Cause Analysis and Solution of Damage of Magnesium Chromium Brick Circulating Pipe in RH Vacuum Furnace

1 Preface

The circulating pipe is the most important part of the vacuum chamber of RH furnace. Due to the severe impact of high-speed circulating molten steel, repeated temperature changes, and the particularity of the structure, the refractory of the vacuum chamber circulating pipe becomes the weakest part of the whole RH furnace. The service life of the vacuum chamber is often limited by the circulating pipe.

2. Damage and mechanism analysis of circulating pipe

The steel plant has several RH furnace production lines and different continuous casting machines. In addition, the set capacity of RH furnace of other production lines is higher than 100 t. However, the capacity of the 100 t RH furnace currently used in the steel plant is insufficient. Intermittent production mode is adopted, with a long interval between heats. The top gun of the production intermittent vacuum chamber cannot be effectively baked due to mechanical failure, resulting in large temperature fluctuations in the furnace. This production line uses mechanical pump to vacuum, and the limit vacuum degree is 36 Pa. In order to save the equipment reaction time, the mechanical pump will pre pump the pressure to 30 kPa before processing the molten steel. At the beginning of the treatment, the molten steel rises rapidly and reacts violently under the pre pumping pressure, and the high-temperature molten steel has a severe impact on the inner surface of the circulating pipe. Through tracking the use of the circulating pipe, it is found that the contact part of the first ring brick of the circulating pipe and the immersion pipe is peeled off and melted abnormally to form a circular groove, and the groove gradually expands and deepens during use, as shown in Fig. 1 (a) and (b).

Fig. 1 Service condition of circulating pipe groove

Under normal conditions, the damage of magnesia chrome bricks in circulating pipes is divided into structural peeling and erosion, and the damage mechanism is listed in Table 1.

Table 1 Damage Mechanism of Circulating Pipe

Table 1 Damage Mechanism of Circulating Pipe

Disassemble the used circulating pipe and slice the normal part and groove part, as shown in Fig. 2 (a) and (b). The residual bricks in the two parts are divided into slag layer, permeable layer and original brick layer.

Figure 2 Sectional View of Residual Bricks in Normal and Grooves

The apparent porosity of magnesia chrome brick is large, and the molten slag penetrates into the magnesia chrome brick matrix, which will form a high melting point composite spinel phase during the reaction process to make the slag sticky, but it cannot form a whole and effectively prevent the continuous penetration of the slag [1]. With the penetration of slag, the brick forms slag layer and penetration layer, and the surface of slag layer is uneven, showing dark brown; The thickness of the permeable layer is uniform, showing a greenish gray color. The thickness of slag layer, penetration layer, original brick layer and residual brick shall be measured after 70 heats are used for normal parts of circulating pipe. The data are listed in Table 2. The damage rate of magnesia chrome bricks at normal parts of circulating pipes is 0.714 mm/furnace.

Table 2 Damage Data of Normal Parts of Circulating Pipe

It can be seen from Fig. 2 (b) that there are obvious cracks parallel to the heating surface between the slag layer, the permeability layer and the original brick layer, and the cracks tend to flake off. There is a certain density difference between the penetration layer formed by slag erosion to the interior of the brick and the original brick layer; When the temperature of RH furnace changes, due to the difference in the expansion coefficient between the penetration layer and the original brick layer, a certain volume change occurs inside the magnesia chrome brick; In addition, the valence state of iron permeating into the magnesia chrome brick changes with temperature [2]. The magnesia chrome brick eroded by steel slag expands differently when heated, which makes the slag layer, the penetration layer and the original brick layer have transverse cracks, thus causing the structure of the magnesia chrome brick to peel off. The thickness of slag layer, penetration layer, original brick layer and residual brick at this position are measured respectively, and the data are listed in Table 3. The damage rate of magnesia chrome bricks in the circulating pipe at this position is 1.714 mm/furnace, which is obviously higher than that of the normal position.

Table 3 Damage Data of Circulating Pipe Groove

3. Cause analysis of circulating pipe damage 3.1 Material cause

After disassembling the off-line circulating pipe, it was found that the magnesia chrome brick of the same circulating pipe only had abnormal melting loss near the mating surface with the immersion pipe, and other parts were normal, which indicated that the magnesia chrome brick had no quality problem, and it was necessary to find the cause from the structure.

