To address the current challenges of nozzle service failure and inability to enhance continuous casting production efficiency caused by erosion of refractory materials at the slag line region of submerged nozzles, this study focuses on the ZrO₂-C submerged nozzle commonly used in continuous casting of low-carbon aluminum-killed steel at a certain steel mill. Samples were collected from the eroded refractory materials at the nozzle slag line region, and a detailed analysis was conducted on the causes and mechanisms of erosion. Results indicate that the erosion and necking of the nozzle’s slag line region stem primarily from the protective slag’s erosion, dissolution, and reaction with the external slag line materials. Therefore, effectively preventing or minimizing contact and reaction between the slag line refractories and protective slag is crucial for suppressing slag line erosion.
With the continuous advancement of iron and steel metallurgical technology, the scale and scope of continuous casting processes have expanded significantly. Currently, the vast majority of steel production relies on continuous casting processes [1,2]. As the critical functional refractory material connecting the ladle and mold in continuous casting, the submerged nozzle prevents molten steel oxidation and splashing, regulates flow conditions within the mold, and prevents slag entrapment [3,4]. Simultaneously, the slag line region of the nozzle is constantly subjected to alternating erosion and abrasion from molten steel and protective slag. As refractory materials in this area continuously dissolve and peel away, a “neck-down” phenomenon gradually forms at the slag line. This not only compromises the safety of continuous casting operations but also introduces inclusions from detached slag line material that directly impact molten steel quality, particularly affecting the quality of high-grade steel ingots. At present, erosion of refractory materials at the nozzle slag line has become a critical challenge causing nozzle service failure and limiting continuous casting production efficiency. Effectively controlling and preventing erosion at the nozzle slag line remains an ongoing research and improvement topic in continuous casting technology.
Based on this, this paper takes the ZrO₂-C-based continuous casting submerged nozzle commonly used for low-carbon aluminum-killed steel at a certain steel mill as an example. Samples were collected from the eroded refractory materials at the slag line of the nozzle, and the causes and mechanisms of erosion were analyzed in detail.
1.Experiment
To analyze the erosion behavior and mechanisms of the slag line in continuous casting nozzles for high-quality steels, field sampling of nozzles was conducted at a low-carbon aluminum-killed steel continuous casting platform in a steel mill. The primary chemical compositions of the refractory materials and protective slag at the slag line region of the corresponding ZrO₂-C-based submerged nozzle are shown in Table 1.
After casting completion, the macroscopic morphology of the refractory material at the nozzle slag line is shown in Figure 1. As seen in Figure 1, significant erosion and necking have occurred at the nozzle slag line. At the most severely necked section, the slag line material is nearly completely eroded. If continuous casting time is further extended, this nozzle is at high risk of direct fracture.
2.Analysis Following Erosion of the Water Inlet Slag Line
2.1 Slag Line Surface
Figure 2 shows the microstructure of the eroded nozzle slag line material surface, with the corresponding elemental composition listed in Table 2. After erosion, the outer wall of the nozzle slag line becomes denser and relatively smoother. The distribution of elemental content indicates that the eroded slag line surface consists entirely of zirconium-containing compounds formed by the reaction between ZrO₂ from the original slag line material and the protective slag. Specifically, the smooth regions of the slag line surface primarily consist of adhered calcium silicate, along with minor amounts of newly formed compounds such as calcium zirconate resulting from reactions with zirconia within the slag line. Additionally, due to the fluidity of the molten protective slag during continuous casting, the eroded slag line surface also contains residual protective slag deposits from slag flow and erosion.
In areas subjected to more intense erosion, the slag line experiences accelerated erosion and dissolution rates. Consequently, stable calcium silicate deposits cannot form on its outer surface. Furthermore, large zirconia particles within the slag line become directly exposed to the protective slag (point 5), gradually reacting with it to dissolve or even detach. At this point, the composition of the adherent material on the slag line’s outer wall becomes similar to that of the protective slag, containing a small amount of molten steel entrained at the protective slag-molten steel interface.
