEN METAL CONTAINER, AND LINING STRUCTURE OF MOLTEN METAL CONTAINER - Google Patents

Abstract

To impart excellent durability to a MgO-C brick upon using the same in a lining structure of an intermittently operated molten metal container.SOLUTION: There is provided a method of choosing a MgO-C brick for use in a lining structure of an intermittently operated molten metal container, comprising a step of choosing a MgO-C brick whose test piece shows permeability of not lower than 10.0×10msubsequent to repeating processes of heating a test piece of the MgO-C brick up to 1600°C, keeping the test piece at 1600°C for 3 hours, slowly cooling the test piece down to 500°C, and keeping the test piece at 500°C for 3 hours, 10 times under a reduction atmosphere, and a step of choosing a MgO-C brick according to its apparent porosity subsequent to reduction calcination.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a method for selecting an MgO-C brick, a method for operating a molten metal container, and a lining structure of the molten metal container.

  MgO-C brick (magnesia carbon brick) is a brick composed mainly of magnesia and graphite, and is widely used for lining structures of molten metal containers such as converters because of its excellent corrosion resistance and spall resistance. Yes. In MgO-C brick, it is known that durability is improved by densifying the structure.

  For example, Patent Document 1 discloses a highly durable MgO-C brick, which is made more dense after heat reception by optimizing the particle size composition of the magnesia raw material and the particle size composition of graphite. Techniques for providing are described. According to Patent Document 1, the MgO-C brick is made denser, thereby reducing the permeability to the outside air, and suppressing the penetration of slag, hot metal, and molten steel, thereby preventing the oxidation resistance of MgO-C brick. And corrosion resistance can be improved.

JP 2014-166943 A

  However, for example, when using MgO-C brick for the lining structure of the converter, even if the MgO-C brick densified as described above is used, the expected durability cannot be obtained depending on the operating conditions. was there. Specifically, in the case of intermittent operation in which the waiting time from when the molten steel is discharged to when the next molten iron is charged is long, and during which a large temperature drop occurs on the operating surface, MgO − constituting the operating surface C bricks may be worn more than expected.

  The present invention relates to a novel and improved method for selecting an MgO-C brick, a molten metal container capable of obtaining good durability when using MgO-C brick for a lining structure of a molten metal container for intermittent operation. It is an object of the present invention to provide an operation method and an MgO-C brick.

According to an aspect of the present invention, there is provided a method for selecting an MgO-C brick used in a lining structure of a molten metal container that performs intermittent operation, the step of heating a test piece of MgO-C brick to 1600 ° C, a test piece The test piece was kept at 1600 ° C. for 3 hours, the test piece was slowly cooled to 500 ° C., and the test piece was held at 500 ° C. for 3 hours under a reducing atmosphere 10 times. A MgO-C brick including a step of selecting an MgO-C brick having a size of 10.0 × 10 ⁻¹⁵ m ² or more and a step of selecting an MgO—C brick based on an apparent porosity after reduction firing. A selection method is provided.

  In the above selection method, in the step of selecting MgO-C brick based on the apparent porosity after reduction firing, MgO-C brick having an apparent porosity of less than 10% after reduction firing at 1400 ° C. for 30 hours is selected. May be.

According to another aspect of the present invention, there is provided a method of operating a molten metal container having a lining structure formed of MgO-C brick, including a step of performing intermittent operation using the molten metal container, and includes a MgO-C brick. The air permeability after repeating the step of heating to 1600 ° C., the step of holding at 1600 ° C. for 3 hours, the step of gradually cooling to 500 ° C., and the step of holding at 500 ° C. for 3 hours under a reducing atmosphere is 10 times. There is provided a method for operating a molten metal container having an apparent porosity of 10.0 × 10 ⁻¹⁵ m ² or more and reduction firing at 1400 ° C. for 30 hours, which is less than 10%.

