Analysis and Control of Internal Surface Scaling Defects in JS25Mn Steel Pipes

Literature Overview

This paper, published in the journal "Iron and Steel Vanadium Titanium" in 2016 (Vol. 37, Issue 5, pp. 153-156), presents a systematic investigation into the internal surface scaling (peeling) defects observed in JS25Mn steel pipes produced by Pangang Group. The study was conducted by researchers from the Pangang Group Research Institute, affiliated with the State Key Laboratory of Comprehensive Utilization of Vanadium and Titanium Resources, in collaboration with the Pangang Group Titanium Extraction Steelmaking Plant. The authors employed metallographic examination, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) to identify the root causes of the defect and propose effective control measures. The work is particularly valuable for engineers dealing with medium-carbon low-alloy steel pipes where internal surface integrity is critical for downstream applications such as mechanical processing and pressure vessel fabrication.

Root Cause Analysis

The study identifies two primary mechanisms responsible for the internal surface scaling defects in JS25Mn steel pipes. The first mechanism involves high-melting-point non-metallic inclusions, particularly titanium-containing compounds, which remain dispersed throughout the steel matrix even after secondary refining. These inclusions create localized stress concentrations during subsequent rolling or forming operations, initiating micro-cracks that propagate to the pipe surface. The second mechanism involves the segregation and accumulation of low-melting-point non-metallic inclusions at locations of center porosity within the continuous casting (CC) billet. During the rolling process, these accumulated inclusions are forced toward the surface, resulting in visible scaling or peeling.

The following table summarizes the key defect characteristics and their metallurgical origins:

The authors conducted detailed SEM observations of the defect regions and found that the high-melting-point inclusions were predominantly TiN and titanium oxide particles with sizes ranging from 5 to 30 micrometers. These particles were identified through EDS mapping, which confirmed the presence of titanium, nitrogen, and oxygen in the inclusion phases. The low-melting-point inclusions were primarily MnS and various silicate oxides, which tended to migrate toward the center of the CC strand during solidification due to the mushy zone dynamics.

Process Optimization and Control Measures

The corrective actions proposed in the study are centered on two main pillars: improving steel cleanliness through vacuum treatment and optimizing the continuous casting process parameters to enhance billet internal quality.

For steel cleanliness improvement, the authors implemented a dual vacuum degassing process. The first stage involved argon stirring under vacuum to reduce dissolved hydrogen and nitrogen levels. The second stage employed vacuum induction melting or vacuum arc remelting techniques to remove non-metallic inclusions through flotation. The target cleanliness level was specified as follows:

For CC process optimization, the key adjustments included reducing the casting speed from 1.2 m/min to 0.9 m/min, optimizing the mold level control system to maintain a stable meniscus, and adjusting the secondary cooling water distribution to promote more uniform solidification. The reduced casting speed allowed for better inclusion flotation and more complete solidification before the strand exited the mold. The optimized secondary cooling profile reduced the center porosity rate from approximately 8% to less than 2% of the billet cross-section.

Engineering Practice Implications

From an engineering practice perspective, this study highlights several important lessons for steel pipe manufacturers. First, the quality of the CC billet is the foundation of pipe surface quality, and any investment in billet quality improvement will yield significant returns in reducing downstream defect rates. Second, titanium-containing inclusions in medium-carbon low-alloy steels should be considered as a primary concern for surface quality, particularly when the steel is subjected to significant deformation during rolling. Third, the combination of vacuum treatment and CC parameter optimization represents a cost-effective approach to solving surface defect problems without requiring major capital investment in new equipment.

In practical applications, the authors recommend implementing a comprehensive quality control system that includes:

1. Pre-pouring cleanliness monitoring using inline inclusion sensors or periodic laboratory analysis
2. Billet ultrasonic testing to detect center porosity before rolling
3. Statistical process control (SPC) of CC parameters to maintain consistent billet quality
4. Surface inspection of finished pipes using eddy current or magnetic particle testing to verify defect elimination

The user feedback confirmed that the optimized production process effectively eliminated the internal surface scaling defects, and the product performance met the end-user requirements for downstream mechanical processing and fabrication. This case study demonstrates the effectiveness of a systematic root cause analysis approach combined with targeted process optimization in resolving persistent quality issues in steel pipe manufacturing.

Study Insights and Reflections

The study provides a clear demonstration of how metallurgical fundamentals can be applied to solve practical manufacturing problems. The identification of two distinct defect mechanisms, each requiring different control strategies, underscores the importance of thorough defect analysis before implementing corrective actions. A blanket approach to improving cleanliness alone would not have addressed the center porosity-related inclusion segregation problem, and vice versa. The integration of vacuum treatment with CC process optimization represents a holistic solution that addresses both mechanisms simultaneously.

One area for further investigation would be the long-term stability of the optimized process under varying production conditions, such as changes in raw material composition or seasonal variations in ambient temperature. Additionally, the economic analysis of the vacuum treatment investment versus the cost savings from reduced defect rates and improved yield would provide valuable data for decision-making in other steel plants. Overall, this paper serves as an excellent reference for engineers facing similar surface quality challenges in medium-carbon low-alloy steel pipe production.