Improving the Yield Rate of CPE Seamless Steel Tube Production

Literature Overview

The paper by Hang Nai-qin, Chen Ding-xun, Wang Bo-wen, Liu Sheng, Duan Jia-tian, and Kang Zhi-yong, published in the Journal of Wuhan University of Science and Technology (2009, Vol. 32, No. 5, pp. 453-457), addresses a critical production efficiency issue in seamless steel tube manufacturing. The research was conducted jointly by Wuhan University of Science and Technology and the Hanyang Steel Pipe Plant of Wuhan Steel Group. The central finding is that cutting loss is the primary factor reducing the yield rate of the CPE (Continuous Piercing Extrusion) unit, and systematic optimization of the process can improve the annual comprehensive yield rate by 8 percent.

Root Cause Analysis of Yield Loss

The authors conducted a thorough analysis of the CPE production process to identify the sources of yield loss. The CPE process involves continuous piercing of the billet followed by extrusion through a forming mandrel, and the resulting tube undergoes sizing and tension reduction before final cutting. Through a systematic investigation, the following key loss mechanisms were identified:

The thickened end is a characteristic feature of the tension reduction process, where the tube wall thickens at the exit due to the combination of forward tension and the geometric constraint of the sizing rolls. If this thickened region is not minimized, it results in significant material loss when the affected portion must be cut off to meet dimensional specifications.

Process Optimization Measures

The research team implemented several targeted improvements to address the identified loss mechanisms:

Sizing and Tension Reduction Optimization

The sizing and tension reduction deformation schedule was re-optimized to reduce the length of the thickened end. This involves adjusting the roll gap settings, the tension force applied at the exit, and the deformation distribution across the sizing passes. By distributing the deformation more evenly and controlling the exit tension more precisely, the thickened region was shortened significantly, reducing the amount of material that needs to be trimmed.

Piercing Mandrel and Head Cutting Improvements

The piercing mandrel die (顶管缩口设备) was redesigned to improve the quality of the tube head formation. The method of cutting the tube head after piercing was also improved to reduce the amount of material removed. These improvements work together to minimize the scrap generated at the beginning of the tube, which is a significant contributor to overall yield loss.

Cutting Precision Enhancement

The cutting system was upgraded to improve the accuracy of the final tube cutting. Better cutting precision means that fewer tubes are rejected due to length tolerance violations, directly improving the yield rate.

Results and Engineering Significance

The combined effect of these optimizations resulted in an 8 percent improvement in the annual comprehensive yield rate of the CPE unit. This is a substantial improvement in industrial terms, translating directly to significant cost savings and reduced material waste. For a production unit processing hundreds of thousands of tons of seamless tubes annually, an 8 percent yield improvement represents millions of yuan in annual savings.

Key Process Parameters Before and After Optimization

Technical Insights from a Materials and Process Perspective

From a metallurgical standpoint, the CPE process produces seamless tubes with a distinctive microstructure characterized by a fibrous flow pattern aligned with the tube axis. The deformation schedule during sizing and tension reduction directly influences the final grain structure and mechanical properties. An overly aggressive reduction schedule can lead to excessive strain localization and potential cracking, particularly in high-carbon or alloy steels. Therefore, the optimization of the deformation schedule must balance yield improvement with the maintenance of acceptable mechanical properties and surface quality.

The thickened end phenomenon is fundamentally a consequence of the plastic deformation behavior of the tube material under combined tension and compression loading in the sizing rolls. The material at the exit experiences a state of plane strain with tensile stresses in the axial direction and compressive stresses in the radial direction. The thickness increase is governed by the volume constancy principle and the stress state at the roll-tube contact interface. Understanding this deformation mechanism is essential for optimizing the sizing parameters to minimize thickening while maintaining dimensional accuracy.

Implications for Practice

This study demonstrates that systematic process analysis and targeted optimization can yield significant improvements in production efficiency without requiring major capital investment in new equipment. The approach of identifying specific loss mechanisms and addressing each one through process parameter adjustment and equipment modification is a model for continuous improvement in steel tube manufacturing. Engineers and plant managers should adopt a similar methodology when facing yield rate challenges in their own production lines. The PDCA (Plan-Do-Check-Act) cycle is well suited for implementing such improvements, starting with a thorough analysis of loss sources, followed by targeted interventions, verification of results, and standardization of the improved process.