Evaluation method for magnesia carbon bricks

Abstract

To provide a novel evaluation method for assessing the serviceability of magnesia carbon bricks, including the impact resistance to mechanical impact due to scrap loading.SOLUTION: An evaluation method of this invention includes steps of: dropping a weight 2 to a brick surface 11 parallel to a pressing direction P during uniaxial pressing; and measuring the length of a crack extending from a penetration mark 12 formed on the brick surface 11 to the inside of the brick.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for evaluating magnesia carbon bricks. [Background technology]

[0002] Magnesia carbon bricks are widely used as refractory linings for steelmaking furnaces such as converters and electric furnaces due to their excellent corrosion resistance and thermal shock resistance. In converter operation, a typical application of magnesia carbon bricks, the converter is tilted to one side and scrap and molten pig iron are charged into the converter, and oxygen is blown into it to decarburize and adjust the composition, turning the molten pig iron into molten steel. Once blowing is complete, the converter is tilted to the other side and the molten steel is discharged into a ladle through a tap hole. When charging scrap, the converter is tilted and the scrap is placed into a trough-shaped scrap chute, which is then tilted so that it slides down into the converter. When molten pig iron is received into the converter, the converter is tilted in the same way and the molten pig iron in the molten pig iron ladle is poured into the converter through a throat at the top of the molten pig iron ladle. When scrap and molten iron are charged into a converter, the area where the scrap collides and the molten iron is poured is subjected to mechanical shocks and high thermal loads from the charging of scrap and molten iron, and as such exhibits unique damage patterns compared to other areas, and is therefore called the charging wall.

[0003] Thus, the charging wall of a converter is not only exposed to molten steel and molten slag like other side wall parts during blowing, but is also subjected to mechanical shocks caused by the charging of scrap, which causes rapid damage and often determines the life of the furnace. For this reason, magnesia-carbon bricks with higher durability are desired, but no method has been established to evaluate the durability of magnesia-carbon bricks, including their resistance to mechanical shocks caused by the charging of scrap, and therefore no guidelines have been established for developing magnesia-carbon bricks with higher durability, making the development of such bricks difficult.

[0004] Patent Document 1 describes a method for evaluating the impact resistance of magnesia carbon bricks, in which an inverted cone-shaped weight of 850 g is dropped from a position 1,150 mm above the surface of the magnesia carbon brick, and the weight is measured after 20 repeated drops. However, simply measuring the volume of damage does not allow evaluation of the durability of the magnesia carbon brick, including its impact resistance to mechanical shocks caused by charging scrap. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Special Publication No. 62-9553 Summary of the Invention [Problem to be solved by the invention]

[0006] An object of the present invention is to provide a new evaluation method for evaluating the durability of magnesia-carbon bricks, including the impact resistance to mechanical impact caused by the insertion of scrap. [Means for solving the problem]

[0007] The inventors of the present invention conducted detailed observations of magnesia-carbon bricks used in the charging wall of a converter and found that the working surface of the magnesia-carbon brick had indentations (dents) that were thought to be caused by the mechanical impact of scrap charging, and that cracks extended from the indentations into the brick. Furthermore, because the cracks contained bare metal, it was inferred that the cracks had developed during operation, specifically, due to the mechanical impact of scrap charging. Therefore, the inventors conducted impact tests simulating the mechanical impact of scrap charging, focusing on the length of the cracks that extended from the indentations into the brick. They found a correlation between the length of the cracks and the durability of the brick in an actual furnace. In other words, the longer the crack, the more likely the working surface of the brick would spall, either due to the crack itself or due to its further expansion during use. The present inventors have also found that the depth of the penetration mark is correlated with durability. That is, it is considered that the deeper the penetration mark, the greater the chipping of the brick surface when scrap collides with the brick or the greater the wear when molten iron hits the brick.

