Analysis and Prevention of Abnormal Stopping Accidents of Tundish Stoppers in Continuous Casting Machines

This paper analyzes and studies the causes of abnormal stopping accidents of stoppers in tundishes during continuous casting machine production, based on production practice. Targeted optimization measures are proposed to effectively prevent such accidents.

1 .Introduction

The stopper, as one of the main control components of the tundish in a continuous casting machine, controls the flow rate of molten steel from the tundish to the copper mould tube, thus achieving stable control of the crystallizer level control system. When the crystallizer level control system detects the molten steel level in the copper mould tube, it compares it with the set level. If there is a deviation between the actual level and the set level, the control system will automatically compensate. When the molten steel level in the copper mould tube is higher than the set value, the stopper will descend to reduce the opening. If the molten steel level in the copper mould tube is lower than the set value, the stopper will rise to increase the opening and replenish the molten steel in the crystallizer. Our plant mainly produces aluminum deoxidized killed steel. During continuous casting, occasional instances of the stopper rod rising occasionally occur. Generally, applying calcium lines to the stopper rod location can alleviate the rising, and timely replacement of the ladle with a fresh batch of molten steel can resolve the stopper rod rising issue and prevent casting stoppages. However, in this particular incident, the stopper rod died within just 5 minutes, causing a production halt. To prevent similar incidents from recurring, we conducted an analysis of the accident and developed corresponding preventative measures, which effectively prevented similar accidents from happening again.

2.Production Process and Chemical Composition of Molten Steel

The production process for low-carbon SPCC steel on the No. 2 continuous casting machine of this plant is as follows: molten iron → converter → LF refining → slab continuous casting machine. Four heats were cast in this batch. Samples of the four heats of molten steel produced in the batch affected by the production accident were taken and their chemical composition was statistically analyzed, as shown in Table 1.

Table 1 Molten Steel Composition / %

Table 1 shows that the composition of C, Si, Mn, P, S, Als, and Ca in all heats of this casting were within the standard requirements, with no abnormal components.

3.Production Process Parameters

3.1 Smelting Process Parameters

The smelting process of all heats of this casting was statistically analyzed, as shown in Table 2. Table 2 shows that the fourth heat of steel in this casting, which was stopped by the stopper rod, had a refining cycle of 26 minutes in the LF furnace. This is significantly shorter than the average refining time of 35 minutes for low-carbon steel in the past, resulting in inadequate refining, incomplete removal of inclusions, and excessive aluminum deoxidation inclusions. Furthermore, the third heat of this casting cycle was a Class D ladle. As a new ladle, the MgO content in the refractory material of the slag line on the ladle wall was over 60%, resulting in high MgO content in the slag. This makes it prone to reacting with Als in the molten steel to form magnesium-aluminum spinel. During the final slag addition at the end of casting, the continuous casting yield of this heat was 127.3 t. There is a possibility that complex inclusions such as magnesium-aluminum spinel may enter the tundish and accumulate on the stopper head during casting, leading to uncontrolled stopper rise.

3.2 Stopper Position Changes

The casting curve of the heat that was stopped due to the accident was analyzed, as shown in Figure 1.

Table 2 Production Process Parameters

Figure 1. Changes in Stopper Rod Position and Liquid Level During the Accident

As shown in Figure 1, casting of the fourth heat began at 13:48. At 14:18, the stopper rod position started rising from 55 mm, rapidly increasing at 14:23, and reaching 100 mm at 14:24, completely out of control, leading to production stoppage. Before the rapid rise in stopper rod position, the liquid level in the copper mould tube did not fluctuate significantly, indicating that the rise in stopper rod position was a sudden and isolated incident. The reason for the short-term rise in stopper rod position was the inability to replenish molten steel in the crystallizer for a short period, indicating that the stopper rod head was temporarily blocked at the nozzle bowl or the inner wall of the nozzle.

3.3 Electron Microscopic Analysis of the Stopper Rod Head and Inclusions

Samples were taken from the stopper rod in the tundish after the casting accident. The head morphology is shown in Figure 2, and the cross-section is shown in Figure 3. As shown in Figure 2, a layer of white powdery adhering material was attached to the surface of the stopper rod head, but the thickness of the adhering material was about 3 mm. The cross-section was then observed. As shown in Figure 3, there is a dense, grayish-white layer between the surface of the stopper rod and the powdery adhering material.

Figure 2. Morphology of the stopper head

Figure 3. Cross-section of the stopper rod

Scanning electron microscopy (SEM) energy dispersive spectroscopy analysis was performed on the cross-section of the stopper rod head from the inside out. The results are shown in Figure 4. Figure 4 shows that the components of the stopper rod head cross-section from the inside out are: stopper rod refractory containing Mg, Ca, Al, and O. The surface residue is mainly calcium aluminate, with a thickness of 3–4 mm. This indicates that the instantaneous stopper rod expansion was not related to the short-term accumulation and elevation of the residue on the stopper rod head.

