Zhuojin Pipe Fitting Co., LtdImproving the Yield Rate of CPE Seamless Steel Pipe Production
Literature Overview
This paper by Hang, Chen, Wang, Liu, Duan, and Kang (2009), published in the Journal of Wuhan University of Science and Technology, focuses on a practical production engineering problem in seamless steel pipe manufacturing using the Continuous Piercing and Extrusion (CPE) mill. The authors identified cutting loss as the primary factor reducing the yield rate and proposed systematic improvements to the sizing, tension reduction, and cutting processes. The collaborative effort between Wuhan University of Science and Technology and Wuhan Steel Group Hanyang Steel Pipe Plant exemplifies the integration of academic research with industrial practice.
Root Cause Analysis of Yield Rate Reduction
The CPE process involves piercing a steel billet, extruding it to form a hollow shell, and then reducing the wall thickness through a tension reduction stand. The yield rate is fundamentally limited by the material losses at each stage, with cutting operations being the most significant contributor. The authors conducted a detailed analysis of the production process to quantify the losses at each stage:
| Process Stage | Primary Loss Mechanism | Estimated Impact on Yield Rate |
|---|---|---|
| Piercing and extrusion | Piercing cap thickness and extrusion head loss | Moderate |
| Sizing rolls | Surface defects requiring trimming | Minor |
| Tension reduction | Thickened ends due to non-uniform deformation | Major |
| Post-extrusion cutting | Head and tail cutting of extruded pipe | Major |
| Final cutting and inspection | Rejection of defective sections | Variable |
The thickened ends produced during tension reduction are a critical issue. In the tension reduction stand, the pipe is simultaneously drawn and reduced in wall thickness by a combination of rolling and axial tension. Due to the non-uniform contact between the rolls and the pipe, the ends of the pipe experience less deformation than the central section, resulting in thicker walls at the ends. These thickened ends must be cut away to meet dimensional specifications, directly reducing yield.
Process Optimization Measures
The authors proposed and implemented several improvements that collectively increased the annual comprehensive yield rate by 8 percent, a significant improvement in a high-volume production environment. The key measures included:
- Sizing and tension reduction deformation schedule optimization: By adjusting the rolling schedules in the sizing stands and the tension reduction stand, the authors achieved more uniform deformation along the pipe length, thereby reducing the length of thickened ends. This requires careful coordination between the rolling force distribution and the axial tension applied to the pipe.
- Improvement of the extrusion cap cutting equipment: The original cutting equipment was modified to improve cutting accuracy and reduce the length of material removed from each pipe end. Precision in cutting directly translates to reduced material waste.
- Modification of post-extrusion head cutting method: The method used to cut the head of the extruded pipe was refined to minimize the length of the section removed while still ensuring that all defective material is eliminated.
- Enhancement of segmented cutting precision: Improving the accuracy of the cutting operations that divide continuous pipe into individual lengths reduces the number of rejected sections due to dimensional non-conformance.
Engineering Practice and Quality Control Integration
The 8 percent improvement in yield rate represents a substantial economic benefit in seamless pipe production, where raw material costs constitute a major portion of the total production cost. From a quality control perspective, the improvements must not compromise the mechanical properties or dimensional accuracy of the final product. The tension reduction deformation schedule optimization, in particular, must be validated through mechanical property testing of the reduced sections to ensure that the reduced wall thickness meets the minimum requirements of the applicable standard, such as GB/T 17395 for cold-rolled or cold-drawn seamless steel tubes.
In my experience with seamless pipe production lines, the optimization of the tension reduction schedule is a delicate balance between achieving the target wall thickness and maintaining adequate deformation uniformity. Over-aggressive reduction schedules can lead to localized thinning, surface cracking, or excessive work hardening in the pipe wall, all of which may result in product rejection during non-destructive testing. The systematic approach taken by the authors, combining process analysis with equipment modification and schedule optimization, provides a replicable methodology for other CPE production lines seeking similar yield improvements.
Study Reflection and Implications
This paper is a prime example of how engineering optimization in production processes requires a holistic understanding of the entire manufacturing chain. The yield rate improvement was not achieved through a single change but through a coordinated set of modifications that address the root causes of material loss. The methodology employed is consistent with the PDCA (Plan-Do-Check-Act) cycle, where the analysis phase identified the root causes, the implementation phase applied the improvements, and the verification phase confirmed the 8 percent yield improvement. For engineers working in seamless pipe manufacturing, this paper underscores the importance of detailed process analysis and the potential for significant cost reduction through systematic optimization of existing production equipment and schedules.