Three-Dimensional Finite Element Simulation of Cross-Sectional Wall Thickness Variation During Hot Continuous Rolling of Steel Pipes

Literature Overview

The paper by Zhao Zhiyi, Xie Jianxin, Hong Huiping, Yu Yong, and Pan Feng, published in the Journal of Plasticity Engineering (Vol. 16, No. 2, 2009, pp. 109-112), presents a three-dimensional finite element (3D FEM) simulation of the hot continuous rolling process for seamless steel tubes. The study focuses on a 20 steel (carbon structural steel) pipe with dimensions of Φ119.0 mm × 9.25 mm, manufactured using a full-floating mandrel 8-stand continuous tube mill. The research was conducted in collaboration with Baosteel Co., Ltd. Baoshan Branch, combining academic modeling with industrial practice.

Process Description and Technical Parameters

The hot continuous rolling process for seamless tubes involves the following key steps:

1. Billet heating: The steel billet is heated to a rolling temperature of approximately 1100-1200°C in a reheating furnace.
2. Piercing: The heated billet is pierced to form a hollow shell (blank) using a piercer mill.
3. Continuous rolling: The hollow shell passes through 8 consecutive rolling stands, each reducing the wall thickness and adjusting the outer diameter.
4. Cooling and sizing: The rolled tube is cooled and sized to final dimensions.

The specific parameters for the simulated process include:

Simulation Methodology

The 3D FEM simulation employed a rigid-plastic finite element approach, which is well-suited for metal forming processes. The key modeling assumptions include:

• The material is modeled as rigid-plastic, with the flow stress defined by a temperature-dependent constitutive equation
• The tooling (rolls and mandrel) is modeled as rigid bodies
• Friction at the roll-tube and mandrel-tube interfaces is characterized by a friction coefficient
• The deformation is quasi-static, with inertia effects neglected

The simulation output includes:

• Equivalent plastic strain at each stand exit
• Wall thickness variation at each stand exit
• Outer diameter change at each stand exit
• Rolling force at each stand
• Mandrel force at each stand

Key Results and Analysis

The simulation results revealed several important process characteristics:

1. Wall Thickness Reduction Distribution: The majority of wall thickness reduction occurs in stands 1 through 6. Stands 7 and 8 contribute minimally to wall reduction. This distribution is typical of continuous tube mills, where the initial stands perform heavy reduction and the final stands perform sizing and finishing.
2. Outer Diameter Variation: Similar to wall thickness reduction, the outer diameter change is concentrated in the first six stands. By stands 7 and 8, the tube cross-section has achieved a uniform and circular shape.
3. Rolling Force Trend: During the stable continuous rolling phase, the rolling force decreases progressively from stand 1 to stand 8. This trend is attributed to the decreasing reduction per stand and the increasing temperature of the tube (due to adiabatic heating), which reduces the flow stress.
4. Mandrel Force: The mandrel force exceeds the rolling force at each stand. This is because the mandrel must overcome the internal friction and the resistance to internal deformation of the tube. The inner surface of the tube experiences higher contact pressure and plastic strain than the outer surface.
5. Inner Surface Condition: The simulation indicates that the inner surface of the tube undergoes significant plastic deformation and contact stress. This finding has direct implications for mandrel lubrication and surface quality control.

Engineering Practice Implications

The simulation results provide valuable guidance for process optimization and quality control:

• Mandrel Lubrication: Given the high contact pressure and plastic strain on the inner surface, proper mandrel lubrication is critical. Inadequate lubrication can lead to excessive friction, increased rolling force, surface defects on the inner tube wall, and premature mandrel wear. The authors recommend the use of high-temperature lubricants (such as graphite-based or water-based emulsions) and regular mandrel maintenance.
• Stand Configuration: The distribution of reduction across stands should be optimized to ensure uniform deformation and minimize residual stresses. Excessive reduction in early stands can lead to edge cracking, while insufficient reduction in final stands can result in dimensional inaccuracies.
• Quality Monitoring: The simulation results can be used to predict wall thickness variation and dimensional accuracy, enabling proactive quality control. In practice, wall thickness is measured using ultrasonic testing (UT) at each stand exit, and the measured values should be compared with the simulation predictions to validate the process model.
• Defect Prevention: Common defects in hot rolled seamless tubes include wall thickness variation, ovality, internal cracks, and surface scale. The simulation can help identify the stands most prone to each defect type, enabling targeted corrective actions.

Study Insights and Reflections

The paper demonstrates the effectiveness of 3D FEM simulation as a tool for understanding and optimizing the hot continuous rolling process. The good agreement between simulated and measured results validates the modeling approach and provides confidence in using simulation for process development and troubleshooting.

One key insight from the study is the importance of the mandrel force. In many process analyses, the focus is on the rolling force, but the mandrel force is equally important for the quality of the inner surface. Engineers should pay close attention to mandrel force monitoring and control during production.

Another important consideration is the temperature evolution during rolling. The adiabatic heating effect can significantly influence the flow stress and deformation behavior. In practice, the rolling temperature should be monitored and controlled to ensure consistent product quality.

The study also highlights the value of collaboration between academia and industry. The involvement of Baosteel Co., Ltd. provided practical process data and validation opportunities, which are essential for developing reliable simulation models.