According to still another aspect of the present invention, a molten metal container that is intermittently operated is a lining structure formed of MgO-C brick, and the MgO-C brick is heated to 1600 ° C at 1600 ° C. The air permeability after repeating the step of holding for 3 hours, the step of gradually cooling to 500 ° C., and the step of holding for 3 hours at 500 ° C. in a reducing atmosphere 10 times or more is 10.0 × 10 ⁻¹⁵ m ² and There is provided a lining structure of a molten metal container having an apparent porosity of less than 10% after reduction firing at 1400 ° C. for 30 hours.

  As described above, according to the present invention, when the selected MgO—C brick is used for the lining structure of a molten metal container that performs intermittent operation, good durability can be obtained.

It is sectional drawing of the converter which used the MgO-C brick which concerns on one Embodiment of this invention for the lining structure. It is a flowchart which shows the selection method of the MgO-C brick which concerns on one Embodiment of this invention. It is a graph which shows the relationship between the repetition frequency of heat processing, and an air permeability. It is a graph which shows the relationship between the repetition frequency of heat processing, and an air permeability. It is a graph which shows the relationship between the repetition frequency of heat processing, and an air permeability. It is a graph which shows the relationship between the apparent porosity after reduction | restoration baking, and the erosion amount index | exponent in a rotation erosion test (normal). It is a graph which shows the relationship between the apparent porosity after reduction | restoration baking, and the erosion amount index | exponent in a rotation erosion test (intermittent). It is a graph which shows the relationship between the air permeability after repeated heat processing, and the erosion amount index | exponent in a rotation erosion test (normal). It is a graph which shows the relationship between the air permeability after repeated heat processing, and the erosion amount index | exponent in a rotation erosion test (intermittent). It is a graph which shows the relationship between the air permeability after repeated heat processing, and the apparent porosity after reduction baking.

  FIG. 1 is a cross-sectional view of a converter using MgO—C brick according to an embodiment of the present invention for a lining structure. Referring to FIG. 1, a converter 1 which is an example of a molten metal container according to the present invention has a lining structure 2 formed of MgO-C brick. The temperature of the working surface 2s of the lining structure 2 is about the same as the hot metal during the hot metal treatment by the converter 1, specifically, for example, about 1600 ° C. to 1800 ° C., but after the treated molten steel is discharged. In the standby time until the molten iron is charged next, the temperature falls below the above temperature.

  Here, when the operating rate of the converter 1 is high and the standby time is short, the temperature drop is small. In this case, the operating surface 2s is maintained at a temperature similar to that during processing, for example. On the other hand, when the operating rate of the converter 1 is low and the standby time is long, the temperature drop is large. In this case, the operating surface 2s is allowed to cool to a temperature that is significantly lower than the above temperature. For example, the operating surface 2s may be gradually cooled to a temperature of about 800 ° C. or a lower temperature of about 500 ° C. during which the visible radiation of the attached slag becomes insignificant during the waiting time.

  In the present specification, an operation in which the temperature drop generated on the operating surface 2s of the lining structure 2 exceeds a predetermined range while the converter 1 is not charged with hot metal or molten steel is referred to as intermittent operation. In the molten metal container other than the converter 1, the intermittent operation can be defined similarly.

  As described above, in the converter 1 that performs intermittent operation, even when densified MgO-C brick is used, the MgO-C brick constituting the working surface 2s of the lining structure 2 is worn more than expected. May occur. As a result of observing the worn part of the working surface 2s, the present inventors have found that there are many voids in the MgO-C brick in the vicinity of the working surface 2s. In the converter 1 that performs intermittent operation, it is considered that when the hot metal is charged again after the standby time, the operating surface 2s is subjected to a large thermal shock, whereby the operating surface 2s is peeled off from the above-described gap. .

