External Folding Defect Analysis and Process Control in 37Mn5 Steel Pipe

Literature Overview

The paper by Wei Jun and colleagues, published in Special Steel (Volume 31, Issue 1, 2010, pages 30-32), investigates the root cause of external folding defects in 37Mn5 steel pipes with dimensions of Phi 85 mm x 8 mm. The study, conducted jointly by the University of Science and Technology Beijing and Chengde Jianlong Special Steel Co., Ltd., combines metallurgical microstructural analysis with process optimization to identify and mitigate a critical quality issue in seamless steel pipe manufacturing. The chemical composition of the investigated steel is 0.39% C and 1.30% Mn, which places it in the medium-carbon manganese structural steel category commonly used for mechanical tubing and structural applications.

Core Technical Findings

The fundamental discovery of this research is that brittle inclusions located at grain boundaries, specifically silicates and SiO2 particles with dimensions of approximately 20 micrometers, are the primary cause of external folding formation in 37Mn5 steel pipes. During the hot rolling and piercing operations inherent to seamless pipe production, these brittle intergranular inclusions act as stress concentrators. Under the intense radial compressive and tangential tensile stresses experienced at the pipe outer surface during rolling, microcracks initiate at these inclusion sites and propagate along the grain boundaries, ultimately manifesting as external folds.

The metallurgical mechanism can be understood through the following sequence: during the final stages of continuous casting, oxygen dissolved in the molten steel reacts with silicon and other deoxidation products to form silicate inclusions. When the refining and deoxidation practices are insufficient, these inclusions remain entrapped at austenite grain boundaries. As the billet undergoes hot working, the strain energy concentrates at these brittle interfacial regions, leading to delamination and folding on the outer surface of the rolled pipe.

Process Control Parameters and Optimization

The authors propose a comprehensive set of metallurgical processing controls to suppress inclusion formation and ensure clean steel chemistry. The following table summarizes the critical process parameters and their target values:

Engineering Practice Integration

From a practical standpoint, this study provides a clear PDCA (Plan-Do-Check-Act) framework for steel pipe manufacturers dealing with external folding defects. The Plan phase involves metallurgical analysis to identify the specific inclusion chemistry and morphology responsible for the defect. The Do phase implements the refined processing parameters outlined above, with particular emphasis on the LF white slag refining protocol and the soft argon stirring practice. The Check phase relies on systematic inclusion rating per ASTM E45 or equivalent standards, ensuring that the A, B, C, and D class levels remain within the specified limits. The Act phase involves continuous refinement of the process window based on statistical analysis of inclusion data across multiple heats.

In my own engineering experience, the interplay between slag thickness control and inclusion behavior is often underestimated. A top slag layer exceeding 50 mm during tapping creates a substantial interface area for slag-metal interaction, increasing the probability of slag entrainment into the liquid steel. Furthermore, the requirement for white slag (FeO ≤ 1%) during LF refining is critical because high FeO content in the slag increases the oxygen potential, promoting the reformation of silicate inclusions even after initial deoxidation. The combination of white slag refining with extended holding time and soft argon stirring creates the thermodynamic and kinetic conditions necessary for large, spherical inclusions to float out of the melt efficiently.

Key Questions and Reflections

Several important questions arise from this study that warrant further investigation. First, the study focuses on silicate and SiO2 inclusions, but what role do manganese silicate (MnSiO3) and manganese aluminum silicate (MAS) inclusions play in the folding mechanism? Given that the steel contains 1.30% Mn, manganese-bearing silicates are likely present and may contribute differently to crack initiation due to their distinct morphology and interface strength with the austenite matrix. Second, the study does not address the effect of casting speed or secondary cooling intensity on inclusion morphology and distribution, which are known to influence the degree of inclusion elongation and grain boundary alignment. Third, the transition from clean steel chemistry to actual pipe quality depends heavily on the downstream rolling and finishing operations, and the interaction between residual inclusion content and rolling reduction ratio deserves systematic study.

Study Insights and Implications

The most significant implication of this research is the establishment of a quantifiable relationship between metallurgical cleanliness parameters and a specific macroscopic defect mode. Rather than treating external folding as a purely mechanical or rolling process issue, the authors demonstrate that it is fundamentally a steel quality problem rooted in inclusion control. This perspective shift is valuable for quality management systems in steel pipe plants, as it directs corrective action toward upstream metallurgical operations rather than solely toward rolling parameter adjustments. For engineers involved in steel pipe procurement and qualification, this study underscores the importance of requiring suppliers to provide inclusion rating data alongside conventional mechanical property certificates, particularly for applications where surface integrity and fatigue resistance are critical.