LF Refining Slag System Optimization for 20 Steel Pipe Industrial Trial

Literature Overview

This study by Zhu Fuqiang et al. (2020), published in the journal Special Steel (Vol. 41, No. 5, pp. 45-47), addresses a critical metallurgical challenge in the production of 20 steel pipe — the optimization of the LF (Ladle Furnace) refining slag system at Zhongtian Iron and Steel Group. The research team, comprising engineers from Zhongtian Steel and metallurgical scientists from Anhui University of Technology, conducted both thermodynamic and kinetic analyses of the existing slag system before designing an improved formulation. The work was conducted on a 100 t LF, which is a representative capacity for modern continuous casting operations feeding pipe mills.

Existing Slag System Analysis

The conventional slag system used for 20 steel pipe production at the studied facility consisted of the following composition ranges:

The wide compositional spread in SiO₂ (4–17%) and CaO (40–65%) indicates significant process variability, which inherently limits the consistency of inclusion removal performance. The study's kinetic and thermodynamic analysis revealed that this broad window allowed the slag viscosity to fluctuate, sometimes falling below the optimal range for inclusion flotation. The authors identified that the existing slag system did not consistently maintain the desired slag viscosity of 1.5–2.0 Pa·s, which is the critical parameter governing the flotation kinetics of non-metallic inclusions from the molten steel.

Optimized Slag System Design

Based on the thermodynamic and kinetic modeling, the authors proposed a refined slag composition:

The key design philosophy was to raise the basicity (CaO/SiO₂ ratio) from approximately 2.4–16.3 in the original system to a more controlled 3.9–10.8 in the optimized system, while simultaneously increasing the Al₂O₃ content to enhance the slag's capacity to adsorb alumina-type inclusions. The increased Al₂O₃ content serves a dual purpose: it acts as a structural component that stabilizes the slag's liquidus temperature and improves the slag's wetting behavior toward oxide inclusions, thereby promoting their detachment from the steel surface and subsequent flotation.

Industrial Trial Results

The industrial trial results demonstrated that the optimized slag system achieved a significant improvement in inclusion control. The most notable finding was that Class B (round, dendritic, or clustered oxide inclusions) ratings were consistently maintained at ≤1.0 per the relevant inclusion rating standards (typically GB/T 10561 or ISO 4967). This is a meaningful improvement for 20 steel pipe, where inclusion morphology and distribution directly affect the material's fatigue resistance, formability during pipe rolling, and resistance to hydrogen-induced cracking during hydrostatic testing.

From a process engineering perspective, the narrower compositional window of the optimized slag system also translates to more consistent LF operation. Operators can more reliably achieve the target viscosity of 1.5–2.0 Pa·s, which reduces the trial-and-error approach to slag adjustment during production. This consistency is particularly important for pipe-grade steels, where the downstream pipe rolling and forming operations are highly sensitive to the cleanliness and homogeneity of the ingot or strand.

Engineering Practice Implications

In practical pipe manufacturing, the quality of the steel feedstock is the foundation upon which all subsequent processing steps depend. For seamless pipe production via the piercing-rolling route, inclusions that are too large or improperly shaped can initiate surface defects during rolling, leading to scrap or rework. For welded pipe production (ERW, HFW, LSAW), inclusion-rich zones can become preferential sites for weld cracking during the welding process, particularly in the heat-affected zone where thermal cycling creates additional microstructural complexity.

The study's approach — combining thermodynamic modeling with kinetic analysis before industrial trial — represents a sound engineering methodology. The PDCA cycle is clearly evident: the existing process was analyzed (Plan), the optimized slag was designed and trialed (Do), results were evaluated against inclusion rating criteria (Check), and the findings can be institutionalized into standard operating procedures (Act). This methodology is directly transferable to other steel grades used in pipe production, such as API 5L grades, 16Mn, Q345, and low-alloy high-strength grades.

Key Reflections

One important insight from this study is the recognition that slag optimization is not merely about achieving a target basicity but about controlling the entire compositional window to maintain consistent physical properties. The viscosity constraint of 1.5–2.0 Pa·s is not arbitrary — it represents the optimal balance between sufficient fluidity for inclusion flotation and adequate surface tension for slag-steel interface stability. In my own experience with pipe-grade steelmaking, I have observed that operators often focus narrowly on the CaO/SiO₂ ratio while neglecting the Al₂O₃ content, which can lead to inconsistent results even when basicity is nominally controlled.

Another practical consideration is the interaction between the LF slag system and the downstream continuous casting process. The slag composition optimized in this study must be compatible with the mold powder used in continuous casting. If the LF slag leaves a thick layer of modified slag on the steel surface, the transition to the mold powder must be managed carefully to avoid re-inclusion or surface defects on the strand. This interface management is often overlooked in slag optimization studies but is critical for pipe-grade steel quality.

The study's focus on Class B inclusions is appropriate for 20 steel pipe, where the primary concerns are formability and resistance to cracking during forming and welding. However, for higher-grade pipe applications such as CRA (Corrosion Resistant Alloy) pipe or low-temperature service pipe, additional attention to Class C (elongated oxide inclusions) and sulfide inclusion ratings would be necessary. The methodology presented here provides a solid framework that can be extended to these more demanding applications.

This study exemplifies how systematic metallurgical optimization, grounded in both thermodynamic and kinetic understanding, can deliver measurable improvements in steel quality for pipe manufacturing. The industrial trial approach, conducted at 100 t scale, provides confidence that the results are directly applicable to production conditions rather than being limited to laboratory-scale observations.