To extend the lifespan of ladle linings, new materials must be specifically designed based on the challenges inherent in current ladle operations. These new materials must possess excellent thermal shock resistance, adequate resistance to desulfurizer erosion, and sufficient thermal insulation. Research and development primarily focus on aluminum-carbon materials, specifically those designed for the molten iron environment. In recent years, fired aluminum-silicon carbide carbon bricks have gained widespread adoption in major steel mills due to their excellent thermal shock and erosion resistance. However, because they contain carbon materials such as graphite, exposed areas like the slag line are susceptible to oxidation, becoming porous and susceptible to erosion, significantly reducing their erosion resistance. Furthermore, the presence of carbon increases thermal conductivity, leading to high furnace shell temperatures and significant heat loss. This can lead to both capping and bottoming, making treatment difficult and hindering production. The average temperature drop during ladle transportation at one steel mill is approximately 190°C, slightly higher than at other mills. Some mills have reduced this temperature drop by adding caps to their ladles, but this issue remains a significant problem in mills with open-top ladle operations and long turnover routes. Based on this situation, clay silicon carbide bricks with good matching to the ladle process and excellent related performance were developed. Their thermal shock resistance and thermal insulation performance are better than those of aluminum-chromium silicon carbide bricks and aluminum silicon carbide bricks.
Clay Silicon Carbide Bricks for Ladle Lining
The main raw materials for clay silicon carbide bricks are pyrotechnics, andalusite, Guangxi white mud, silicon carbide, and alumina powder. The antioxidant is metallic silicon, and the binder is sulfite pulp wastewater. The purity of the main raw material, pyrotechnics, is ≥43.0% w(Al2O3) and has a particle size of 3 μm to 0 mm. Andalusite has a w(Al2O3) content of ≥69.0% and a particle size of 3 mm to 0 mm. Guangxi mud has a w(Al2O3) content of ≥30.0% and a particle size of 200 mesh. Silicon carbide has a w(SiC) content of ≥90.0% and a particle size of 200 mesh. Alumina powder has a w(Al2O3) content of ≥99% and a particle size of 3 μm to 6 μm.
To extend the service life of the ladle lining, the first step is to improve the thermal shock resistance of the working layer bricks. To combat thermal stresses generated by rapid thermal cycling, a suitable lining structure should incorporate effective stress buffering and release mechanisms to prevent the propagation of thermal shock microcracks and the resulting delamination of the lining. Under thermal shock, crack initiation, growth, and propagation in porous refractories such as aluminum silicon carbide bricks are related to the fracture surface energy. To improve the thermal shock resistance of the material, a high elastic modulus and low strength should be ensured. The lower compressive strength of clay silicon carbide bricks compared to aluminum silicon carbide bricks improves their thermal shock resistance. Furthermore, due to their higher apparent porosity than aluminum silicon carbide bricks, the evenly distributed pores effectively buffer and release thermal stress, significantly reducing the occurrence of delamination cracks and delamination.
Under normal circumstances, due to the higher porosity of clay silicon carbide bricks compared to aluminum silicon carbide bricks, their looser structure reduces their corrosion resistance, making them more susceptible to desulfurization agents and slag. However, in actual use, due to its high porosity, the surface of the clay silicon carbide brick is prone to accretion of a layer of slag. When the ladle cools down after emptying, the slag solidifies within the voids, partially filling the pores. This slag layer blocks some of the pores and provides a protective barrier, ensuring that the clay silicon carbide brick also has good slag resistance. Furthermore, the slag layer protects the brick lining from direct impact of the high-temperature molten iron, providing a buffering effect and reducing damage to the brick lining caused by thermal shock stress.
The relatively high apparent porosity and low bulk density of clay silicon carbide bricks ensure their relatively good thermal insulation properties, with the lowest gas thermal conductivity. The evenly distributed pores in clay silicon carbide bricks reduce their thermal conductivity, minimizing the temperature drop caused by heat conduction at the bottom and opening of the ladle. This helps to reduce slag and capping at the bottom and opening of the ladle.
Practical Applications of Clay Silicon Carbide Bricks
When aluminum silicon carbide bricks were used in 150-ton hot metal ladles, their service life was as low as 220 heats. The lining of the ladles severely peeled due to thermal shock, resulting in an uneven surface and dense, narrow, layered cracks visible on the exposed bricks. After the clay silicon carbide bricks were put into use, the excellent thermal shock resistance significantly reduced the spalling of the brick lining, resulting in a smooth and orderly surface. Furthermore, their excellent thermal insulation properties alleviated the problem of bottom and cover buildup in the ladles, reducing the occurrence of blasting and mechanical stress damage to the lining.
Once the clay silicon carbide bricks were put into use, the average service life of the 150-ton hot metal ladle reached 350 heats, significantly extending its service life. Furthermore, because the clay silicon carbide bricks are cheaper per unit than aluminum silicon carbide bricks, and their low bulk density and light weight mean each bale weighs less, their service life is extended. As a result, the overall cost of using the 150-ton hot metal ladle has been significantly reduced since their introduction. Due to the excellent performance and significant cost reduction of clay silicon carbide bricks, they have completely replaced aluminum silicon carbide brick ladle linings in the 150-ton ladle system and are still in use today.
Based on the above analysis, the following conclusions are drawn:
- (1) Thermal shock is the main factor leading to damage to the ladle lining. Clay silicon carbide bricks have excellent thermal shock resistance and can significantly reduce the layered cracks and layered peeling caused by rapid cooling and heating during the transportation and iron-bearing process of the 150-ton ladle.
- (2) After being put into use, clay silicon carbide bricks can ensure relatively good slag resistance and meet the ladle lining’s corrosion resistance requirements for desulfurizers and molten iron slag.
- (3) The thermal insulation effect of clay silicon carbide bricks is better than that of aluminum silicon carbide bricks, which improves the phenomenon of the bottom and mouth of the 150-ton ladle being solidified.
