JP7720197B2 - Evaluation method for magnesia carbon bricks - Google Patents

Description

The present invention relates to a method for evaluating magnesia-carbon bricks.

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.

As such, the charging wall of a converter is not only exposed to molten steel and molten slag like other sidewall parts during blowing, but is also subjected to mechanical impacts from scrap charging, which causes rapid damage and often determines the furnace's lifespan. For this reason, there is a demand for magnesia-carbon bricks with higher durability. However, because there is no established method for evaluating the durability of magnesia-carbon bricks, including their resistance to mechanical impacts from scrap charging, there are no guidelines for developing magnesia-carbon bricks with higher durability, and development is currently proving difficult.

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

Special Publication No. 62-9553

The problem that this invention aims to solve is to provide a new evaluation method for evaluating the durability of magnesia carbon bricks, including their impact resistance to mechanical impact caused by the addition of scrap.

The present inventors conducted detailed observations of magnesia-carbon bricks used in the charging wall of a converter and found indentations (dents) on the working surface of the magnesia-carbon brick, which were believed to be caused by mechanical impacts due to the charging of scrap, and cracks extending from the dents 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 impacts caused by the charging of scrap. Therefore, the present inventors conducted impact tests simulating the mechanical impacts caused by the charging of scrap, focusing on the length of the cracks extending from the dents into the brick. They found a correlation between the length of the cracks and the durability of the bricks in an actual furnace. In other words, the longer the cracks, the more likely the working surface of the bricks is to spall off, either due to the crack itself or due to further crack propagation 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.

The present invention is based on the new findings of the present inventors, and specifically, is a method for evaluating magnesia carbon bricks obtained through a uniaxial pressing process, characterized in that in an XYZ Cartesian coordinate system, the brick to be evaluated is positioned so that the pressure direction during uniaxial pressing 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, and a drop test is carried out so that the tip of the weight hits the center of the brick surface, and the length of a crack that progresses into the brick from the penetration mark formed on the brick surface is measured.

The present invention makes it possible to evaluate the durability of magnesia-carbon bricks, including their resistance to mechanical shock caused by the addition of scrap. This provides a guideline for developing highly durable magnesia-carbon bricks. Specifically, by using the reduction of the crack length obtained using the evaluation method of the present invention as one of the guidelines, it is possible to develop highly durable magnesia-carbon bricks.

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

In this invention, a test was conducted to simulate 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 Figure 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 1,000 m. During manufacturing, the brick is uniaxially pressed so that the circumferential side surfaces 33 and 34 serve as the pressing surfaces. This magnesia-carbon brick also contains 15% by mass of scaly (flake) graphite. As shown in Figure 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 orient 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.

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 was obtained by the same manufacturing process as that of magnesia carbon bricks generally used for the charging wall of a converter or the lining of an electric furnace. That is, the brick was 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.

In evaluating 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 therein.
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 face with the smallest area of the rectangular brick was selected as the evaluation target face. This is because the crack will extend in the direction in which the weight falls, so it is important to ensure a sufficient length in this direction.

There are no particular restrictions on the shape of the bricks to be evaluated in this evaluation method; they can be any shape, including drumstick-shaped, rectangular, cubic, and cylindrical. Furthermore, the bricks to be evaluated can be either cut from larger bricks or uncut. However, it is best to standardize the shape of the bricks to be evaluated for comparison, so that the effect of shape on crack propagation is the same. Furthermore, as the size of the bricks to be evaluated increases, the evaluation equipment also increases. To ensure that the evaluation equipment is of a practical size, the bricks to be evaluated can be rectangular, with dimensions of 50 to 200 mm in both length and width and 150 to 500 mm in length.

In this embodiment, the pressure direction P during uniaxial press molding of the brick being 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 press molding is the XY plane and XZ plane in the XYZ Cartesian coordinate system.

When the weight 2 is dropped onto the 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 in 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.

