Preface

The stopper rod is a type of shaped refractory material used in the production process of the continuous casting machine. It can ensure the stability of the liquid level in the crystallizer, control the linear flow rate of molten steel, and ensure the surface quality and internal quality of the ingot. In actual production, the stopper rod plays a vital role in the stability of steelmaking production. Problems with the stopper rod, such as erosion, breakage, and nodules, will cause excessive fluctuations in the liquid level of the crystallizer molten steel, which may cause the continuous casting machine to shut down prematurely or be forced to be pulled down in severe cases.

Analysis of the problem of stopper rod fracture

The slab continuous casting machine of a certain steel plant frequently encountered the problem of stopper rod breakage during steel casting, which seriously affected the normal production of the continuous casting machine. According to the statistics of accidents, the proportion of stopper rod head falling off during the baking of the tundish or the pouring was 20%, and the rest was that the stopper rod head fell off during the pouring process or the stopper rod broke at a position 650-700mm below the upper edge, and the fracture was “V” shaped, as shown in Figure 1. According to statistics, there were 12 stopper rod accidents in 2021, an average of 1 per month. Each accident caused the continuous casting machine to shut down unplanned, and the tundish needed to be baked again, the machine needed to be prepared, and the pouring started, which disrupted the normal production plan. Therefore, it is urgent to solve the stopper rod problem, stabilize the production of the continuous casting machine, and reduce cost losses.
Through statistical classification of stopper rod accidents, it is believed that the main factors affecting stopper rod breakage and stopper rod head falling are thermal shock, strength and stopper rod design and installation problems.
2.1 Thermal shock problem
From the analysis of the material design of the body and head of the stopper rod and the manufacturing process of the stopper rod, it is believed that the material of the stopper rod currently used meets the process requirements. The steel plant mainly uses carbon structural steel, high carbon steel and some medium carbon alloy steel, which has little erosion on the stopper rod. The stopper rod has no slag line and the scouring of the stopper rod head is not serious. The normal steel pouring is 15 hours, the stopper rod head has good flow control and uniform erosion. The design of the material is basically reasonable. From the fracture of the stopper rod head, the position is 50-60mm, and the fracture is flush. First of all, the problem of stopper rod manufacturing should be solved. The manufacturing process needs to be adjusted and strengthened. The uneven composition of the raw materials used to manufacture the stopper rod leads to stress, which is the direct cause of the stopper rod fracture.
The baking of the on-site tundish is tracked, and the tundish baking has the problem of low baking effect. The tundish baking uses converter gas, the calorific value is generally 1300×4.1868kJ/m³, the baking time is 3h, the stopper rod is in an open state during the tundish baking, the baking temperature is 1000℃, the tundish is not sealed during baking, and the flame overflows seriously during baking. Poor baking effect of stopper rod will lead to uneven thermal stress of stopper rod refractory, thus generating micro cracks. During the casting process, the stopper rod works in an extreme environment, and any small defects will be magnified.
2.2 Strength Issues
The problem of stopper rod slag line fracture usually occurs 1~2 furnaces after changing the water nozzle during steel casting, after bonding recovery, and during the quick change of the tundish. Analysis shows that when the stopper rod is closed during the steel casting process, it is subjected to lateral and longitudinal forces to crack and then break.
Analysis of the strength of the stopper rod body shows that the strength of the stopper rod is low in the hot state, and it is easy to break when molten steel is flushed or stressed, so the strength of the stopper rod body needs to be strengthened.
2.3 Design and installation of stopper rod
The installation of the stopper rod on site was tracked and analyzed as follows: ① There are problems with the stopper rod design. When the stopper rod is opened for steel pouring, the stopper rod is 200mm higher than the tundish cover. According to the normal design, the stopper rod is 50mm higher than the tundish cover to meet the requirements. The stopper rod is too long. When the stopper rod is closed in an emergency, it is subjected to huge external force and is prone to breakage. In particular, the slag line is the position where the stopper rod changes diameter, and this part is weaker; ② There are problems with the installation of the stopper rod. The main reason is that the stopper rod is installed offline, and there may be shaking damage during the process of putting the tundish online. The stopper rod installation requires a “gnawing head”. The existence of the “gnawing head” causes lateral force when the stopper rod is closed, causing cracks in the stopper rod head or the weaker part of the stopper rod. When the stopper rod is closed during the steel pouring process, the operator changes the mode from automatic to manual, and then presses the gate handle hard to close the stopper rod, which also aggravates the damage to the stopper rod.

