In modern continuous casting processes, the monolithic tundish stopper rod serves as a critical functional refractory component (Figure 1). Working in tandem with the submerged entry nozzle or upper nozzle, it performs the vital task of precisely controlling the flow of molten steel, with its primary function being the maintenance of a stable molten steel level within the mold. The performance of the stopper rod directly impacts the continuity of the casting process and the quality of the final steel product; issues such as rapid erosion of the rod tip, localized spalling, or breakage can lead to fluctuations in the mold level and a decline in cast strand quality, or even result in casting interruptions that severely disrupt production schedules. Given that stopper rods operate in harsh environments characterized by high temperatures and intense corrosive forces-and cannot be replaced mid-run-their service life directly dictates the tundish's operational cycle. Consequently, developing long-lasting stopper rod tips has become a key factor in enhancing continuous casting efficiency
Fig. 1 Macroscopic photographs of the stopper rod before (a) and after (b) service.
Currently, stopper rod heads are predominantly manufactured using Al₂O₃-C refractory materials. However, with the rapid development of China's iron and steel industry, the limitations of these materials in the smelting of special steels and clean steels have become increasingly apparent, primarily in the following respects: First, during the casting of calcium-treated steel, the Al₂O₃ matrix reacts with CaO in the molten steel at approximately 1400°C to form a low-melting-point calcium aluminate liquid phase; this causes chemical erosion of the rod head, thereby shortening its service life. Consequently, the resistance of Al₂O₃-C materials to calcium erosion has become a critical factor limiting the service life of stopper rods. Second, the Al₂O₃ matrix may also react with alloying elements such as manganese in the steel, further exacerbating structural degradation. Furthermore, as the capacity of continuous casting tundishes continues to increase, the length of stopper rods used for slab continuous casting has grown significantly, currently reaching over 1.5 meters. Such long stopper rods are subjected to greater bending moments caused by their own weight at high temperatures, placing higher demands on their high-temperature mechanical strength to prevent breakage during installation and operation. Given these shortcomings of Al₂O₃-C stopper rod materials, developing alternative refractory materials with superior performance has become a key research focus in the fields of refractories and continuous casting technology.
Against this backdrop, MgO-C refractories have garnered increasing attention due to their unique performance advantages and are regarded as a potential alternative to Al₂O₃-C materials. These refractories utilize high-purity magnesia-typically produced via dead-burning or electro-fusing processes-as their primary raw material. The characteristics of the primary crystalline phase, periclase, directly determine the material's core performance; it offers not only exceptional refractoriness but also outstanding resistance to alkaline slag erosion. Thanks to these properties, MgO-C refractories are widely employed in critical areas such as ladle slag lines, electric arc furnace linings, and basic oxygen converters. The following section provides a systematic overview of the performance advantages of MgO-C stopper rod materials compared to their Al₂O₃-C counterparts.
Resistance to molten steel erosion
From the perspective of flow-control zone dynamics, the erosion of the stopper rod stems primarily from the unique fluid dynamic conditions of its operating environment. The flow-control point is defined as the geometric point of tangency between the multi-arc stopper rod and the curved inlet (bowl) of the submerged entry nozzle (SEN) when in the closed position; the regions immediately above and below this point constitute the flow-control zone. Under process conditions involving a tundish melt level of 1 m and a steel throughput of 2.5 t/min, numerical simulations using the finite element method were conducted to analyze the flow field and pressure distribution of the molten steel surrounding the stopper rod; the results are shown in Figure 2(a). The simulations reveal that the flow velocity of the molten steel increases significantly at and below the flow-control point-reaching up to 5 m/s-while the pressure drops sharply, creating a distinct negative-pressure zone. A systematic analysis of the pressure distribution along the stopper rod's centerline-extending from the tundish melt surface to the SEN outlet (Figure 2(b))-reveals a region of significant high-vacuum negative pressure near the flow-control point, particularly just below it. In this region, the molten steel forms a high-speed jet; this flow pattern, combined with the intense negative-pressure environment, constitutes the key dynamic mechanism driving erosion at the stopper rod tip. The synergistic effect of continuous shear stress exerted by the high-speed flow on the material surface and cavitation induced by the negative pressure significantly accelerates material loss. Once erosion alters the geometry of the stopper rod tip, flow-control performance is directly compromised, manifesting as reduced control precision, a shortened service life, and potential operational safety hazards. Utilizing MgO-C refractory material has emerged as an effective solution to mitigate this dynamic erosion; its superior thermodynamic stability resists damage from the molten steel, thereby extending the stopper rod's service life and enhancing the reliability of the continuous casting process.
MgO does not react with the vast majority of components in molten steel; notably, it does not react with calcium to form low-melting-point compounds, thereby avoiding adverse effects on flow control and steel cleanliness. MgO possesses a melting point as high as 2800°C, whereas the melting point of FeO is 1370°C; the two form neither a low-melting eutectic phase nor extensive solid solutions. Similarly, MnO and MgO form only limited solid solutions and do not produce a low-melting-point liquid phase. Consequently, MgO aggregates maintain high chemical stability even in molten steel with high oxygen or manganese content. Furthermore, from the perspective of interfacial chemistry, the wetting behavior between the solid and liquid phases can be characterized by Equation (1):
Figure 2. Contour maps of velocity and pressure fields in the flow control zone of the stopper rod.
Research indicates that at the temperature of molten steel, when MgO comes into contact with C, the MgO is reduced to generate Mg vapor, as described by reaction (2). This Mg vapor diffuses outward through the pores and micro-cracks within the material. Upon reaching the material surface, the Mg vapor reacts with dissolved oxygen ([O]) in the molten steel or with oxide inclusions (such as FeO and MnO) via an oxidation process, represented by reaction (3). These reactions proceed rapidly in the surface region, producing solid MgO particles. These newly formed MgO particles aggregate, grow, and sinter on the material surface, gradually creating a continuous, dense MgO-enriched layer that effectively inhibits further penetration and erosion of the refractory material by the molten steel.