Zhuojin Pipe Fitting Co., LtdAnalysis and Control of Internal Surface Scaling Defects in JS25Mn Steel Pipes
Literature Overview
The paper authored by Li Hongguang, Chen Tianming, Chen Liang, and Guo Dianrong, published in Iron, Steel, Vanadium and Titanium in 2016 (Vol. 37, No. 5, pp. 153-156), presents a comprehensive defect analysis and control strategy for internal surface scaling (peeling) defects in JS25Mn steel pipes. The authors are affiliated with the Pangang Group Research Institute (National Key Laboratory for Comprehensive Utilization of Vanadium-Titanium Resources) and Pangang Group Titanium Extraction Steelmaking Plant.
Defect Description and Classification
Internal surface scaling in seamless steel pipes manifests as thin layers of oxide or inclusion-rich material that can be lifted or peeled from the inner pipe surface. This defect is particularly detrimental in applications where internal surface integrity is critical, such as hydraulic tubing, automotive structural components, and chemical processing equipment. The defect in JS25Mn pipes was identified during customer end-use testing, where the scaled material was found to loosen during mechanical processing or service loading.
Root Cause Analysis
The authors conducted a systematic metallurgical investigation using macroscopic examination, optical microscopy, scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The analysis revealed two primary mechanisms contributing to the scaling defect:
Mechanism 1: High-Melting-Point Inclusions
The steel contained high-melting-point non-metallic inclusions, particularly titanium-bearing compounds (TiN, TiO, Ti₂O₃). These inclusions have melting points significantly higher than the steel matrix and remain solid throughout the steelmaking and casting process. When located near the pipe inner surface after piercing and rolling, they create weak interfaces that facilitate delamination.
Mechanism 2: Segregation at Center Porosity
Low-melting-point non-metallic inclusions (such as MnS, SiO₂, and complex sulfide oxides) segregated and accumulated at the center porosity zones of the continuous casting billet. During the subsequent piercing and rolling operations, the center region of the billet becomes the inner surface of the pipe. Consequently, inclusion-rich center porosity areas are transferred to the pipe ID, creating localized weak zones prone to scaling.
| Defect Factor | Type | Location in Billet | Effect on Pipe |
|---|---|---|---|
| TiN, TiO₂ | High-melting-point inclusions | Distributed throughout | Weak interfacial bonding at pipe ID |
| MnS, SiO₂ complexes | Low-melting-point inclusions | Center porosity zone | Segregation at pipe ID |
| Center porosity | Structural defect | Billet center | Transferred to pipe ID after piercing |
Control Measures and Solutions
The authors implemented a two-pronged control strategy:
Steelmaking Optimization
- Vacuum degassing treatment: Implementation of vacuum treatment for the molten steel significantly reduced dissolved oxygen and nitrogen content, thereby decreasing the formation of oxide and nitride inclusions. The oxygen content was reduced from the typical 15-20 ppm to below 10 ppm.
- Refined slag control: Modified the refining slag composition to enhance inclusion modification and flotation efficiency.
- Temperature control: Optimized tapping temperature and reheating temperature to minimize secondary oxidation during transfer.
Continuous Casting Process Optimization
- Mold flux optimization: Adjusted the mold flux composition and viscosity to improve inclusion flotation and reduce entrapment.
- Casting speed adjustment: Modified casting speed profiles to reduce center porosity formation.
- Secondary cooling optimization: Refined the secondary cooling water distribution to promote equiaxed grain formation and reduce center shrinkage.
Engineering Significance
The case study demonstrates a fundamental principle in seamless pipe manufacturing: the internal surface quality of the finished pipe is directly determined by the internal quality of the continuous casting billet, particularly the center region. The piercing operation transforms the billet center into the pipe bore, making center porosity and center segregation the most critical factors for ID quality.
From a quality control perspective, this case highlights the importance of:
- Billet internal quality inspection (UT scanning for center porosity)
- Inclusion analysis as a root cause tool
- The relationship between steelmaking cleanliness and final product surface quality
- The necessity of process optimization at the source rather than attempting to repair defects downstream
Study Insights
This paper provides an excellent example of the PDCA (Plan-Do-Check-Act) cycle applied to defect elimination. The systematic approach—starting from defect identification, through metallurgical root cause analysis, to process modification and verification—serves as a model for any quality improvement project in steel pipe manufacturing. The emphasis on steel liquid cleanliness as a root cause factor reinforces the well-established principle that "clean steel makes clean pipes." For engineers involved in pipe procurement, this literature underscores the importance of requiring billet-level quality documentation and inclusion rating specifications in supply contracts.