  In general, it is considered that the gap inside the brick is caused by the disappearance of magnesia and graphite that have caused the MgO-C reaction under high temperature and reduced pressure, and the expansion and contraction of the MgO aggregate. However, when the working surface 2s of the MgO-C brick is densified (more specifically, when the slag infiltrates the working surface of the MgO-C brick), the above MgO-C reaction or the slag in the furnace The ratio of the CO gas generated by the reaction between the component and graphite staying inside the densified layer of the working surface 2s without being released into the furnace increases. It is considered that the voids observed at the above-mentioned worn portions were formed by the action of the staying CO gas.

  As described above, it is effective to improve the oxidation resistance and corrosion resistance by densifying the MgO-C brick structure to improve the durability of the MgO-C brick. Therefore, the present inventors have studied a method for preventing the separation of the working surface 2s caused by the generation of voids as described above while maintaining such a densification effect. As a result, the MgO-C brick forming the lining structure 2 has a certain air permeability, and the CO gas does not stay inside the working surface 2s and is diffused inside the brick, thereby preventing the generation of voids. I came up with the possibility.

  MgO-C bricks have almost no air permeability at the time of manufacture, but when the MgO-C reaction as described above occurs under the heat load in the converter 1, gaps are generated in the structure, and in the case of intermittent operation. As the MgO aggregate repeatedly expands and contracts, the gap is expanded to form a vent hole, and the air permeability is increased. As described above, when the working surface 2s is densified and the CO gas generated by the MgO-C reaction stays immediately below the dense layer to generate voids, the working surface 2s is peeled off. If the gap generated inside the -C brick is expanded by expansion and contraction of the MgO aggregate, etc., it has the function of diffusing CO gas into the MgO-C brick as a vent hole, and the gap immediately below the dense layer It is thought that generation can be suppressed. Based on such knowledge, the MgO—C brick according to the present embodiment is selected by a selection method as described below.

  FIG. 2 is a flowchart showing a method for selecting an MgO—C brick according to an embodiment of the present invention. Referring to FIG. 2, the selection method of MgO-C brick includes step S1 of selecting MgO-C brick based on the air permeability after repeated heat treatment, and MgO-C brick based on the apparent porosity after reduction firing. And selecting step S2.

In step S1, MgO-C brick is subjected to repeated heat treatment simulating the intermittent operation in the converter 1 as described above, and the air permeability is measured. Based on the result, MgO- suitable for the lining structure 2 is measured. This is the step of selecting C brick. Specifically, the step of heating the MgO-C brick test piece to 1600 ° C, the step of holding the test piece at 1600 ° C for 3 hours, the step of gradually cooling the test piece to 500 ° C, and the test piece at 500 ° C After repeating the step of holding for 3 hours 10 times in a reducing atmosphere, the air permeability was measured by the method prescribed in JIS R2115, and the MgO-C brick having an air permeability of 10.0 × 10 ⁻¹⁵ m ² or more was measured. Select one suitable for use.

  Step S2, for example, as described in Japanese Patent Application Laid-Open No. 2014-166943, after MgO-C brick is preliminarily reduced and fired and brought close to the state when used as the lining structure 2 in the converter 1, This is a step of measuring the apparent porosity as an index of the fineness of the structure, and selecting an MgO—C brick suitable for the lining structure 2 based on the result. Specifically, after the reduction firing of the MgO-C brick test piece was performed at 1400 ° C. for 30 hours, the apparent porosity was measured by the method defined in JIS A1509-3, and the apparent porosity was less than 10%. A certain MgO-C brick is selected as suitable for use. Here, the time for reduction firing is not limited to 30 hours, but since MgO-C brick is unfired brick, volatilization of components generated during use in an actual machine by performing reduction firing for a long time. In addition, it is possible to reproduce the tightening due to MgO—C reaction, metal reaction, and the like, and it is possible to more accurately evaluate the state of the open pores on the surface after the operation in the actual machine is repeated.