[0008] The present invention is based on the new findings of the present inventors, and specifically, a method for evaluating magnesia carbon bricks obtained through a uniaxial pressing process. And , In an XYZ orthogonal coordinate system, the evaluation target brick is arranged so that the pressure direction during uniaxial press molding of the evaluation target brick is the X-axis direction, the brick surface onto which a weight with a sharp tip is dropped is the XY plane, and the dropping direction of the weight is the Z-axis direction, and a drop test is carried out so that the tip of the weight hits the center of the brick surface, It is characterized by measuring the length of a crack that propagates from a penetration mark formed on the brick surface into the brick interior. [Effects of the Invention]

[0009] According to the present invention, it is possible to evaluate the durability of magnesia-carbon bricks, including the impact resistance to mechanical shocks caused by charging scrap. This makes it possible to obtain guidelines for developing magnesia-carbon bricks with high durability. Specifically, by using the reduction of the length of cracks obtained by the evaluation method of the present invention as one of the guidelines, it is possible to develop magnesia-carbon bricks with high durability. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a conceptual diagram of a method for evaluating magnesia carbon bricks according to one embodiment of the present invention. [Figure 2] Photograph showing an example of a penetration mark formed on a brick surface. [Figure 3] A photograph showing an XZ cross section passing through the deepest point of the penetration mark in Figure 2. [Figure 4] A perspective view of a magnesia carbon brick for a converter. [Figure 5] An enlarged perspective view of a scaly graphite particle. DETAILED DESCRIPTION OF THE INVENTION

[0011] In the present invention, a test was conducted simulating internal cracks that would occur when scrap collided with the working surface 31 of a magnesia-carbon brick 3 used in the charging wall of a converter, as shown in FIG. 4 . This magnesia-carbon brick 3 is a long, narrow, dovetail-shaped brick with a working surface width W1 of 150 mm, a back surface width W2 of 170 mm, a working surface and back surface height H of 150 mm each, and a length L of 1000 mm. During manufacturing, the brick was uniaxially pressed so that its circumferential side surfaces 33 and 34 served as the pressing surfaces. This magnesia-carbon brick also contained 15% by mass of scaly (flake) graphite. As shown in FIG. 5 , the scaly graphite 4 has a very thin, flake-like particle shape. Therefore, during uniaxial pressing, the largest surface 41 of the scaly graphite tends to be oriented perpendicular to the pressing direction P. Therefore, cracks originating from the working surface 31 tend to propagate into the brick along a plane perpendicular to the pressing direction P.

[0012] FIG. 1 conceptually shows a method for evaluating magnesia carbon bricks, which is one embodiment of the present invention. The magnesia carbon brick 1 to be evaluated is obtained by the same manufacturing process as the magnesia carbon bricks generally used for the charging walls of converters and the linings of electric furnaces. That is, the bricks are obtained by adding an organic binder to a raw material mixture containing magnesia and scaly graphite as the main raw materials, kneading the mixture, pressing it in one direction to form it (uniaxial pressing process), and then heat treating it.

[0013] To evaluate the magnesia carbon brick 1 obtained through the uniaxial pressing process in this way, a weight 2 is dropped onto the brick surface 11 parallel to the pressing direction P during uniaxial pressing, as shown in Fig. 1. The reason why the brick surface 11 parallel to the pressing direction P during uniaxial pressing is used as the evaluation target surface is that, as described above, in an actual furnace, the brick surface 11 parallel to the pressing direction P during uniaxial pressing becomes the working surface and that scaly graphite (flake graphite) is oriented thereon. In Figure 1, there are four brick faces parallel to the pressure direction P during uniaxial pressing, but any face can be selected. In Figure 1, the side with the smallest area of the rectangular brick was selected as the evaluation target face. The reason for this is that cracks extend in the direction in which the weight falls, so it is important to ensure a sufficient length in this direction.

[0014] In this evaluation method, there are no particular restrictions on the shape of the brick to be evaluated; it can be any shape, such as a drumstick, rectangular parallelepiped, cube, or cylinder. Furthermore, the brick to be evaluated can be either cut from a larger brick or uncut. However, it is best to standardize the shape of the bricks to be evaluated for comparison purposes to ensure that the effect of shape on crack propagation is the same. Furthermore, since the larger the brick to be evaluated, the larger the evaluation device must be. To ensure that the evaluation device is of a practical size, the brick to be evaluated can be a rectangular parallelepiped with a length and width of 50 to 200 mm and a length of 150 to 500 mm.

[0015] In this embodiment, the pressure direction P during uniaxial pressing of the brick to be evaluated is the X-axis direction in the XYZ Cartesian coordinate system, as shown in Figure 1, and the brick surface 11 parallel to the pressure direction P during uniaxial pressing is the XY plane and XZ plane in the XYZ Cartesian coordinate system.

[0016] When a weight 2 is dropped onto a brick surface 11, the tip of the weight 2 penetrates the brick surface 11, forming a penetration mark 12. Figure 2 shows a photograph of an example of an actual penetration mark 12, and Figure 3 shows a photograph of an XZ cross section 13 (see Figure 1) passing through the deepest point of this penetration mark 12. As can be seen from Figure 3, there is a crack 14 that propagates from near the deepest point of the penetration mark 12 formed on the brick surface 11 toward the interior of the brick.