Improvement measures

3.1 Improvement of stopper rod fracture problem
In view of the above problems, the current stopper rods are considered to be improved. The specific solutions are as follows:
(1) Under the premise of meeting production requirements, the material is fine-tuned to appropriately improve the thermal shock resistance of the stopper rod and strengthen the neck strength of the stopper rod. The adjustment idea is: SiC powder is introduced into the production ingredients of the tundish stopper rod, focusing on the stopper rod head and slag line position, and the stopper rod is designed with a composite structure to improve the comprehensive mechanical properties of the stopper rod and reduce the linear expansion coefficient. When 5% SiC powder is added, the room temperature and high temperature flexural strength of the magnesium carbon rod head material are 8.2MPa and 9.4MPa respectively, and the average linear expansion coefficient is 7.1×10-6/℃. No obvious cracks are observed after three thermal shock cycles at 1100℃.
(2) Adjustment of stopper rod length. The current stopper rod is too long, reaching 1750mm, which increases the difficulty of stopper rod production. It will also amplify the lateral force generated by the flow of molten steel during use, causing the rod body to break after being stressed. Combined with the site, the stopper rod length is adjusted to 1650mm.
(3) The stopper rod head was optimized by changing its shape from spherical to conical to improve the flow control accuracy of the stopper rod. At the same time, the size of the stopper rod head was changed from 60 mm to 45 mm.
3.2 Improvement of stopper rod installation method
At present, the stopper rod is installed in the repair area, and the vibration during the transportation of the tundish will have an adverse effect on the stopper rod. Improve the existing stopper rod installation process, change from offline installation to online installation before the tundish is baked, and reduce the impact of the tundish transportation process; in addition, before debugging the stopper rod, check whether the shutter beam is deformed, observe whether the stopper rod is straight, cancel the “gnawing head” during installation and debugging, and strictly align it to avoid the stopper rod from being broken due to the lateral force when the stopper rod controls the flow.
3.3 Optimization of stopper baking system
Optimize the stopper rod baking system to address the thermal shock problem. Seal the tundish edge when the tundish is covered offline, and strictly seal before baking to improve the tundish baking effect; when the tundish starts to bake, the stopper rod is in a closed state, and the stopper rod is opened after the tundish is baked for half an hour, which is conducive to the baking of the stopper rod and the upper water inlet. Revise the tundish baking system and stipulate a reasonable baking time. Long-term baking shortens the life of the tundish and even wastes the tundish. The fire cannot be stopped during the baking process, otherwise the tundish will be disposed of as a waste; strictly control the opening of the gas valve according to the tundish baking curve to ensure that the temperature rises to above 1000℃ during the 1~2h baking period.
3.4 Improvement of stopper operation during pouring and steel pouring
In view of the habit of “watching the rod” during the existing tundish baking process, the stopper rod is checked by stopping baking before pouring. If there is a small deviation in the stopper rod, the stopper rod is fine-tuned to reduce the negative impact caused by stopping baking and adjusting the stopper rod vigorously during baking.
During the process of changing the water inlet, bonding recovery, and quick change of the tundish, the mode is first changed from “automatic” to “manual”, the operator holds the stopper rod gate handle, and the stopper rod is closed by its own gravity, and then the operator presses the gate handle to close the stopper rod and perform subsequent operations.

Implementation process and results

According to the above measures, it is implemented in two steps: first, improve the tundish baking system, strengthen the tundish sealing, improve the stopper rod baking effect, and improve the stopper rod installation and operation; second, optimize the stopper rod, and the manufacturer designs the stopper rod improvement drawings, makes the mold and stopper rod, and has the conditions for field testing.
The improved stopper rod is tested on site, and the stopper rod baking, pouring and steel pouring processes are tracked. It is required to lift the stopper rod out every time the machine is shut down. The casting steel types include Q235, Q345, 45#, 50# and other steel types. The condition of the stopper rod head is observed, the flow control is good, and the stopper rod head is uniform.
The implementation of the above measures has achieved certain results. The continuous casting machine shutdown accidents caused by stopper rod breakage have gradually decreased. In 2022, the steel plant had only 2 stopper rod breakage accidents, which was 10 times lower than in 2021, and the use of stopper rods has stabilized.

conclusion

By controlling the amount of additives added during the production of the stopper rod and optimizing the shape of the stopper rod, the thermal stability and flow control accuracy of the stopper rod can be improved, which is conducive to stabilizing the liquid level of the crystallizer, reducing the number of stopper rod switching, and improving the use effect of the stopper rod.
Through the improvement and optimization of on-site stopper rod installation, baking and use, the problem of stopper rod fracture due to thermal shock and mechanical damage is reduced, making the use of tundish stopper rods more stable.