Hot stamping of complex shaped boron steel panels

Abstract

In the present work, a new grip design for the Gleeble Materials-Simulator has been developed to reduce the long-standing problem of the temperature gradient during thermo-mechanical tensile testing. The deformation behaviour of 22MnB5 boron steel for a range of temperatures and strain rates were evaluated using the new test grip. Deformation results showed higher strain hardening (n-value) behaviour at low temperature compared to that at high temperature deformation, which increases the drawability of the material at low temperature hot stamping conditions. Moreover, hot stamping at high temperatures takes a longer time to cool the material below the martensitic finish temperature, which increases the die holding time and decreases productivity. To improve the formability and productivity of the hot stamping processes, the new hot stamping process at low temperatures was demonstrated. An automotive B-Pillar component was hot stamped at a wide range of temperatures and forming speeds, which included temperatures much lower than that of a normal hot stamping temperature range. To understand the influence of tool-workpiece contact pressure, tool and tool surface temperatures, and the effect of initial work-piece stamping temperature on in-die cooling time and tool surface temperature were investigated. A series tests of low-temperature hot stamping and heat transfer at different initial workpiece temperatures and contact pressure were carried out. The results confirmed that hot stamping could be performed at a lower temperature (500 °C) without compromising the part quality. It could also benefit the reduction of in-die quenching time by over 50%, which would increase productivity for automotive mass production. In the process, material microstructural and thermomechanical behaviours can be carefully controlled to improve drawability and at the same time, to maintain or even enhance the post-form properties. Finally, unified viscoplastic constitutive equations were discussed, and material constants were determined. The viscoplastic constitutive material model was input into the finite element code, LS-DYNA, through a user defined subroutine and used for the simulation of hot stamping processes. The implemented viscoplastic constitutive model was accurate enough to predict the thickness distribution of a hot stamped product.