  3 to 5 are graphs showing the relationship between the number of repetitions of the heat treatment and the air permeability, using Examples 1 to 16 in Examples described later as samples. 3 to 5, it can be seen that the air permeability increases in each example until the heat treatment cycle is repeated approximately 10 times, and the magnitude relationship of the air permeability is switched between the examples. On the other hand, after the heat treatment cycle is repeated approximately 10 times, the increase of the air permeability in each example becomes moderate, and it is understood that the magnitude relationship of the air permeability does not change between the examples. .

  This is because, when the magnesia and graphite that have caused the MgO-C reaction disappear and gaps are formed, the contact portion between the remaining magnesia and graphite decreases, so that the MgO-C reaction occurs as the heat treatment cycle is repeated. This is considered to be because it becomes difficult to form a new gap inside the brick. Further, since the amount of expansion of magnesia due to heat is limited, it is considered that expansion of the gap due to thermal expansion and contraction of magnesia converges as the heat treatment cycle is repeated.

Considering the above points, in this embodiment, the air permeability measured after repeating the heat treatment cycle 10 times is used for selection of the MgO-C brick. As shown in the graphs of FIGS. 3 to 5, in each example, the air permeability was a predetermined value (10.0 × 10 ⁻¹⁵ m ² or more in Examples 1 to 6) when the heat treatment cycle was repeated 10 times. ). Also, if the air permeability is substantially constant even if it is less than 10 times depending on the conditions of the sample, use the value of the air permeability when the change in air permeability is stable after repeating the heat treatment cycle twice or more for selection. Also good.

  In this embodiment, by forming the lining structure 2 of the converter 1 using the MgO—C brick selected as suitable for use in Steps S1 and S2 described above with reference to FIG. 2. Even in the converter 1 that performs intermittent operation, peeling of the working surface 2s of the lining structure 2 can be prevented, and good durability can be obtained.

  Next, examples of the present invention will be described. In addition, the Example demonstrated below is only the example of conditions employ | adopted in order to confirm the feasibility and effect of this invention, and this invention is not limited to the conditions of the following Examples.

  The following table | surface shows the composition of the MgO-C brick which concerns on Example 1- Example 16 used as the object of selection in the Example of this invention. Between each example, the particle size composition of magnesia and graphite was kept constant while the mass ratio of magnesia and graphite (magnesia 87 mass%, graphite 13 mass%) and the amount of metallic aluminum added (0.5 mass% on the outer shell) were constant. Is changing. For each example, after determining the suitability for use according to the selection method described above as an embodiment of the present invention, the converter 1 that performs intermittent operation is used for the lining structure 2 and the actual machine test is performed. A wear rate of 2 s was measured. Moreover, about each example, the rotary erosion test which simulated both normal operation (operation which can ignore the temperature fall of the working surface during standby time) and intermittent operation was implemented, and the erosion amount index in each case was calculated.

  In each of Examples 1 to 6, the ratio of coarse magnesia (particle size of 1 mm or more) to magnesia and graphite is 50% by mass, and magnesia of fine particles (particle size of less than 0.075 mm). Is 2 mass% or less. Such a magnesia particle size configuration is a configuration in which there are more coarse particles and fewer fine particles than general densified MgO-brick. According to the knowledge of the present inventors, the larger the coarse-grained magnesia, the easier it is to form a gap between particles due to thermal expansion, and the air permeability increases as this gap becomes a vent hole. Moreover, since the surface area becomes smaller with respect to the volume of magnesia as the amount of fine magnesia is smaller, the generation of CO gas due to the MgO—C reaction is suppressed.

  In addition, in all of Examples 1 to 6, the average particle diameter of graphite exceeds 0.3 mm. The larger the particle size of the graphite, the smaller the surface area with respect to the volume of the graphite. Therefore, the generation of CO gas due to the MgO—C reaction and the reaction with the slag component in the furnace is suppressed.