3.2 The top gun cannot be baked when the heat is intermittent

This RH furnace production line has insufficient capacity, and the intermittent time between heats is long. The RH furnace top gun runs up and down frequently, and the production will be interrupted due to gun jamming. Therefore, gun baking is not selected among the current heats, but this will lead to a rapid drop in the furnace temperature, a rapid rise in the furnace temperature after the start of the next furnace smelting, and large fluctuations in the temperature. The boundary between the penetration layer and the original brick layer will produce great stress, which will cause cracks parallel to the hot surface, thus causing material cracking and peeling [3].

3.3 Floating of circulating pipe bricks and sinking of impregnated pipe bricks

The circulating pipe brick floats mainly due to the following three reasons.

(1) The density of circulating pipe bricks is 3.3 g/cm3, and the density of liquid steel is 7 g/cm3. The density difference is large. The circulating pipe bricks are buoyant by liquid steel, and have a tendency to float.

(2) When laying the circulating pipe and the lower groove, the reserved gap between the first ring brick and the refractory brick at the bottom of the vacuum chamber is small. When the refractory brick is heated and expanded, a large extrusion stress will be generated between the bottom slot brick and the first ring brick and it will be in an unstable state. Even if it is subject to a small upward external force, it is easy to induce the middle part to rise upward, which is shown as the first ring brick floats up [4].

(3) The high-speed upward flowing molten steel will generate upward friction on the riser circulating pipe brick, which will cause the riser circulating pipe brick to float.

The sinking of the impregnated pipe brick is mainly caused by the defects in the assembly process of the impregnated pipe, the lack of brick support plate or the defects in the pouring process of the castable between the brick and the steel liner, and the extrusion stress generated during the use process.

After disassembling the impregnating pipe and circulating pipe on site, no floating or sinking of the circulating pipe brick was found. After use, the upper surface of the impregnating pipe brick remained level with the flange surface, and no delamination was found inside the circulating pipe brick, as shown in Figure 3 and Figure 4.

Fig. 3 Upper Surface of Immersion Pipe

Fig. 4 Internal Conditions of Circulating Pipe

3.4 Thermal expansion

Make a sample of this batch of magnesia chrome bricks and measure its linear expansion rate from room temperature to 1500 ℃, as shown in Figure 5. It can be seen from the figure that the linear expansion rate of magnesia chrome brick increases with the increase of temperature, reaching 1.69% at 1450 ℃.

Fig. 5 Linear expansion rate of magnesia chrome brick

The expansion of magnesia chrome brick at high temperature produces extrusion stress, which causes the extrusion of the contact surface between the circulating pipe and the immersion pipe, and cracks at the corners of the circulating pipe brick. Due to the erosion and peeling of molten steel, the grooves gradually develop during the alternate use of cold and hot, leading to their early damage.

4 Methods to solve the groove defect of circulating pipe

(1) Reasonably allocate production, improve the use efficiency of RH furnace, increase refining furnace times, shorten the intermittent time, and take measures to lower the top gun during the intermittent period to reduce the temperature fluctuation in the furnace.

(2) Monitor the melting loss status of the groove of the circulating pipe, and conduct spray repair during the shutdown period between two heats to slow down the development of the groove.

(3) During the construction of circulating pipe and lower groove, reasonable expansion joints shall be left locally to slow down the local peeling caused by the expansion and extrusion of magnesia chrome bricks. The specific measures are as follows: ① Fill 2-3 mm chrome corundum fireclay between the circulating pipe and the impregnating pipe; ② Fill 5-7 mm magnesia chrome ramming material between the circulating pipe and the first layer of the working layer; ③ The working layer below the rib plate is changed from dry masonry to wet masonry; ④ The gap between the rib plate and the working layer is filled with fiber cotton instead of ramming material.

By adopting the above methods, the circulation pipe will not have grooves during use, and the service life of each set of vacuum chamber will be increased from 70 heats to about 100 heats, extending the service life of refractory materials and reducing the production cost.

5 Conclusion

The damage reason and mechanism of circulating pipe of RH furnace are analyzed, and the method to solve the groove defect of circulating pipe is found. The successful solution to this problem has extended the service life of the vacuum chamber, reduced the production cost and created economic benefits for the enterprise.