Numerical modelling of Direct Hot Tube Rotary Draw Bending of 22MnB5 High Strength Steel

The use of High Strength Steel (HSS) tubular components in structural frames allows a drastic increase of the stiffness-to-weight ratio and, therefore, may open new possibilities to the design lightweight components for the transport industrial sector. However, the diffusion of these elements is still limited by the lack of manufacturing technologies capable of efficiently forming and bending HSS hollow profiles. To overcome the limits of traditional room temperature processes, an innovative rotary draw bending process carried out at high temperature is proposed, in which the profile cross section is quickly heated to the target temperature and, then, bent and selectively cooled to induce controlled microstructure changes. The paper presents a fully coupled thermo-mechanical-metallurgical numerical model to support the process design, which was applied to 22MnB5 tubes. The model was used to identify the optimal thermal and mechanical parameters to increase the material formability and to obtain the target microstructure in the final part. All the process steps, namely heating, handling, forming, and cooling, were modelled, and the results were experimentally validated by industrial trials carried out on a bending pilot plant. The results prove the possibility of an accurate prediction of both the final shape and the distribution of the microstructural and mechanical properties along the bend.