Damage to Ladle Slide Gates
With the advancement and widespread use of secondary refining and continuous casting processes, there has been an increasing demand for precise control of molten steel flow between ladles and tundishes, along with stricter safety requirements for ladle protection during casting operations. Advanced ladle sliding gate systems meet these demands and are therefore widely adopted by large steel enterprises. As the core component that directly controls the flow of molten steel in the sliding gate system, the slide gate is subjected to prolonged contact with high-temperature molten steel during operation and experiences intense thermal shock during repeated use. Therefore, it must possess high strength, excellent corrosion resistance, superior thermal shock resistance, high-temperature endurance, and minimal creep deformation.
Part 1: Common Damage Phenomena and Analysis of Slide Gates
The ladle sliding gate system used in a particular No. 3 Steelmaking and Continuous Rolling Plant adopts a two-layer structure, as shown in Figure 1. Based on operational experience, the factors contributing to the failure of slide gates during service can be categorized into three main types: damage due to thermal shock, thermochemical corrosion, and improper operation.
1.1 Thermal Shock Damage
Before service, the slide gate is at a relatively low temperature. However, during the casting process, it comes into contact with high-temperature molten steel in a short period of time. The resulting significant temperature difference exerts intense thermal shock on the slide gate body. At this point, tensile stress is generated outside the casting hole of the slide gate. Once this stress exceeds the strength of the slide gate material, radial microcracks form, as shown in Figure 2. These cracks facilitate the diffusion, accumulation, and penetration of external molten steel, slag, and oxygen, which in turn intensify chemical corrosion.
1.2 Thermochemical Corrosion
During service, the slide gate comes into contact with high-temperature molten steel and slag, triggering a series of chemical reactions that result in thermochemical corrosion. This type of corrosion tends to degrade the high-temperature wear resistance of the slide gate's working surface, leading to surface layer spallation. Consequently, the slide gate's sealing performance deteriorates, causing increased gaps. These gaps further promote air aspiration and subsequent oxidative corrosion. The alternating effects of these processes can ultimately lead to severe accidents, such as molten steel leakage from the slide gate in extreme cases.
Based on variations in steel grades and slag compositions, common thermochemical corrosion phenomena can be classified into the following categories during slide gate service.
1.2.1 Chemical Corrosion in Calcium-Treated Steels
In the production of aluminum-killed steel and silicon-aluminum-killed steel, calcium treatment is performed by feeding Ca-Al wire or Ca-Si wire into the molten steel during the refining process to improve its castability. When producing these steel grades, the eroded area of the slide gate exhibits a distinct "horseshoe" shape. The primary cause is that [Ca] and CaO in the molten steel react with Al₂O₃ and SiO₂ in the slide gate to form low-melting-point compounds. Particularly during the casting process, the flow of molten steel within the slide gate hole tends to create a negative pressure zone, as illustrated in Figure 3. Under the influence of this negative pressure, calcium vapor directly reacts with the aspirated oxygen to form CaO, which accumulates in this region, leading to the "horseshoe"-shaped erosion.
1.2.2 Chemical Corrosion in High-Manganese Steels
When casting high-manganese steel grades, such as pipeline steel, the slide gate experiences significant hole expansion, with a maximum expansion of up to 5 mm per heat. Additionally, the corrosion of the slide gate's contact surface is relatively severe, accompanied by spallation of the surface contact layer and intensified cracking. This is due to the following chemical reactions between MnO in the high-manganese molten steel and Al₂O₃, SiO₂ in the slide gate:
MnO + SiO₂ → MnO·SiO₂
MnO + Al₂O₃ → MnO·Al₂O₃
These reactions cause the decomposition of corundum and zircon mullite-the primary materials in the slide gate that provide corrosion and thermal shock resistance. Consequently, the erosion of molten steel is intensified, leading to abnormal expansion of the gate hole.
1.2.3 Chemical Corrosion by Slag
In the late stage of ladle casting, due to the suction effect caused by molten steel flow, part of the steel slag is drawn into the nozzle, resulting in slag corrosion on the slide gate. The main characteristics of this corrosion include hole expansion, surface corrosion, and intensified cracking. Steel slag has a complex composition, primarily consisting of CaO, SiO₂, Al₂O₃, MgO, MnO, FeO, Cr₂O₃, CaF₂, and others. Most of these oxides can react with Al₂O₃ and SiO₂ in the slide gate to form low-melting-point compounds. Additionally, FeO, MnO, and other oxides may react with carbonaceous raw materials in the slide gate, causing decarburization. This makes the surface structure of the slide gate loose and ultimately leads to damage.
1.3 Operational Factors
Based on practical experience, the operational factors that cause damage to slide gates can be categorized into three types: improper slide gate installation, unreasonable flow control during casting, and oxygen lancing of the nozzle.
(1) Improper Slide Gate Installation:
If the slide gate is not properly leveled or exhibits warping when installed in the sliding mechanism, or if the clamping of the slide gate is loose, significant external stress will be generated during service, leading to overall damage to the slide gate.
(2) Unreasonable Flow Control During Casting:
Unreasonable flow control during the casting process is likely to cause spallation, corrosion, and steel entrapment on the working surface of the slide gate, as shown in Figure 4. A review of flow control operations in production reveals that the main causes of slide gate damage are excessive or overly frequent movement of the slide gate. In particular, the number of slide gate damages caused by manual flow control is higher than that caused by computerized automatic flow control, indicating that human factors in operations are also an important cause of slide gate damage.