In the measurement of the air permeability in each example, in accordance with JIS R2115, an MgO—C brick test piece having the same shape as that of the apparent porosity was sealed in a SiC partition box filled with coke powder, Using a reducing atmosphere to heat the partition box to 1600 ° C., hold the partition box at 1600 ° C. for 3 hours, slowly cool the partition box to 500 ° C., and hold the partition box at 500 ° C. for 3 hours The air permeability after repeated slow cooling to room temperature for each cycle was measured 10 times or more. As a result, in all of Examples 1 to 6, the air permeability after the 10th time was 10.0 × 10 ⁻¹⁵ m ² or more. Therefore, the MgO-C brick according to Examples 1 to 6 is selected as suitable for use in step S1 of the selection method according to the embodiment of the present invention described with reference to FIG.

  On the other hand, in the measurement of the apparent porosity in each example, in accordance with JIS A1509-3, the MgO-C brick test piece was formed into a cylindrical shape having a diameter of 50 ± 0.5 mm and a height of 50 ± 0.5 mm. After sealing in a SiC partition wall box filled with coke powder, firing was performed at 1400 ° C. for 30 hours, and after cooling to room temperature, the apparent porosity was measured. As a result, in all of Examples 1 to 6, the apparent porosity was less than 10%. Therefore, the MgO-C brick according to Examples 1 to 6 is selected as suitable for use even in step S2 of the selection method according to the embodiment of the present invention described with reference to FIG.

  On the other hand, Examples 7 to 16 are examples that are not selected as suitable for use in any of the steps of the selection method according to the embodiment of the present invention described with reference to FIG.

Specifically, Examples 7 to 15 are examples in which the apparent porosity after reduction firing is less than 10%, but the air permeability after repeated heat treatment is less than 10.0 × 10 ⁻¹⁵ m ² ( (Not selected in step S1). Speaking of the composition of the MgO-C brick, in Examples 7, 8, and 15, the ratio of magnesia of fine particles (particle size less than 0.075 mm) exceeds 2% by mass. In Examples 9, 13, and 14, the average particle size of graphite is less than 0.3 mm. In Examples 10 to 12, Example 14, and Example 15, the proportion of magnesia of coarse particles (particle size of 1 mm or more) is less than 50% by mass.

On the other hand, Example 16 is an example in which the air permeability after repeated heat treatment exceeds 10.0 × 10 ⁻¹⁵ m ² , but the apparent porosity after reduction firing exceeds 10% (not selected in Step S2). In Example 16, the proportion of magnesia of fine particles exceeds 2% by mass, and the average particle size of graphite is less than 0.3 mm.

  The result of the actual machine test using the converter 1 in each of the above examples will be described. The converter 1 used as an actual machine has a capacity of 300 t and performs intermittent operation. Specifically, in the converter 1, after the molten steel is discharged, the temperature of the working surface 2s is reduced to about 500 ° C. during the standby time until the molten iron is charged next. In the actual machine test, in the converter 1, the initial thickness of the lining structure 2 formed of the MgO-C brick according to each example and the thickness after the 1000-charge (ch) hot metal (ch) were measured (the portion with the greatest wear) were measured. The wear rate (mm / ch) was calculated by dividing the difference by the number of charges.

  According to the observations by the present inventors, in the converter 1 that performs the intermittent operation as described above, the operation surface 2s is peeled off at a rate of once per 6ch on average. Moreover, the thickness of the layer of the working surface 2s which peels at once is about 3 mm on average. From such observation results, it was assumed that the wear rate averaged 0.50 mm / ch (= 3 mm / 6 ch) at the site where the operating surface 2 s was peeled off due to the intermittent operation of the converter 1. On the other hand, the working surface 2s is worn mainly due to melting damage at a portion where the working surface 2s is not peeled, but the wear rate at this portion is 0.25 mm / ch on average.

  According to the results of observation and consideration by the inventors as described above, in Examples 1 to 6, the wear rate was 0.25 mm / ch to 0.34 mm / ch. It is considered that no peeling occurred. On the other hand, in Examples 7 to 16, the wear rate was 0.39 mm / ch to 0.88 mm / ch, and in most cases, it is considered that peeling of the working surface 2s occurred.

