In the previous article, we discussed the continuous casting challenges of high-aluminum TRIP/TWIP steels. Today, we focus on an internal defect in continuous casting slabs that significantly affects the performance of downstream rolled products: centerline porosity. Centerline porosity refers to the presence of pores in the central region of the slab. If these pores cannot be welded shut during subsequent rolling, they lead to lamination, seams, and seriously affect the steel's density, impact toughness, and fatigue life. For products like heavy plates, seamless tube rounds, and forging blooms, controlling centerline porosity is a key quality focus. How can you effectively control centerline porosity in continuous casting slabs to improve internal density and mechanical properties? Wuxi WeiDa Cored Wire Co.,Ltd provides a comprehensive solution based on solidification end optimization and soft reduction technology.
The Formation Mechanism of Centerline Porosity: The "Final Relic" of Solidification Shrinkage
The formation of centerline porosity is closely related to the solidification process of the slab. As the slab solidifies layer by layer from the surface toward the center, the remaining liquid phase at the center undergoes solidification shrinkage in the final stage. If there is insufficient liquid phase to compensate for this shrinkage, pores – centerline porosity – form in the central region. The severity of centerline porosity depends on:
•Solidification rate: Slower solidification rates lead to more developed dendrites and greater likelihood of "bridging," hindering feeding.
•Steel superheat: Higher superheat results in longer solidification time and more severe porosity.
•Slab cross-sectional size: Larger cross-sections have longer solidification times and greater porosity tendency.
•Steel grade characteristics: High carbon steels and alloy steels have wider solidification ranges and are more prone to porosity.
The "Twin" Relationship Between Centerline Porosity and Centerline Segregation
Centerline porosity and centerline segregation often appear as "twins." The presence of porosity provides "accumulation" space for enriched solutes, exacerbating segregation; conversely, low-melting-point phases caused by segregation affect feeding, worsening porosity. Therefore, controlling centerline porosity and controlling centerline segregation are two sides of the same coin.
Our Solution: Synergy of Electromagnetic Stirring, Soft Reduction, and Composition Optimization
First, optimize electromagnetic stirring parameters to expand the equiaxed crystal zone. Mold electromagnetic stirring (M-EMS) and final electromagnetic stirring (F-EMS) can break dendrites and increase the proportion of equiaxed crystals. A wider equiaxed crystal zone means fewer opportunities for "bridging" and feeding channels. We recommend optimizing stirring intensity (200-400A) and frequency (2-5Hz) based on steel grade and slab cross-section.
Second, implement dynamic soft reduction technology. Soft reduction is one of the most effective means of controlling centerline porosity. Applying reduction force at the solidification end of the slab can:
•Mechanically close pores: Directly porosity.
•Compensate for solidification shrinkage: Reduce pores caused by volume shrinkage.
•Promote of enriched solutes: Also improve segregation.
We recommend controlling total reduction between 6-12mm based on steel grade and cross-section, and optimizing the reduction zone to apply the main reduction in the region with solid fraction 0.6-0.9.
Third, reduce steel superheat. Superheat is a fundamental factor affecting centerline porosity. We control target superheat within 15-25°C, which can significantly shorten solidification time and reduce porosity formation without sacrificing casting speed.
Fourth, optimize composition through cored wire technology. Although cored wire cannot directly control the solidification process, it can porosity by optimizing steel composition:
•Expand the equiaxed crystal zone: Feed titanium, boron, or rare earth cored wires to form fine heterogeneous nucleation sites, promoting equiaxed crystal formation.
•Refine grains: Microalloying elements (Nb, V, Ti) form fine carbonitrides that refine grains, indirectly improving porosity.
•Modify sulfides: Calcium treatment modifies MnS into globular CaS, reducing pores caused by sulfide aggregation.
Fifth, optimize the secondary cooling regime. Inappropriate secondary cooling can cause reheating of the slab surface (temperature recovery), exacerbating centerline porosity. We provide optimization services for dynamic secondary cooling models, adjusting water spray rates in different cooling zones in real-time based on steel grade and casting speed to avoid temperature recovery and ensure uniform slab temperature.
From "Improvement" to "Elimination"
Complete elimination of centerline porosity is very difficult, but it can be controlled at very low levels. For most products, a centerline porosity rating of 0.5-1.0 (according to YB/T 4003 standard) is sufficient to meet service requirements. For demanding products like oil pipeline steel and high-pressure vessel steel, a rating of 0 (no visible porosity) is required. Wuxi WeiDa's comprehensive solution has helped numerous customers achieve stable control of centerline porosity ratings below 0.5.
If you wish to improve the internal density of your continuous casting slabs to meet more stringent non-destructive testing standards, please visit https://www.weidamaterials.com/ to obtain our professional information on centerline porosity control technology.