Three-Dimensional Finite Element Simulation of Wall Thickness Variation During Hot Continuous Rolling of Steel Tubes

Literature Overview

The paper by Zhao Zhiyi, Xie Jianxin, Hong Huiping, Yu Yong, and Pan Feng, published in Journal of Plasticity Engineering (Vol. 16, No. 2, 2009, pp. 109-112), presents a three-dimensional finite element simulation of the hot continuous rolling process for seamless steel tubes. The study focuses on a 20 steel grade seamless tube of specification Φ119.0 mm × 9.25 mm, produced through an eight-stand full-floating mandrel continuous rolling mill. The simulation captures the evolution of equivalent plastic strain, blank tube wall thickness, rolling force, and mandrel force across all eight stands, providing a detailed process map that is validated against measured production data.

Core Technical Findings

The simulation results reveal several important process characteristics that are critical for seamless tube manufacturing:

The most significant finding is that the mandrel force exceeds the rolling force at all stands, indicating that the inner surface of the blank tube experiences higher compressive loads and greater plastic strain than the outer surface. This has direct implications for mandrel design, lubrication strategy, and the prediction of internal defects such as surface folds or internal cracks.

Interpretation of Technical Points

In seamless tube hot continuous rolling, the full-floating mandrel system allows the mandrel to move freely in the rolling gap, guided by the interaction between the tube wall and the mandrel surface. The three-dimensional finite element simulation captures the complex three-dimensional deformation state that occurs as the tube passes through each stand, including the non-uniform strain distribution through the wall thickness and around the circumference.

The observation that wall thickness reduction and outer diameter change are concentrated in the first six stands is consistent with the typical rolling schedule design, where the initial stands are configured with larger reduction ratios to achieve the primary deformation, while the finishing stands (stands 7 and 8) are configured for final sizing and surface finishing. The decreasing trend of rolling force with stand number reflects the reduction in material cross-section and the corresponding decrease in deformation resistance.

Mandrel Force and Inner Surface Considerations

The finding that mandrel force exceeds rolling force is of particular metallurgical significance. The inner surface of the blank tube is in direct contact with the mandrel, and the high compressive forces can lead to:

• Surface folding or wrinkling if the material flow is not properly controlled.
• Increased surface roughness on the inner diameter.
• Potential initiation of internal cracks if the compressive stress is excessive and the material is not adequately lubricated.
• Mandrel wear and galling, which can transfer debris to the tube inner surface.

The study recommends reasonable lubrication of the mandrel surface to mitigate these issues. In practice, this involves the use of high-temperature lubricants (such as graphite-based or ceramic-based coatings) that can withstand the elevated temperatures of the rolling process (typically 1000-1200°C for 20 steel) while providing adequate separation between the mandrel and the tube surface.

Process and Standards Analysis

The seamless tube manufacturing process is governed by several international and national standards that specify dimensional tolerances, mechanical properties, and surface quality requirements. For the 20 steel seamless tube of Φ119.0 mm × 9.25 mm specification, the relevant standards would include:

• GB/T 8163: Steel pipes for fluid transport, specifying dimensional tolerances and mechanical properties.
• ASTM A106: Seamless and welded carbon steel pipes for high-temperature service.
• EN 10216-1: Technical delivery conditions for seamless and welded steel tubes for pressure purposes.

The simulation results provide process data that can be used to verify compliance with these standards. For example, the predicted wall thickness uniformity and roundness at the exit of stand 8 can be compared against the dimensional tolerance requirements specified in the applicable standard. The rolling force and mandrel force data can be used to verify that the rolling mill equipment is adequately sized for the specified reduction schedule.

Integration with Engineering Practice

For seamless tube manufacturers, the ability to predict wall thickness variation and rolling forces through finite element simulation is invaluable for process optimization. The simulation results can be used to:

• Design the rolling schedule (reduction distribution across stands) to achieve uniform wall thickness and roundness.
• Predict the maximum rolling force and mandrel force to verify equipment capacity and safety margins.
• Optimize the mandrel lubrication strategy based on the predicted inner surface strain distribution.
• Identify potential defect initiation sites (such as high-strain zones on the inner surface) and implement preventive measures.
• Reduce trial-and-error during process development, saving time and material costs.

The validation of the simulation against measured production data, with good agreement reported by the authors, confirms the reliability of the finite element model for engineering applications. However, it is important to note that the accuracy of the simulation depends on the constitutive model used for the steel grade at elevated temperatures, the accuracy of the boundary conditions, and the representation of the rolling mill geometry.

Key Questions and Reflections

Several questions arise from this study that merit further consideration. First, the simulation focuses on a single specification (Φ119.0 mm × 9.25 mm, 20 steel); the applicability of the model to other specifications, materials, or rolling configurations requires verification. Second, the study does not address the influence of rolling speed on the deformation process and final product quality; in practice, rolling speed is a critical process parameter that affects both the deformation behavior and the temperature evolution of the tube. Third, the long-term wear behavior of the mandrel under repeated rolling cycles is not addressed, which is important for production planning and maintenance scheduling.

Another reflection concerns the relationship between the simulated strain distribution and the final microstructure and mechanical properties of the tube. The high plastic strain on the inner surface could lead to grain refinement and potentially improved mechanical properties, but excessive strain could also lead to recrystallization anomalies or texture development that affects formability and fatigue resistance.

Study Insights and Implications

This study demonstrates the power of three-dimensional finite element simulation as a tool for understanding and optimizing the hot continuous rolling process for seamless steel tubes. The detailed process map of wall thickness variation, rolling force, and mandrel force across all eight stands provides engineers with actionable insights for process design and optimization. The finding that the inner surface experiences higher loads and strains than the outer surface highlights the importance of mandrel surface quality and lubrication in achieving defect-free products. For seamless tube manufacturers, the adoption of finite element simulation as a standard engineering tool can significantly improve process control, reduce defect rates, and enhance product quality consistency.