In the previous article, we discussed microinclusion control technology for bearing steel. Today, we focus on a common problem during continuous casting that severely affects slab surface quality: slag entrapment. Slag entrapment occurs when molten steel flow in the mold entrains mold flux into or beneath the solidifying shell. The entrained slag droplets become large exogenous inclusions in the slab, leading to surface cracks, subsurface slag spots, and ultimately surface defects in the final rolled product. For high-end products like automotive panels, tinplate, and thin-gauge strips, slag entrapment is a "zero tolerance" defect. How can you effectively prevent slag entrapment during continuous casting to ensure slab surface quality and internal cleanliness? Wuxi WeiDa Cored Wire Co.,Ltd provides a comprehensive solution based on mold flow field optimization and mold flux property control.
The Formation Mechanism of Slag Entrapment: The "Consequence" of Turbulence and Shear
Slag entrapment primarily occurs near the meniscus of the mold. After molten steel exits the submerged entry nozzle (SEN) ports, it forms complex flow patterns within the mold. When the steel flow velocity is too high, the flow field is unstable, or the mold flux properties are mismatched, the liquid slag layer can be "torn" or "entrained" by the steel, forming slag droplets that become trapped in the solidifying shell. There are three main types of slag entrapment:
•Shear entrapment: High-speed return flow of steel scours the meniscus, "shearing" liquid slag into the steel.
•Vortex entrapment: Steel flow forms vortices that "suck" liquid slag deep into the steel.
•Argon bubble-carried entrapment: Argon bubbles injected through the SEN carry liquid slag into the steel as they float up.
The "Drivers" of Slag Entrapment: Which Factors are at Play?
The main factors leading to slag entrapment include:
•Incorrect SEN port angle: Ports with too small a downward angle cause steel flow to impact the meniscus; too large an angle may cause the return flow zone to be too high.
•Excessively high casting speed: Higher casting speed means higher steel flow rate and greater risk of entrapment.
•Inappropriate mold flux properties: Flux with too low viscosity or density is more easily entrained.
•Excessively high argon flow rate: Too high an argon flow rate exacerbates turbulence at the meniscus.
•Mold level fluctuation: When level fluctuations exceed ±5mm, the risk of slag entrapment increases dramatically.
Our Solution: Flow Field Control and Mold Flux Matching
Wuxi WeiDa provides a complete set of slag entrapment prevention solutions, covering equipment parameters, operating practices, and consumables.
First, optimize submerged entry nozzle design. The SEN port angle, shape, and immersion depth are for controlling the mold flow field. We recommend optimal SEN parameters based on steel grade and casting speed. For low carbon steels, we recommend SENs with ports angled 15-20° downward; for medium carbon steels, ports angled 20-25° downward; for high carbon steels, ports angled 25-30° downward. Additionally, we recommend controlling SEN immersion depth between 120-180mm.
Second, customized mold flux selection. Mold flux properties directly affect slag entrapment tendency. Our continuous casting mold fluxes can be customized based on your steel grade and casting speed:
•Viscosity: For high-speed (>1.5m/min) low carbon steels, select low viscosity (0.8-1.2 poise) flux to ensure appropriate liquid slag layer thickness and reduce shear force.
•Density: Select flux with appropriate density to form a stable liquid slag layer on the steel surface that is not easily entrained.
•Crystallization behavior: For steel grades prone to entrapment, select weakly crystalline flux, which has a more stable liquid slag layer.
Third, precisely control argon flow rate. Argon injected through the SEN prevents nozzle clogging, but excessive argon flow is a major cause of slag entrapment. We recommend using a controllable, pulsed argon injection mode, controlling the argon flow rate within the range of 3-8 liters/minute, to minimize disturbance to the meniscus while ensuring clogging prevention.
Fourth, stabilize mold level control. Use an electromagnetic level control system to keep mold level fluctuations within ±3mm. At the same time, maintain a stable casting speed, avoiding changes.
Fifth, optimize SEN immersion depth and alignment. SEN immersion depth should be adjusted dynamically based on casting speed and mold flux properties. Too shallow immersion exacerbates meniscus disturbance; too deep immersion causes the return flow zone to rise. Also, ensure alignment accuracy between the SEN and the mold to prevent flow asymmetry that leads to entrapment.
Online Monitoring and Rapid Response for Slag Entrapment
Even with properly set process parameters, anomalies can still occur. We recommend equipping mold level fluctuation monitoring systems and thermal cameras to monitor meniscus conditions in real-time. Once abnormal fluctuations are detected, rapid adjustments to casting speed, argon flow rate, or flux addition frequency can nip slag entrapment defects in the bud.
The Last Line of Defense for High-Quality Slabs
Slag entrapment is one of the most common surface quality defects during continuous casting, but it is also completely preventable. With Wuxi WeiDa's systematic approach – covering SEN design, flux selection, argon control, and level stabilization – you can significantly reduce the incidence of slag entrapment and produce high-quality slabs free of entrapment defects.
If you are troubled by slag entrapment defects and wish to improve slab surface quality and rolled product yield, please visit https://www.weidamaterials.com/ to obtain our professional technical information on mold metallurgy and slag entrapment prevention.