The wall thickness eccentricity is one of the most important weaknesses that appears in seamless tubes production, since this imperfection is subsequently transferred downstream through the manufacturing stages until the final product. For this reason, in this article a finite element model of the rotary tube piercing (RTP) process is developed aimed at analysing the wall thickness eccentricity imperfection. Experimental data extracted from the industrial process is used for the validation of the model, including operational process variables like power consumption and process velocity, and deformation variables as elongation and longitudinal torsion, originated by axial and shear strain respectively. The cause of longitudinal torsion is also analysed. The two most important conclusions derived from this study are: (I) the longitudinal torsion of the tube is a crucial parameter for the correct model validation, and (II) the combined effect between the uneven temperature distribution of the billet and the plug bending deformation is identified as the major cause of the wall thickness eccentricity flaw.

In the numerical simulation of hot forming processes, the correct description of material flow stress is very important for the accuracy of the results. For complex manufacturing processes, such as the rotary tube piercing (RTP), constitutive laws based on both power and exponential mathematical expressions are commonly used due to its inherent simplicity, despite the limitations that this approach involves, namely, the use of accumulated strain as a state parameter. In this paper, a constitutive model of the P91 steel derived from the evolution of dislocation density with strain, which takes into account the mechanisms of dynamic recovery (DRV) and dynamic recrystallization (DRX), is proposed for the finite element (FE) analysis of the RTP process. The material model is developed in an incremental manner to allow its implementation in the FE code FORGE®. The success of this implementation is confirmed by the good correlation between results of the simulation and experimental measurements of the manufactured tube (elongation, twist angle, mean wall thickness and eccentricity). In addition, this incremental model allows addressing how the restoring mechanisms of DRV and DRV occur during the RTP process. The analysis puts into evidence that DRV and DRX prevail over each other cyclically, following an alternating sequence during the material processing, due mainly to the effect of the strain rate on the material.

Resumen En el proceso de colada continua contribuyen complicados fenómenos físicos con un marcado efecto en la calidad final del producto y en la productividad del proceso. Además, los distintos fenómenos físicos no aparecen de forma aislada, sino que interactúan unos con otros de manera acoplada, afectando a la calidad del semiproducto. El correcto diseño y ajuste es, por lo tanto, una tarea que requiere un gran conocimiento tanto de los distintos fenómenos que intervienen como de sus interacciones. En este trabajo se presenta un novedoso método de simulación que resuelve el proceso en su conjunto, siendo las variables del proceso las únicas condiciones de contomo. Mediante la utilización conjunta de herramientas comerciales (FLUENT y ABAQUS) y una serie de modelos de desarrollo propio se resuelven las interacciones entre los distintos fenómenos. Palabras clave Colada continua. Modelización. Fundentes. Solidificación. Refrigeración.

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