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With the increasing demand for steel quality by users, higher requirements are placed on the purity of steel. It is generally believed that oxygen, sulfur, phosphorus, hydrogen, nitrogen and non-metallic inclusions are the main factors affecting the purity of steel. There is a direct relationship between the refractory of steelmaking and the purity of steel.
Flow steel bricks are usually used in the bottom casting steel casting method. Because they are in direct contact with the molten metal and are subjected to scouring and chemical action, which seriously affects the quality of the casting, it is necessary to conduct in-depth research. In this paper, the surface morphology, elemental composition and reaction layer thickness of high alumina brick and mullite flow brick for cast steel reacted with molten metal were studied. The reasons affecting the quality of molten steel were analyzed.
1 Interaction between flow brick and molten steel
The reasons that affect the purity of steel are mainly divided into two categories, one is due to the steelmaking operation process, such as melting and deoxidation process; the second is caused by refractory materials for steelmaking, such as flow steel bricks, nozzle bricks, steel bricks, etc. .
Flow steel bricks are usually used in the bottom casting steel casting method. When the molten steel flows from the steel water tank to the steel ingot mold, it must pass through the flow steel brick. Due to the high temperature of the molten steel, the convection steel brick will have an impact effect, thereby causing the flow. The steel brick structure partially falls off, and the molten steel also interacts with the flow steel brick, and the reaction product is mixed into the molten steel, which affects the quality of the steel ingot.
2 test and performance test
The test process was carried out at the production site: the molten steel was preheated to 1527 ° C for casting, the casting time was 8 min, the diameter of the soup channel was 60 mm, and the flow steel brick of 10 mm × 10 mm × 10 mm was cut out from the surface of the soup channel after the casting was completed. Microstructure, surface elements and line scan analysis of flow bricks of different materials were carried out by SEM.
The physical and chemical properties of the high alumina and mullite flow steel bricks used in the test are shown in Table 1.
Results and Discussion Figures 1 and 2 are SEM images of high alumina brick and mullite brick after reaction with molten steel. It can be seen from the figure that the surface of the high-alumina brick is large and the structure is relatively loose. The surface of the mullite brick is fine, the structure is dense, and the surface has needle-like crystals, which is a network cross structure. This dense structure can hinder the further erosion of molten steel convection steel bricks. Therefore, in the process of flowing molten steel through the flow steel brick, the molten steel is more likely to react with the high alumina brick refractory material.
The surface of the flow brick was subjected to elemental analysis using an electron probe of SEM. Figure 3 and Figure 4 show the energy spectrum analysis of the surface of high alumina brick and mullite brick, respectively. Table 2 shows the element content of the surface. It can be seen that after reacting with molten steel, the Al element content of the high alumina brick substrate is 12.65% 埘), the Fe element content is 9.03% (Bin), and the Al element content of the mullite substrate surface is as high as 32.81% (Bin). The content of Fe is only 2.12% (w), which indicates that the reaction degree of molten steel and high alumina brick is stronger than that of mullite, and high alumina brick is more likely to pollute molten steel.
After this experiment, the following points are drawn:
1. After contact with high-temperature molten steel, the surface of the high-alumina brick is large and the structure is relatively loose. The surface of the mullite is fine and the structure is relatively dense; the surface structure of the mullite has needle-like crystals and has a network cross structure.
2. After reacting with molten steel, the Al element content on the surface of the high alumina brick substrate is 12.65% (w), while the Al element content on the surface of the mullite matrix is as high as 32.81% (w). Mullite is more likely to form a dense layer of alumina, which hinders further erosion of the molten steel.
3. The phenomenon of cracking in high-aluminum bricks is mainly due to the high content of Si in high-aluminum bricks and the presence of a certain amount of K element. The liquid viscosity decreases, the fluidity increases, and the high-temperature strength decreases rapidly. The aluminum bricks are cracked.
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