A damage-coupled unified viscoplastic constitutive model for prediction of forming limits of 22MnB5 at high temperatures

Hot stamping of the boron steel 22MnB5 has been widely used to produce high strength parts in automobile industry. The forming limits of 22MnB5 sheets are governed by the damage evolution in hot stamping. The aim of this work is to formulate a constitutive model to predict the forming limits of 22MnB5 sheets under hot stamping condition according to continuum damage mechanics (CDM). The authors developed a set of damage coupled viscoplastic constitutive equations to describe the thermal deformation behavior of 22MnB5 steel under uniaxial tensile state. The material constants of uniaxial constitutive equations were determined from the tensile testing data at a temperature range of 600-900 degrees C and a strain rate range of 0.01-10 s(-1). The uniaxial damage coupled constitutive equations were generalized to the plane stress state by introducing a multiaxial damage factor considering the effect of stress state on damage evolution. Hot Nakajima testing was conducted to get the forming limits of 22MnB5 sheets at different deformation temperatures and punch speeds. The material constants of multiaxial damage factor were calibrated and confirmed from the experimental data of Nakajima testing. The Nakajima test was numerically simulated by implementing the established constitutive equations via the user material subroutine VUMAT. The prediction by the numerical simulation agrees with the experimental results. Using the determined plane stress damage-coupled constitutive model, the forming limits of 22MnB5 sheets can be predicted under the hot stamping condition. (C) 2017 Elsevier Ltd. All rights reserved.