2.2 Slag Line – Protective Slag Interface
Figure 3 shows the microstructure of the interface between the slag line and the protective slag after continuous casting. The elemental composition at corresponding points is listed in Table 3. The surface of the nozzle slag line refractory material after continuous casting is covered by a layer of adhered protective slag, with a clearly defined boundary between the slag line and the adhered protective slag. The adhered protective slag exhibits a composition similar to the original protective slag, containing minor amounts of molten steel components entrained at the protective slag-molten steel interface. Furthermore, due to contact and reaction between the nozzle slag line material and the protective slag during continuous casting, large zirconia particles within the slag line gradually react, dissolve, and incorporate into the protective slag. Regarding the slag line material, zirconia particles exposed at the interface exhibit significant cracking and fragmentation due to erosion reactions with the protective slag. This progressive erosion of the slag line material at the slag line-protective slag interface deepens and intensifies as continuous casting time increases. Ultimately, this leads to the necking phenomenon depicted in Figure 1, posing a severe threat to continuous casting production safety.
2.3 Inside the slag line
Figure 4 shows the microstructure of the residual slag line after erosion. As illustrated, during continuous casting, erosion of the nozzle slag line by the protective slag primarily affects the refractory materials on the surface layer of the slag line, with minimal direct erosion or penetration impact on the internal refractory materials. After continuous casting, the connection between materials in the nozzle slag line remains tight. The interior of the slag line still consists primarily of structurally intact large zirconia particles and graphite materials, containing some impurities and a small amount of fine zirconia particles.
3. Erosion Process and Mechanism of the Nozzle Slag Line During Continuous Casting
Based on the corresponding analysis results of slag line erosion behavior, the erosion process and mechanism of the slag line material outside the nozzle can also be summarized as shown in Figure 5. As illustrated in Figure 5, when the slag line material outside the nozzle comes into contact with the protective slag and molten steel, the slag line undergoes a series of erosion behaviors including decarburization, dissolution, and chemical reactions. Furthermore, during continuous casting, the protective slag also exhibits a certain degree of fluidity. Thus, while undergoing erosion, the slag line simultaneously endures continuous scouring by the protective slag. On the slag line surface, reaction products formed between the protective slag and the surface, along with unstable refractory materials, gradually detach under slag scouring and migrate into the protective slag. However, for the slag line material itself, the intrinsic concentration of both the slag line and its surface materials remains unchanged throughout the erosion process. Therefore, according to Fick’s Law and the Nernst equation [5,6,7], as the protective slag continuously erodes and scours the slag line material, the erosion behaviors of the nozzle slag line material itself—including decarburization, dissolution, and chemical reactions—will progressively worsen and intensify.
Additionally, during slag line erosion, varying surface tension gradients exist between the slag line and the protective slag. According to the Marangoni effect, shear stresses also occur at the interface between the slag line and the protective slag [8]. Furthermore, under the influence of the Marangoni effect, the erosion rate of the protective slag at the slag line continuously accelerates. Moreover, during continuous casting, the gradual dissolution of carbon-containing materials (such as graphite) within the slag line material improves the wettability and contact area between the slag line material and the protective slag. This further accelerates the erosion rate and reaction speed of the protective slag on the slag line material during continuous casting. Ultimately, this leads to a continuous deterioration of the erosion process of refractory materials at the nozzle slag line, accompanied by a rapid reduction in slag line dimensions. Therefore, addressing the current erosion issues of ZrO₂-C nozzle slag lines requires two approaches: on one hand, preventing or avoiding contact and reaction between the refractory materials at the slag line and the protective slag; on the other hand, suppressing the loss of carbon from the refractory materials at the slag line.
During continuous casting, the erosion, dissolution, and reaction of protective slag on the external slag line material of the nozzle are key factors causing erosion and necking issues in the slag line region of submerged nozzles. As carbon-containing refractories, the decarburization behavior of refractory materials in the slag line region also facilitates the penetration and erosion of protective slag into these materials. Therefore, effectively preventing or avoiding reactions between the refractory materials at the slag line and the protective slag, as well as mitigating decarburization of the slag line itself, is crucial for resolving erosion of the refractory materials at the slag line of the submerged nozzle.