[0017] In this embodiment, the depth of the penetration mark 12 and the length of the crack 14 are measured on an XZ cut plane 13 passing through the deepest point of the penetration mark 12, which corresponds to FIG. 3 . Although the length of the crack 14 can be measured on a cut plane other than the XZ cut plane 13, it is preferable to measure the length of the crack 14 on a cut plane that intersects with a YZ plane passing through the deepest point of the penetration mark 12 and passes through the deepest point of the penetration mark 12 for the following two reasons (the XZ cut plane 13 in this embodiment is an example). First, the crack 14 starts near the deepest point of the penetration mark 12 and propagates toward the inside of the brick. Second, in the magnesia carbon brick 1 obtained through the uniaxial pressing process, the planes of the scaly graphite are aligned in a direction perpendicular to the pressing direction P (X-axis direction) during uniaxial pressing. Therefore, the crack 14 tends to propagate along the YZ plane, which is perpendicular to the pressing direction P (X-axis direction) during uniaxial pressing.

[0018] In this embodiment, the direction in which the weight 12 is dropped is the Z-axis direction in an XYZ Cartesian coordinate system, i.e., the vertical direction, as shown in FIG. 1 . The height at which the weight 12 is dropped and the weight and shape of the weight 12 can be appropriately determined so that the mechanical impact caused by the dropping of the weight 12 is equivalent to the mechanical impact caused by the charging of scrap in an actual furnace. In other words, in an actual furnace, penetration marks are observed on the working surface of a magnesia carbon brick, and cracks extending from these penetration marks into the brick are observed. Therefore, the shape of the brick to be evaluated, the height at which the weight 12 is dropped, and the weight and shape of the weight 12 can be appropriately determined so that a similar phenomenon can be reproduced. In this embodiment, the weight 12 has a pointed tip. In this case, the tip of the weight 12 can typically be conical. When dropping a weight with a pointed tip, a cylindrical guide can be used to accurately impact the tip with the center of the brick to be evaluated.

[0019] As described above, magnesia carbon bricks are obtained by heat treatment after the uniaxial pressing process, but when evaluating them, it is preferable to perform a pre-treatment of reduction firing at about 1400°C, taking into consideration the conditions of use in an actual furnace. The firing time is not particularly limited, but about 10 hours is sufficient. When dropping the weight 12 onto the brick surface 11 of the magnesia carbon brick 1, the magnesia carbon brick 1 may be restrained by a restraining jig on its periphery, taking into consideration the conditions of use in an actual furnace. For example, metal plates can be placed on each side and clamped to restrain the brick. [Example]

[0020] Table 1 shows the results of measuring the depth of the penetration marks formed on the brick surface and the length of the cracks that propagated from the penetration marks into the brick for the magnesia carbon bricks (hereinafter simply referred to as "bricks") of Examples 1 to 4 in the manner shown in FIG. 1, as well as the results of evaluating the amount of wear in an actual furnace (the charging wall of a converter) for each brick. Here, the penetration mark depth refers to the linear distance extending vertically from the surface of the brick to be evaluated to the deepest point of the penetration mark, and the crack length refers to the linear distance from the deepest point of the penetration mark to the tip of the crack. For example, in FIG. 3, the crack length is the linear distance from point A (the deepest point of the penetration mark 12) to point B (the tip of the crack 14). Table 1 also shows the physical properties of each brick.

[0021] The bricks of Examples 1 to 3 were obtained by adding an organic binder to the raw material composition shown in Table 1, kneading the mixture, uniaxially pressing the mixture, and heat-treating it at 250°C, while the brick of Example 4 was obtained by adding an organic binder to the raw material composition shown in Table 1, kneading the mixture, uniaxially pressing the mixture, heat-treating it at 1000°C, and impregnating it with tar. These bricks are rectangular parallelepipeds measuring 100 x 100 x 230 mm, and were uniaxially pressed in the pressing direction P shown in Figure 1.

