Following our previous discussion on rare earth treatment for heavy rail steel, which highlighted the importance of inclusion morphology control on material properties, today we focus on a small but extremely harmful defect in continuous casting slabs: pinholes. These tiny pores, typically less than 2mm in diameter, are often distributed on or just beneath the slab surface. They cannot be welded shut during subsequent rolling, leading to lamination in plates, wire breakage, or leakage in tubes. For high-end products like automotive outer panels, food-grade tinplate, and high-pressure cylinder steel, pinholes are unacceptable and fatal defects. How can you fundamentally prevent pinhole defects in continuous casting slabs to ensure product density and airtightness? Wuxi WeiDa Cored Wire Co.,Ltd provides systematic solutions ranging from molten steel degassing to protective casting.
The Root Cause of Pinholes: The "Imprint" of Bubbles
Pinholes are essentially tiny bubbles that have been "frozen" inside the slab during solidification. These bubbles primarily come from three sources: First, hydrogen and nitrogen in the molten steel. When the hydrogen or nitrogen content exceeds its solubility at that temperature, the supersaturated gas precipitates to form bubbles. Second, argon entrapment. Gases used for stirring or protection (like argon) during refining or continuous casting, if not effectively removed, can also be captured by the solidification front. Third, gas generation from mold flux decomposition. Mold fluxes with high moisture or carbonate content can decompose and generate gases upon contact with high-temperature molten steel. Once these bubbles contact the solidifying shell, they leave their "imprint" on the surface, forming pinholes.
Our Solution: Source Prevention and Process Optimization
Wuxi WeiDa offers a complete strategy for pinhole prevention. First, rigorous degassing treatment. During ladle refining, our high-efficiency synthetic slag cored wires rapidly form a low-oxygen, high-basicity refining slag, creating optimal conditions for vacuum degassing (VD/RH) . Simultaneously, we recommend using high-purity aluminum wire for deep deoxidation, as lowering the oxygen content significantly increases the solubility of hydrogen and nitrogen in the molten steel, thereby suppressing their precipitation.
Second, optimize argon stirring practices. Argon is a double-edged sword: appropriate stirring promotes inclusion flotation and gas removal; excessively strong stirring can entrain argon bubbles deep into the molten steel, where they are captured by the solidifying shell before they can float out. Our technical team can help you optimize argon flow rate and stirring time to find the best balance for "degassing" rather than "gas entrapment."
Third, choose low-gas-source mold fluxes. Wuxi WeiDa recommends using high-quality continuous casting mold fluxes with low moisture and low carbonate content. We strictly control the loss on ignition of raw materials during mold flux production, ensuring they do not generate additional gases upon contact with molten steel. This is particularly important for casting high-aluminum steels (e.g., TRIP steel, TWIP steel), as high-aluminum steel reacts with SiO₂ in the mold flux, generating gases.
Fourth, prevent steel reoxidation. If molten steel contacts air during transfer from tundish to mold, it will reoxidize, forming Al₂O₃ and absorbing nitrogen. We provide optimized solutions for long nozzle argon sealing devices and submerged entry nozzles to ensure full protective casting.
Process Monitoring: Rapid Identification and Response
Even with optimal processes, real-time monitoring and rapid response are essential. Wuxi WeiDa's technical services include: assisting customers in establishing rapid detection methods for pinhole defects (e.g., acid etching inspection, ultrasonic testing), and correlating defect characteristics with process parameters. Once a pinhole tendency is detected, we can quickly trace the cause – whether insufficient degassing, excessive argon flow, or mold flux issues – and take targeted corrective actions.
If you are troubled by pinhole defects in continuous casting slabs and aim to produce pore-free, high-density high-end steels, please visit https://www.weidamaterials.com/ to obtain our technical white paper on pinhole prevention in continuous casting.