  Next, the results of the rotary erosion test will be described. The rotary erosion test was carried out in order to compare the durability of the MgO-C bricks according to each example in the case of normal operation and in the case of intermittent operation. In other words, from the results of the rotary erosion test, whether the MgO-C brick according to each example is suitable for normal operation or intermittent operation, is suitable for normal operation but not suitable for intermittent operation, or is intermittent operation for normal operation. It is possible to judge whether it is not suitable.

The rotary erosion test is a test in which the inner surface of a cylinder having a horizontal rotation axis is lined with MgO-C brick, heated with an oxygen-propane burner, and slag is introduced to erode the brick surface. The slag temperature at the time of charging is 1700 ° C., and the slag is replaced every 30 minutes. The slag composition was CaO / SiO ₂ = 3.2, FeO = 24.8%, MgO = 3.5%, and the amount of erosion was calculated by measuring the size of each brick at the center after the test.

  In the rotary erosion test simulating normal operation, the above test was performed for 5 hours. In the rotary erosion test simulating intermittent operation, the above test was carried out for 5 hours, and then the container was gradually cooled after discharging the slag (about 15 hours), and the operating surface temperature dropped to 500 ° C. After confirming this, the above test was carried out again for 5 hours.

  The result (erosion amount) of each rotary erosion test is represented by an index in which the erosion amount in Example 4 where the wear rate in the actual machine test was the highest among Examples 1 to 6 was 100 (the index was The larger the size, the greater the amount of erosion). Therefore, in Examples 1 to 3, 5 and 6, the index is less than 100. On the other hand, in Examples 7 to 13, the amount of erosion in the rotational erosion test simulating normal operation is similar to that in Examples 1 to 6 (index 85 to 104), but in the rotational erosion test simulating intermittent operation. The amount of erosion is significantly higher than those of Examples 1 to 6 (indexes 115 to 184). Therefore, it can be said that the MgO-C brick according to these examples is suitable for normal operation but not for intermittent operation. In Examples 14 to 16, both the erosion amount (index 128 to 166) in the test simulating normal operation and the erosion amount (index 139 to 194) in the test simulating intermittent operation are the same as in Examples 1 to 6. It is much higher. Therefore, it can be said that the MgO-C brick according to these examples is an example that is not suitable for both normal operation and intermittent operation.

  FIG. 6 is a graph showing the relationship between the apparent porosity after reduction firing and the erosion amount index in the rotary erosion test (normal) in each of the above examples. As shown in FIG. 6, a positive correlation is observed between the apparent porosity after reduction firing and the erosion amount index in the rotary erosion test (normal). Here, in the case of normal operation of the converter 1, the melting loss that is the main cause of wear of the operating surface 2 s occurs in the case of intermittent operation as well (in the intermittent operation, as described above in addition to the melting damage). May occur.) From this result, it can be said that for normal operation, MgO—C brick that effectively suppresses melting of the working surface 2s was selected by selection based on the apparent porosity after reduction firing.

  On the other hand, FIG. 7 is a graph showing the relationship between the apparent porosity after reduction firing in each of the above examples and the erosion amount index in the rotary erosion test (intermittent). As shown in FIG. 7, there is no clear correlation between the apparent porosity after reduction firing and the erosion amount index in the rotary erosion test (intermittent). From this result, for intermittent operation, it is difficult to select an MgO-C brick that effectively suppresses peeling of the working surface 2s only by selection based on the apparent porosity after reduction firing. Recognize.

  Therefore, the present inventors performed selection based on the air permeability after repeated heat treatment. FIG. 8 is a graph showing the relationship between the air permeability after repeated heat treatment and the erosion index in the rotary erosion test (normal) in each of the above examples. However, as shown in FIG. 8, there is no clear correlation between the air permeability after repeated heat treatment and the erosion index in the rotary erosion test (normal).

