The opening rate of the continuous casting ladle—the success rate of initiating the pour—is a key indicator of production continuity and stability. However, many steel plants frequently encounter low opening rates in actual operations; this not only hampers production efficiency but can also adversely affect the quality of the cast billets. This article provides an in-depth analysis of the causes behind low opening rates—examining factors such as the performance of free-opening sand, ladle maintenance, and operational practices—and proposes targeted measures for improvement.
1.Performance of Free-Flowing Sand: A Key Factor Determining the Success of Casting Initiation
As the core filling material for ladle slide gates, the physicochemical properties of free-flowing sand directly determine the success of the initial casting opening. Currently, inadequate sand performance is the primary cause of low opening success rates, specifically manifesting in the following ways:
Uncontrolled Sintered Layer Formation
High-quality free-flowing sand must exhibit the characteristics of “no sintering at low temperatures” and “erosion resistance at high temperatures.” If the sand’s thermal conductivity is too high or its sintering temperature too low, heat from the molten steel accelerates the thickening of the sintered layer, preventing the molten steel’s hydrostatic pressure from breaking through it. For instance, due to the low melting point of silica-based sand, the sintered layer thickness can exceed design limits when tapping temperatures surpass 1680°C, thereby reducing the opening success rate.
Insufficient Erosion Resistance
Oxides such as MnO and FeO in the molten steel react with the SiO₂ in the sand to form low-melting-point compounds (e.g., MnO·SiO₂, melting point 1291°C), leading to excessive sintering. Experiments at a steel plant demonstrated that when ordinary silica sand was used, the crust thickness on the inner wall of the nozzle reached 20 mm; however, switching to zirconia-based sand reduced this thickness to less than 5 mm.
Suboptimal Particle Size Distribution
The flowability of the sand depends on particle shape and size distribution. Angular particles tend to form “bridges,” obstructing natural gravity flow. Experiments show that chromite-based sand with spherical particles achieves an angle of repose below 26°—a 30% reduction compared to angular particles—significantly improving packing density. Furthermore, the minimum particle size must be strictly controlled above 0.5 mm to prevent molten steel penetration, which would otherwise increase the bonding strength between particles.
2.Ladle Maintenance: Overlooked Details and Hidden Risks
As the container for molten steel, the maintenance condition of the ladle has a decisive impact on the casting success rate. Common maintenance defects include:
Incomplete slag removal
After casting, the ladle needs to be turned over to remove slag. However, the significant temperature drop at the ladle opening can easily lead to slag hardening. If not cleaned promptly, the hardened slag may roll into the nozzle during ladle turning, mixing with the guide sand to form a solid blockage. Statistics from a steel plant show that for every 10mm increase in the thickness of slag at the ladle opening, the risk of casting failure increases by 15%.
Defects in nozzle seat brick material
The material of the nozzle seat brick directly affects its thermal conductivity. Carbonaceous materials have high thermal conductivity, easily causing the guide sand to overheat and sinter prematurely. After switching to high-alumina seat bricks, a steel plant saw a 50°C reduction in the inner wall temperature of the nozzle, and a significant improvement in the flowability of the guide sand.
Deterioration in the performance of the slide plate mechanism
Excessive gaps or uneven surfaces in the slide plate mechanism can lead to molten steel leakage. The seeping molten steel reacts with the guide sand to form a high-strength solidified layer. Regularly checking the skateboard gap (which should be ≤0.5mm) and using laser cladding technology to repair surface damage can extend the skateboard’s lifespan to more than 120 cycles.
3.Operating procedures: Lack of refined control
Minor deviations in operational procedures can accumulate into systemic risks, specifically manifested as:
Manual Sand Addition Errors
Traditional manual sand addition methods have a hit rate of only 70%-80%, and it’s difficult to guarantee sufficient sand quantity in the nozzle. After a steel plant switched to tubular ladle sand addition technology, the standard deviation of the sand pile height decreased from 15mm to 5mm, and the casting start rate increased by 12%. This technology achieves precise filling through gravity, shortening the sand addition time per ladle to 30 seconds.
Excessive Steel Transfer Time
For every 30-minute extension of the interval between converter tapping and continuous casting start, the sintered layer thickness of the guide sand increases by 0.8mm. By optimizing the LF+RH refining process, a steel plant reduced the transfer time from 180 minutes to 120 minutes, correspondingly increasing the casting start rate to 95%.
Inappropriate Tapping Temperature Control
High-alloy steel grades such as pipeline steel, due to their higher tapping temperatures (typically ≥1650℃), place higher demands on the refractoriness of the guide sand. Using modified chromium-based diverting sand with a Cr₂O₃ content ≥30% can reduce the compressive strength after high-temperature treatment at 1550℃ from 9.35MPa to 2.47MPa, significantly reducing the strength of the sintered layer.
4.System Solution: Multi-dimensional Collaborative Optimization
Improving the initial casting rate requires a three-pronged control system encompassing materials, equipment, and processes:
Material Upgrades
Promoting the use of zirconium-chromium composite sintering sand, and achieving precise control of sintering temperature by adjusting the Cr₂O₃/Al₂O₃ ratio (optimal value 2:1). After implementation at a steel plant, the initial casting rate increased from 82% to 98%, with only a 0.8 yuan increase in cost per ton of steel.
Equipment Upgrades
Introducing an intelligent sand-adding system, integrating laser positioning, automatic weighing, and pressure monitoring functions, achieving a sand-adding error of ≤1%. Combined with nozzle inner wall coating technology (such as boron nitride coating), the coefficient of friction can be reduced to below 0.1.
Process Standardization
Develop the “Steel Ladle Opening Operation Procedure,” clearly defining the control range of key parameters:
Tap Temperature: 1580-1620℃ (dynamically adjusted according to steel grade)
Passing Time: ≤150 minutes (≤120 minutes for special steel grades)
Slide Opening Speed: ≥0.5m/s (fully open in one go)
5.Conclusion
Improving the continuous casting ladle start-up rate is a systematic project requiring collaborative breakthroughs across multiple dimensions, including materials science, equipment engineering, and process control. By introducing high-performance diversion sand, intelligent sand-adding equipment, and standardized operating procedures, a steel plant has achieved a start-up rate of ≥97% for six consecutive months, extending the continuous casting time per heat to over 48 hours. This demonstrates that through technological innovation and management optimization, efficient and stable operation of continuous casting production is entirely achievable, laying a solid foundation for the green and low-carbon transformation of the steel industry.