3.4 Analysis of the Submerged Inlet Gate

Since the tundish inlet is a single-piece gate, the submerged inlet gate was analyzed in response to the shutdown incident. Figure 5 shows that the inner wall of the gate was relatively clean, and there was no internal blockage, ruling out the possibility that blockage caused by nodule formation on the inner wall of the gate led to the stopper rod expansion.

Figure 4. Energy Dispersive Spectroscopy (EDS) Analysis of the Cross-Section of the Residue at the Stopper Rod Head

Sampling of the immersion nozzle bowl in the repair area revealed a large blockage missing from the cold steel. As shown in Figure 6, the blockage occupied 80% of the channel opening, causing a narrow steel channel that could not meet the molten steel supply to the crystallizer. This forced the stopper rod to rise, ultimately stopping the casting process. Since the blockage sample was lost, scanning electron microscopy (SEM) analysis was performed on the surrounding cold steel and adhering material (see Figure 7) to locate the source of the blockage. Figure 7 shows that the adhering material is brittle, with main components of MgO, Al2O3, SiO2, and CaO. Therefore, it was determined that the blockage was caused by magnesium aluminum spinel adhering material falling into the cylindrical area of ​​the immersion nozzle bowl.

4.Cause Analysis

The main reason for the rise in the stopper rod position was that the liquid level in the crystallizer was lower than the set value, causing the stopper rod to rise to supply molten steel. The insufficient liquid level in the crystallizer was mainly due to the blockage of the tundish nozzle, preventing molten steel from flowing into the crystallizer. There are two reasons for tundish nozzle blockage. The first is due to low steel temperature, poor baking of the tundish or nozzle, and condensation of the steel. This is to ensure the temperature of the steel for continuous casting, but the tundish steel for this casting is normal. The second reason is that there are deposits on the inner wall of the nozzle, which narrows the inner diameter of the nozzle and eventually causes blockage. This is especially noticeable when casting aluminum-containing steel. The continuous casting nozzle blockage phenomenon in our plant belongs to the second type after analysis. Based on a comprehensive analysis of the chemical composition, production process, inclusions at the head of the stopper rod, and submerged entry nozzle of the four heats cast in this batch, it can be seen that the refining time of the molten steel in the stopper rod shutdown heat was too short. As for aluminum deoxidized steel, the aluminum deoxidized inclusions themselves could not float to the surface in time. In addition, the content of magnesium-aluminum spinel composite inclusions exceeded the standard, which led to the accumulation and formation of lumps at the head of the stopper rod. As the lumps grew, they fell into the submerged entry nozzle under the flushing of the molten steel and got stuck in the nozzle bowl, causing a short-term blockage of the submerged entry nozzle bowl. Even when the stopper rod was raised to a high position, it was still unable to meet the molten steel supply to the crystallizer, and the casting was eventually forced to stop.

Figure 5 Schematic diagram of the inner wall of the submersible nozzle

Figure 6. Morphology of the molten steel blockage inside the sprue bowl.

Figure 7. Scanning of morphology and composition of molten steel blockage inside the tundish nozzle.

5.Preventive Measures

5.1 Reduce oxygen at the converter endpoint and control slag discharge during converter tapping.

Optimize the converter bottom blowing process to ensure the oxygen content at the converter endpoint is ≤500×10⁻⁶. Improve the slag-blocking process to reduce the amount of slag discharged during converter tapping, alleviate the difficulty of refining, deoxidation, and slag formation, and improve the efficiency of white slag formation. If the molten steel has strong oxidizing properties, it is easy to react with Als during casting to generate Al₂O₃ inclusions, reducing the probability of clogging the inner wall of the tundish nozzle.

5.2 Improve the bottom blowing effect and process of the ladle.

The time interval between Al adjustment and Ca treatment in molten steel must be >6 min. After wire feeding, the soft blowing argon time must be >5 min to ensure that Al₂O₃ inclusions in the molten steel fully float to the surface. Avoid large-volume stirring to prevent secondary oxidation of the molten steel. At the same time, ensure the refining cycle and ensure the purity of the molten steel. During the continuous casting of newly built ladles, perform residual steel operations as much as possible to avoid magnesium aluminum spinel inclusions or slag contamination of the next heat of molten steel with high Mg content.

5.3 Full-Process Protective Casting

Full-process protective casting is crucial for low-carbon aluminum-containing steel casting to prevent secondary oxidation of exposed molten steel, which can lead to Al2O3 inclusions and clogging of the nozzle. Low-level circular pouring should be used when starting the casting process, with appropriate amounts of covering and insulating agents added to isolate the molten steel from air and reduce secondary oxidation.

6 Conclusion

Based on practical production experience, this paper analyzes and studies the occasional abnormal stoppage incidents of the tundish plug during continuous casting. The causes of the incidents were determined, and the abnormal stoppage incidents were effectively prevented by reducing the oxygen at the converter endpoint, controlling slag discharge during converter tapping, improving the bottom blowing effect of the ladle, optimizing the remaining steel process in the new ladle, and implementing full-process protective casting.

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