Defects analysis of the ultra-thick steel pipes using a mixed explicit–implicit FEM

The ultra-thick wall steel pipes are very likely to have quality defects in stretch-reduction hot rolling process, and it is preferred to study these defects by simulation methods. However, traditional FEM often has the problems of convergency difficulty and time consuming for solving complex large deformation problems. Therefore, in this study, a mixed explicit-implicit FEM, was adopted for solving the thermo-mechanical coupled process of the stretch-reduction hot rolling. Multidimensional heat transfer as well as mechanical boundary conditions were acted simultaneously, and the accuracy of the model was validated by industrial experiments. Results showed that the simulation results are very consistent with the actual rolling results. Three typical rolling defects were accurately predicted, i.e., inner hexagon, thickened ends and linear mark. Besides, it is discovered that the uneven distributions of stress, strain and temperature are important causes for the rolling defects, and these influences are also presented and discussed. This paper presents an efficient and precise numerical modelling method so as to provide theoretical guidance for the production of ultra-thick wall pipes.