In the previous article, we discussed how to prevent surface cracks during peritectic steel continuous casting. Today, we focus on a core internal defect in high-carbon steel continuous casting that affects the performance of final products: centerline segregation. High-carbon steels (carbon content >0.50%) are widely used in high-end products such as steel rails, wire rope, tire cord, and spring steel. These applications demand extremely high fatigue performance, wear resistance, and tensile strength from the steel. However, the segregation of carbon, phosphorus, sulfur, and other elements at the center of the continuous casting slab leads to the formation of martensitic or bainitic hard phases and elongated MnS inclusions in the central region, which become crack sources during subsequent rolling or drawing. How can you effectively control centerline segregation in high-carbon steel continuous casting slabs to produce high-quality steel that meets the requirements of high-end applications? Wuxi WeiDa Cored Wire Co.,Ltd provides a comprehensive solution based on electromagnetic stirring, soft reduction technology, and microalloying.
The Formation Mechanism of Centerline Segregation: "Solute Enrichment" at the Solidification End
The solidification of a continuous casting slab progresses from the surface to the center. During this process, solute elements (carbon, phosphorus, sulfur, manganese, etc.) have lower solubility in the solid phase than in the liquid phase, so they are "rejected" into the remaining liquid phase. In the final stages of solidification, "bridging" between dendrites traps the solute-enriched liquid in the central region, forming centerline segregation. High-carbon steels have a much greater tendency for segregation than low-carbon steels due to their high carbon content and wide solidification range. The harm of centerline segregation is mainly reflected in three aspects. First, formation of hard phases. The high-carbon, high-manganese segregation zones form martensite or bainite during cooling. These hard phases have mismatched deformation capacity with the surrounding pearlite structure, making them prone to cracking under stress. Second, aggregation of sulfides. The enrichment of sulfur and manganese in the segregation zone leads to the formation of large, elongated MnS inclusions, which severely damage the transverse properties and fatigue life of the steel. Third, heat treatment response. Segregation leads to hardness non-uniformity during quenching and tempering, affecting the performance consistency of the final product.
Limitations of Traditional Methods
Traditional methods for reducing centerline segregation – such as reducing superheat and increasing secondary cooling intensity – while effective, have limitations. Reducing superheat can reduce the length of columnar crystals and expand the equiaxed crystal zone, but excessively low superheat increases the risk of nozzle clogging. Increasing secondary cooling intensity can accelerate solidification, but excessively strong cooling can cause slab surface cracks. Electromagnetic stirring (EMS) and soft reduction (SR) are currently the two most effective means, but their parameters require optimization based on the steel grade and slab cross-section; otherwise, their effectiveness is limited.
Our Solution: Synergy of Electromagnetic Stirring, Soft Reduction, and Microalloying
Wuxi WeiDa Cored Wire Co.,Ltd, although not a manufacturer of continuous casting equipment, leverages its deep understanding of metallurgical processes to help you maximize the effectiveness of your existing equipment and provide auxiliary optimization through cored wire technology.
First, optimize electromagnetic stirring parameters to expand the equiaxed crystal zone. Mold electromagnetic stirring (M-EMS) and final electromagnetic stirring (F-EMS) are core methods for reducing centerline segregation. M-EMS generates a horizontal rotating flow field in the mold, breaking up columnar crystals and forming more equiaxed crystal nuclei. F-EMS enhances stirring at the solidification end, homogenizing the central composition. We recommend, based on your slab cross-section and steel grade, controlling the M-EMS stirring intensity at 200-400A, frequency 2-5Hz; and the F-EMS stirring intensity at 100-300A, frequency 3-7Hz.
Second, implement dynamic soft reduction technology to mechanically squeeze and promote of enriched solutes. Applying reduction force at the solidification end of the slab (in the region with solid fraction 0.6-0.9) can "push back" the enriched solutes in the central region into the liquid phase while simultaneously closing already-formed porosity. We recommend controlling the total reduction amount between 6-12mm based on the steel grade and cross-section. For high-carbon steels, due to their wide solidification range, the reduction zone should be appropriately extended.
Third, reduce steel superheat to decrease columnar crystal length. Superheat is a fundamental factor affecting the proportion of equiaxed crystals. For high-carbon steels, we recommend controlling the tundish steel superheat at 15-25°C. For every 10°C reduction in superheat, the proportion of equiaxed crystals can increase by 5-10%. However, this needs to be done while ensuring no nozzle clogging. Our tundish heating technology and nozzle clogging prevention measures can help you achieve low-superheat casting.
Fourth, optimize composition through cored wire technology to promote equiaxed crystal formation. This is a unique advantage of Wuxi WeiDa. Although cored wire cannot directly control the solidification process, it can mitigate segregation by optimizing the steel's "intrinsic" quality.
•Rare earth treatment: Globular rare earth sulfides formed by rare earths (cerium, lanthanum) can act as heterogeneous nucleation sites, promoting equiaxed crystal formation while modifying harmful MnS inclusions. Recommended addition: 20-50 grams per ton of steel.
•Titanium microalloying: TiN particles (size 1-3 microns) formed by the reaction of titanium with nitrogen are highly efficient nucleation sites. By feeding ferro titanium cored wire and controlling titanium content at 0.01%-0.03%, the equiaxed crystal zone can be significantly expanded.
•Boron microalloying: Trace amounts of boron (0.001%-0.003% ) can refine the solidification structure, but must be with titanium to prevent BN formation.
Fifth, optimize the secondary cooling regime to avoid "temperature recovery" phenomenon. An inappropriate secondary cooling regime can cause reheating of the slab surface (temperature recovery), exacerbating centerline segregation. We provide optimization services for dynamic secondary cooling models, adjusting water spray rates in different cooling zones in real-time based on your casting speed and steel grade, ensuring uniform slab temperature and avoiding temperature recovery.
Example of Comprehensive Process Parameter Optimization
Taking U75V rail steel (carbon content approximately 0.75%) as an example, Wuxi WeiDa's technical team assisted a customer in optimizing the following parameters: M-EMS stirring current 300A/3Hz, F-EMS stirring current 200A/5Hz, total soft reduction amount 8mm, superheat controlled at 20±3°C, along with feeding rare earth cored wire (30 grams per ton). After implementation, the centerline segregation rating was reduced from 2.5-3.0 to 1.0-1.5 (according to YB/T 4003 standard), and the transverse impact toughness increased by 35% .
From "Passive Acceptance" to "Active Control"
Centerline segregation is a phenomenon that cannot be completely avoided during continuous casting, but it can be controlled at very low levels. With Wuxi WeiDa's systematic solution – electromagnetic stirring parameter optimization, precise dynamic soft reduction control, superheat management, and microalloying assistance – you can minimize the impact of centerline segregation and produce high-quality high-carbon steel that meets the requirements of high-end applications.
If you are producing high-carbon steel and wish to reduce the impact of centerline segregation on product performance, please visit our website https://www.weidamaterials.com/ to obtain the complete technical solution for centerline segregation control in high-carbon steel continuous casting.