Comprehensive study of the seamless tube cross-roll piercing process by means of FEM simulation

Résumé

This dissertation reports the study of the cross-roll piercing (CRP) process for the manufacturing of seamless tubes, aiming to identify the key parameters of the process and to analyse the development of the wall thickness eccentricity defect (WTE) and the undesired redundant deformation in the tube, by means of the process finite element (FE) simulation. First, a preliminary simplified FE model has been proposed for the characterisation of the most important contact present in the process, namely, the contact between the billet and the rolls. The viscoplastic friction model approach with regards to the relative velocity at the interface is concluded as the most appropriate for the roll-billet contact. Then, on the basis of the simplified model, an advanced FE simulation has been developed including all the elements of the piercing mill. A methodology in which industrial data is used for the validation of the FE model has been proposed. The validation is accomplished through the correlation of the power consumed by the rolls, forces exerted by the Dieschers, process time and longitudinal shear deformation with the experimental measures. The analysis of the material state serves to confirm the severe action of the rolls on the material, given that the exterior region of the material is deformed at a higher rate and presents the highest effective strain value. Regarding the temperature distribution in the tube section, a realistic temperature profile has been considered so as to be able to analyse the cause of the WTE defect. The combination of an uneven temperature distribution in the billet cross section and the bending of the plug is identified as fundamental requirement for the development of the WTE defect. Therefore, the suppression of this defect would be possible whenever the plug stiffness were increased enough or the temperature distribution made homogeneous. Regarding the misalignment of the piercing mill elements, their impact on the WTE of the tube is not significant. Furthermore, the redundant shear deformation has been addressed. The longitudinal twisting of the material is considered as an undesired strain that should be minimised, but the circumferential twisting is also associated to the apparition of inside bore defects in the tube. The separation of the Dieschers has a severe impact in the circumferential twisting of the material, whilst the plug position modifies substantially the longitudinal redundant shear deformation. The constitutive model used in this study has shown a good performance. However, high transient loading and deformation conditions have been identified in this industrial process. Therefore, a physically-based constitutive model conceived particularly for those deformation conditions has been developed. First, this formulation has been validated in a compression test simulation, yielding better results than the homologous phenomenological constitutive model. Then, it has been implemented in the simulation of the CRP process, showing its validity for the simulation of complex industrial processes in which transient deformation conditions are present.