  On the other hand, FIG. 9 is a graph showing the relationship between the air permeability after repeated heat treatment and the erosion amount index in the rotary erosion test (intermittent) in each of the above examples. As clearly shown in FIG. 9, there is a negative correlation between the air permeability after repeated heat treatment and the erosion amount index in the rotary erosion test (intermittent). Brick with high air permeability is expected to be inferior in corrosion resistance in normal operation. Surprisingly, it was found that in intermittent operation, brick with higher air permeability has better durability. From this result, it can be seen that the air permeability after repeated heat treatment is effective in selecting an MgO-C brick suitable for intermittent operation.

  FIG. 10 is a graph showing the relationship between the air permeability after repeated heat treatment and the apparent porosity after reduction firing in each of the above examples. As can be inferred from FIGS. 6 to 9 described above, as shown in FIG. 10, there is a clear correlation between the air permeability after repeated heat treatment and the apparent porosity after reduction firing. Absent. From this result, for intermittent operation, the conventional selection based on the apparent porosity could not be selected, but the selection based on the apparent porosity after reduction firing and the selection based on the air permeability after repeated heat treatment. It can be said that it was appropriate to implement. Furthermore, it can be said that MgO—C brick suitable for intermittent operation can be selected more accurately by adding selection based on the apparent porosity after reduction firing.

  Based on the above results, the selection method according to the present invention can effectively select an MgO-C brick suitable for a molten metal container (converter 1) that performs intermittent operation, and was selected by the method. It has been confirmed that MgO-C bricks exhibit good durability when used in the lining structure of a molten metal container (converter 1) that performs intermittent operation.

  The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to such examples. It is obvious that a person having ordinary knowledge in the technical field to which the present invention pertains can come up with various changes or modifications within the scope of the technical idea described in the claims. Of course, it is understood that these also belong to the technical scope of the present invention.

  1 ... converter, 2 ... lining structure, 2s ... working surface.

Claims (4)

A method of selecting a MgO-C brick used for a lining structure of a molten metal container that performs intermittent operation,
A step of heating a test piece of MgO-C brick to 1600 ° C, a step of holding the test piece at 1600 ° C for 3 hours, a step of gradually cooling the test piece to 500 ° C, and a step of heating the test piece at 500 ° C for 3 hours A step of selecting a MgO-C brick in which the air permeability of the test piece is 10.0 × 10 ⁻¹⁵ m ² or more after repeating the holding step 10 times in a reducing atmosphere;
Selecting the MgO-C brick based on the apparent porosity after reduction firing, and selecting the MgO-C brick.   In the step of selecting MgO-C brick based on the apparent porosity after reduction firing, MgO-C brick having an apparent porosity of less than 10% after reduction firing at 1400 ° C for 30 hours is selected. The selection method of MgO-C brick of description. A method for operating a molten metal container having a lining structure formed of MgO-C brick,
Including a step of performing intermittent operation using the molten metal container,
In the MgO-C brick, the step of heating to 1600 ° C., the step of holding at 1600 ° C. for 3 hours, the step of gradually cooling to 500 ° C., and the step of holding at 500 ° C. for 3 hours were repeated 10 times in a reducing atmosphere. A method for operating a molten metal container, wherein the subsequent air permeability is 10.0 × 10 ⁻¹⁵ m ² or more and the apparent porosity after reduction baking at 1400 ° C. for 30 hours is less than 10%. It is a lining structure formed of MgO-C brick of a molten metal container that performs intermittent operation,
In the MgO-C brick, the step of heating to 1600 ° C., the step of holding at 1600 ° C. for 3 hours, the step of gradually cooling to 500 ° C., and the step of holding at 500 ° C. for 3 hours were repeated 10 times in a reducing atmosphere. A lining structure of a molten metal container having a subsequent air permeability of 10.0 × 10 ⁻¹⁵ m ² or more and an apparent porosity of less than 10% after reduction firing at 1400 ° C. for 30 hours.

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