(3) Improper Oxygen Lancing Operation
Oxygen lancing of the ladle nozzle is required during ladle preparation or when molten steel fails to flow during casting. Improper oxygen lancing operations can cause severe oxygen erosion of the slide gate. Specific improper practices that lead to slide gate damage include:
Lancing oxygen before the slide gate is fully aligned, resulting in direct oxygen impact on the working surface.
Lancing oxygen before the tundish flux is completely discharged, which prolongs the lancing time due to difficulty in opening the flow channel.
Misalignment between the oxygen pipe and the flow channel, causing oxygen flow to erode the edge of the slide gate hole and induce hole expansion.
Additionally, other operational irregularities include unreasonable ladle turnover time (which leads to a significant drop in overall ladle temperature and increased thermal shock during reuse) and improper preparation of refractory mortar for the slide gate (such as incorrect proportioning, inadequate mixing, or the presence of impurities).
The aforementioned damage mechanisms interact and reinforce each other during the preparation and service of slide gates, making it difficult to attribute the damage to a single cause. Therefore, to extend the service life of slide gates, a comprehensive analysis and systematic solution are indispensable.
Part 2: Improvement Measures
Based on the analysis of slide gate damage mechanisms, a series of measures for material selection, installation, and application have been formulated, effectively extending the service life of slide gates.
(1) Slide Gate Material Optimization
Select alumina-zirconia-carbon (AZC) slide gates with excellent corrosion resistance and thermal shock resistance. These slide gates use corundum, mullite, and baddeleyite as the main crystalline phases, with carbonaceous materials serving as binders. Consequently, they exhibit high thermal conductivity, a low thermal expansion coefficient, and fine porosity. Furthermore, ZrO₂ undergoes a phase transformation at 1000°C, which helps improve the thermal shock resistance of the slide gate.
(2) Slide Gate Mechanism Improvement
Adopt a horseshoe-shaped clamping structure to ensure uniform stress distribution, avoiding stress concentration defects common in simpler geometric shapes. An eccentric wheel device is used to lock the slide gate, providing a uniform clamping load and enabling operators to easily control the locking torque. The upper and lower slide gates are paired with a spring-loaded thrust frame structure to ensure good surface contact and appropriate clearance between the gates.
(3) Standardized Installation Procedures
Ensure that the refractory mortar for slide gates is correctly proportioned, thoroughly mixed, and free of foreign impurities.
Strictly prohibit the use of dried or caked mortar, as well as mixing used mortar with new mortar.
During installation, ensure the upper nozzle is thoroughly cleaned, and no residues are left in the slide gate frame or mechanism door.
Before installation, carefully inspect the mechanism and slide gate to confirm the normal operation of the horseshoe clamp and eccentric wheel. The slide gate must be free of deformation and cracks, and the flatness of the working surface should be checked.
Handle the slide gate carefully during placement and inspect for unevenness or wobbling after installation.
(4) Proper Operational Practices
During casting, prioritize the use of automatic steel casting programs to avoid manual large-amplitude or frequent opening/closing of the slide gate.
If oxygen lancing is required, ensure the slide gate is fully open, the tundish flux is as fully discharged as possible, and the oxygen pipe is kept as parallel as possible to the molten steel flow channel.
(5) Ladle Preparation Optimization
During ladle preparation and nozzle cleaning, except when replacing the upper nozzle, strictly prohibit oxygen lancing from inside the ladle to the outside.
Conduct oxygen cleaning as quickly as possible.
After preparation, move the slide gate drive mechanism to inspect the working surface of the slide gate. Replace the slide gate immediately if any abnormalities are detected.
Additionally, rationalize the production scheduling to ensure a proper ladle turnover rate, which is an important measure to extend the slide gate's life. If a ladle is left to cool for an excessively long time, replace the slide gate to prevent accidents.
After the implementation of the above measures, the service life of slide gates has been significantly extended (with an annual reduction of 700 sets in slide gate consumption, effectively lowering refractory material costs). Moreover, no molten steel leakage accidents caused by slide gate damage have occurred, leading to a substantial improvement in operational safety.
Part 3: Conclusion
With the development of rapid and efficient smelting-continuous casting technology, extremely stringent requirements have been imposed on the material properties and service performance of slide gates. Based on an extensive review of slide gate damage phenomena in practical applications, the key conclusions are as follows:
(1)Thermal shock damage, thermochemical corrosion, and improper operational factors are the three primary causes of slide gate damage.
(2)The main cause of thermal shock damage is the drastic temperature fluctuation in the working area of the slide gate during ladle turnover and service.
(3)The composition of modern high-value-added steel grades and the processes involved in molten steel treatment are highly prone to inducing thermochemical corrosion of slide gates.
(4)Operational factors during the installation, service, and preparation of slide gates are important external causes of damage.
Targeting these conclusions, the following measures are proposed to extend the service life of slide gates:
(1)Adopt alumina-zirconia-carbon (AZC) slide gates to ensure excellent corrosion resistance and thermal shock resistance.
(2)Utilize reasonably designed slide gate shapes and mechanism structures with uniform stress distribution, which is an effective measure to prolong service life.
(3)Standardize molten steel casting operations, which serve as a necessary guarantee for improving the service life of slide gates.
(4)Proper and thorough cleaning and maintenance of slide gates and sliding mechanisms are also crucial for achieving a long service life.