With the trend toward larger continuous casting equipment, the steel throughput rate of long nozzles has gradually increased—rising from less than 1 t/min to over 7 t/min. This increase in throughput rate causes severe erosion of the inner wall; consequently, inner wall erosion has become the decisive factor determining the service life of high-throughput long nozzles. Table 1 presents the relationship between steel throughput rates and causes of wear/failure for high-throughput long nozzles used both domestically and internationally.
Methods to Extend the Service Life of Long Nozzles
Scholars have investigated the direct, one-to-one relationships between various performance indicators of refractory materials—independent of other complex, confounding factors—and have identified either proportional or inversely proportional correlations. Improving one performance attribute often necessitates a trade-off involving the reduction of others. The relationship between porosity and thermal shock resistance serves as an example: as porosity increases, the resulting voids in the microstructure inhibit crack propagation, thereby enhancing thermal shock resistance; conversely, a decrease in porosity leads to reduced thermal shock resistance (indicating a direct proportional relationship).
Optimization of Carbon Content in Long Nozzles
To enhance the thermal shock stability of long nozzles, a relatively high graphite content is typically employed. However, due to the low strength of graphite, it is susceptible to erosion by high-speed molten steel, thereby reducing the nozzle’s service life. Consequently, optimizing the carbon content—balancing erosion resistance with thermal shock stability—offers a viable solution. A long-life alumina-carbon nozzle has been developed, achieving extended service life by reducing carbon content and enhancing the bonding strength of the binder.
Optimization of particle size composition
Optimizing particle size distribution plays a crucial role in the formulation design of long nozzles. Theories regarding particle packing generally encompass the close packing of particles with discrete sizes and those with continuous sizes. A theory concerning the close packing of discrete-sized particles has been proposed; it posits that the densest packing is achieved when small particles precisely fill the interstices between large particles. Mathematical expressions for the packing of multiple discrete size fractions were subsequently formulated, as shown in Equation (1).
CPFT⁄100=(γlogD−γlogDs)⁄(γlogDl−γlogDs)(1)
In Equation (1), CPFT represents the cumulative percentage by mass of raw material below a certain particle size; γ is the ratio of particle masses between two adjacent size fractions; D is the particle size; Ds is the minimum particle size; and Dl is the maximum particle size.
Andreassen proposed a theory regarding the dense packing of particles with a continuous size distribution. He posited that the form of the particle size distribution remains constant—meaning the distributions are statistically similar—even as more particles are added. This theory is expressed by Equation (2).
CDFT⁄100=(D⁄Dl)q(2)
In Equation (2), $q$ represents the particle size distribution coefficient. A comparison of the two formulas reveals that Equation (2) imposes no lower limit on particle size, implying that the minimum particle size can be infinitely small. By introducing the concept of a finite minimum particle size, the Andreassen equation is modified, resulting in Equation (3).
While discontinuous particle sizing can theoretically yield maximum packing density, practical production faces issues such as particle segregation and economic inefficiency. A superior approach is to select a continuous particle size distribution and optimize the proportions of the various size fractions to achieve high packing density. The highest bulk density in the formed material is achieved when the particle size distribution corresponds to the densest packing arrangement. Therefore, by adjusting the particle size distribution, we can bring properties such as porosity, bulk density, and flexural/compressive strength within the required ranges, resulting in a material with excellent thermal shock resistance and good erosion resistance.
Research indicates that ensuring high thermal shock stability in the “three major components” used in continuous casting typically requires the addition of significant amounts of flake graphite (>20%). However, a higher proportion of coarse particles reduces material uniformity and increases the likelihood of graphite clustering; furthermore, it raises the risk of crack formation at the interfaces between coarse particles and graphite, thereby compromising thermal shock stability. Conversely, an excessive amount of fine powder also impairs thermal shock stability due to reduced strength and a greater tendency for crack propagation. In terms of maximizing thermal shock resistance, the optimal particle size composition features an aggregate-to-fine-powder ratio of 40:60.
Select an appropriate amount of binder
An appropriate amount of binder ensures suitable porosity and strength characteristics. This study investigated the effect of binder content on granulation performance using resin as the binder and a high-speed granulator, while keeping the proportions of aggregates, fines, micro-fines, and graphite constant. The impact was evaluated through sieve analysis of the prepared mixtures, scanning electron microscopy (SEM) observation of specimen morphology, and comparative analysis of physical properties. The results indicate that as the resin content increases, the diameter and quantity of pseudo-granules formed in the mixture also increase. The apparent porosity and bulk density of the specimens are initially governed primarily by the granulation quality but subsequently become increasingly influenced by the resin content itself; meanwhile, the specimens’ cold crushing strength and flexural strength exhibit a gradual upward trend.
Introduce materials with a low coefficient of thermal expansion
Preheat-free long nozzles were developed by incorporating materials such as fused silica and zirconia-mullite, achieving a service life of 4 to 7 hours. Building upon research into foreign technologies, advanced refractory raw materials (such as Sialon) and special binders were utilized; furthermore, manufacturing and firing processes compatible with the formulation technology were experimentally developed. These efforts resolved the conflicting requirements regarding thermal stability, erosion resistance, and corrosion resistance, resulting in preheat-free long nozzles with excellent overall performance. To enhance thermal shock stability, low-expansion refractory raw materials—such as fused silica, zirconia-mullite, and silicon carbide—are typically added to alumina-carbon long nozzles.