[0022] The evaluation items and evaluation methods for the physical properties of each brick are as follows: The corrosion resistance of the samples was evaluated by reducing and firing them at 1400°C for 10 hours and then testing them at 1700°C for 5 hours in a rotary corrosion tester. The corrosion material used was slag with a basicity of 3.4 and T.Fe=18%. The corrosion resistance index was calculated by indexing the dimensional loss (mm) before and after the test, with the brick of Example 1 set at 100. The thermal shock resistance was evaluated by a hot metal immersion spalling test. A sample that had been reduced and fired at 1400°C for 10 hours was immersed in hot metal at 1600°C for 90 seconds, followed by water cooling for 30 seconds. This process was repeated three times. The thermal shock resistance index was calculated by indexing the retention rate of the dynamic modulus of elasticity before and after the test, with the brick of Example 1 set at 100. The retention rate of the dynamic modulus of elasticity was calculated by (dynamic modulus before the test ÷ dynamic modulus after the test) × 100.

[0023] The wear amount in an actual furnace was measured by lining the charging wall of an actual converter with the bricks of Examples 1 to 5, measuring the average remaining dimensions of the bricks after use, and calculating the wear size. The wear rate per charge (mm·ch -1 ) was sought.

[0024] The drop test was carried out as shown in Figure 1. Specifically, a 10 kg weight 12 with a conical tip (with a 90-degree apex angle) was dropped from a height of 1.5 m onto a brick surface 11 parallel to the pressure direction during uniaxial pressing. A cylindrical pipe was used as a guide. The depth of the penetration mark 12 formed on the brick surface 11 and the length of the crack 14 propagating into the brick were measured with a vernier caliper on an XZ cross section 13 passing through the deepest point of the penetration mark 12. Before the drop test, the magnesia carbon brick 1 was subjected to reduction firing at 1400°C for 10 hours as a pre-treatment.

[0025] [Table 1]

[0026] The bricks of Examples 1 to 4 used in this test did not show significant differences in corrosion resistance and thermal shock resistance that would significantly affect the amount of wear in an actual furnace. However, as shown in Table 1, significant differences were observed in the crack lengths measured in the drop test. As shown in Table 1, the smaller the crack length, the smaller the amount of wear in an actual furnace. This indicates that the crack length obtained by the brick evaluation method of the present invention correlates with the durability of the brick in an actual furnace. Specifically, the smaller the crack length obtained by the brick evaluation method of the present invention, the less likely the working surface will spall, improving the durability of the brick in an actual furnace. This suggests that by using the reduction in the crack length obtained by the evaluation method of the present invention as a guideline, it is possible to develop magnesia-carbon bricks with high durability.

[0027] Furthermore, in Examples 3 and 4, the crack lengths are almost the same and there is a large difference in the depth of the penetration marks, but there is a clear difference in the wear rate in an actual furnace. In an actual furnace, it is thought that wear occurs when the surface is scraped off when scrap collides with the brick surface, and when molten pig iron falls, and it is thought that the depth of the penetration marks is correlated with this wear resistance.

[0028] As described above, the evaluation method of the present invention is effective as a method for evaluating magnesia carbon bricks that are subjected to mechanical shocks due to the charging of scrap when used in an actual furnace. [Explanation of symbols]

[0029] 1. Magnesia carbon bricks (bricks to be evaluated) 11 Brick surface (surface to be evaluated) 12 Penetration marks 13 XZ cutting surface (cutting surface) 14 Crack 2 weight 3. Magnesia carbon bricks for converters 31 Operating surface 32 Top side 33 Circumferential side 34 Circumferential side 35 Bottom side 36 Back

Claims

1. A method for evaluating magnesia carbon bricks obtained through a uniaxial pressing process, comprising the steps of: A method for evaluating magnesia carbon bricks, comprising: placing a brick to be evaluated in an XYZ Cartesian coordinate system so that the pressure direction during uniaxial press molding of the brick to be evaluated is the X-axis direction, the brick surface onto which a weight with a sharp tip is dropped is the XY plane, and the dropping direction of the weight is the Z-axis direction; conducting a drop test by impacting the tip of the weight against the center of the brick surface; and measuring the length of a crack that progresses from the penetration mark formed on the brick surface into the brick.

2. A method for evaluating magnesia carbon bricks as described in claim 1, wherein the length of a crack that propagates from the penetration mark formed on the brick surface into the brick is measured on a cross section that intersects with the YZ plane passing through the deepest point of the penetration mark and also passes through the deepest point of the penetration mark.

3. A method for evaluating magnesia carbon bricks as described in claim 1 or claim 2, in which a cylindrical guide is used when dropping the weight.

4. A method for evaluating magnesia carbon bricks described in any one of claims 1 to 3, which measures the depth of penetration marks formed on the brick surface.

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