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 Figure 3. The length of the crack 14 can also be measured on a cut plane other than the XZ cut plane 13. However, for the following two reasons, it is preferable to measure the length on a cut plane that intersects with the YZ plane passing through the deepest point of the penetration mark 12 and passes through the deepest point of the penetration mark 12 (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 interior 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 pressure direction P (X-axis direction) during uniaxial pressing. Therefore, the crack 14 tends to propagate along the YZ plane, which is perpendicular to the pressure direction P (X-axis direction) during uniaxial pressing.

In this embodiment, the direction in which the weight 12 is dropped is the Z-axis direction in the XYZ Cartesian coordinate system, i.e., the vertical direction, as shown in Figure 1. The height at which the weight 12 is dropped, as well as the weight and shape of the weight 12, can be appropriately determined so that the mechanical impact caused by the dropped 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 magnesia carbon bricks, and cracks are observed extending from these penetration marks into the bricks. 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.

As described above, magnesia carbon bricks are obtained by heat treatment after the uniaxial pressing process. However, when evaluating the bricks, it is preferable to perform a pretreatment 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 the weight 12 is dropped 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 may be placed on each side of the magnesia carbon brick 1 and clamped to hold the brick in place.

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, as measured in the manner shown in Figure 1, as well as the results of evaluating the amount of wear in an actual furnace (the charging wall of a converter) when each brick was used. Here, the depth of the penetration mark refers to the linear distance extending vertically from the surface of the brick being evaluated to the deepest point of the penetration mark, and the length of the crack refers to the linear distance from the deepest point of the penetration mark to the tip of the crack. For example, in Figure 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.

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. 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 then 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.

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 1,400°C for 10 hours and then subjecting them to a 5-hour test at 1,700°C in a rotary corrosion tester. The corrosion material used was slag with a basicity of 3.4 and a total iron content of 18%. The corrosion resistance index was calculated by indexing the reduction in size (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 1,400°C for 10 hours was immersed in hot metal at 1,600°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.

The wear rate in an actual furnace was calculated 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 to determine the wear rate per charge (mm·ch ⁻¹ ).

The drop test was conducted 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. Prior to the drop test, the magnesia carbon brick 1 was subjected to reduction firing at 1400°C for 10 hours as a pre-treatment.

The bricks of Examples 1 to 4 used in this test did not show any 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 using the brick evaluation method of the present invention correlates with the brick's durability in an actual furnace. Specifically, the smaller the crack length obtained using the brick evaluation method of the present invention, the less likely the working surface will spall, improving the brick's durability in an actual furnace. This suggests that by using the reduction in the crack length obtained using the evaluation method of the present invention as one of the guidelines, it is possible to develop highly durable magnesia-carbon bricks.

Furthermore, in Examples 3 and 4, the crack lengths were roughly the same and there was a large difference in the depth of the penetration marks, but there was a clear difference in the rate of wear 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 the molten iron falls, and the depth of the penetration marks is thought to correlate with this wear resistance.

As described above, the evaluation method of the present invention is effective as an evaluation method for magnesia carbon bricks, which are subjected to mechanical shocks caused by the charging of scrap during use in actual furnaces.

1. Magnesia carbon bricks (bricks to be evaluated)
11. Brick surface (surface to be evaluated)
12 Penetration mark 13 XZ cross section (cross section)
14 Crack 2 Weight 3 Magnesia carbon brick for converter 31 Working surface 32 Upper surface 33 Circumferential side surface 34 Circumferential side surface 35 Lower surface 36 Rear surface

Claims (4)

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. The method for evaluating magnesia carbon bricks according to 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 cut surface that intersects with a YZ plane that passes through the deepest point of the penetration mark and also passes through the deepest point of the penetration mark. 3. The method for evaluating magnesia carbon bricks according to claim 1, wherein a cylindrical guide is used when dropping the weight. The method for evaluating magnesia carbon bricks according to any one of claims 1 to 3 , further comprising measuring the depth of the penetration marks formed on the brick surface.