In 2021, my country’s crude steel output will reach 1.033 billion tons, accounting for 56% of the world’s crude steel output, making it the world’s largest steel producer. However, there is still a gap between the quality of high-quality steel and special steel compared with developed countries. With the development of science and technology, the requirements for the quality of steel continue to increase, reducing the inclusion content in the steel and improving the cleanliness of the steel are the development directions of this century. Out-of-furnace refining technology has become the most important smelting link for improving the quality of modern steelmaking. The ladle bottom blowing process has been widely used in out-of-furnace refining, and breathable brick is the key functional refractory material for this process.
At present, the ladle is gradually becoming larger, and its high smelting temperature and long-term smelting make the use conditions more and more severe, which puts forward higher requirements for refractory materials for ladle. Due to the special position and environment of the breathable brick in the ladle, the requirements for its life, air blowing success rate, and air permeability are relatively high. High blow-through rate, oxygen-free cleaning, and synchronization with the service life of ladle lining or ladle bottom refractories are the common goals pursued by users, breathable brick manufacturers and researchers.
The intermittent operation process of the breathable brick for ladle is steel connection, air refining, steel pouring, slag pouring, and oxygen cleaning. The breathable brick repeats the above process during use until the end of its life. Among the many factors that affect the service life of breathable bricks, thermal peeling and oxygen cleaning are the important reasons for the damage of breathable bricks. The time and intensity of air blowing and stirring also have a very significant impact on the damage of breathable bricks. The main reasons for the return of molten steel and the frequent replacement of breathable bricks are the peeling of the breathable bricks, the steel in the slits, and the inability to blow.
In field use, it is often found that the lower part of the heating surface of the slot-type corundum-spinel breathable brick in contact with the molten steel is prone to transverse cracks, resulting in thermal spalling from the transverse cracks, which greatly reduces the service life of the breathable brick. Erosion and penetration of slag liquid are one of the important reasons for the damage of breathable bricks. The erosion and penetration of molten steel and slag liquid form a reactive metamorphic layer on the working face of the breathable core, which in turn leads to structural spalling. blow-through rate and service life.
The microstructure analysis of the residual bricks of the ventilated bricks after use shows that the slits and cracks of the permeable bricks used in the LF refining process have obvious slag erosion and penetration, and the slits and cracks of the permeable bricks used in the LF+VD refining process have molten steel penetration. During periodic operation, the expansion of the slit mouth and the cracks caused by thermal shock will aggravate the penetration and erosion of molten steel or slag, and lead to the occurrence of spalling. After the ventilation brick slit is blocked, it must be cleaned by blowing oxygen, which directly affects the blowing rate and service life of the ventilation brick.