Modelling of Plastic Deformation and Fracture in Hot Stamped Steel with Multi-Phase Microstructure

摘要

Hot stamping is an industrialized technique with the aim of improving material properties by heat treatment and forming of a component in a single production step. Within the field of hot stamping the method of tailored material properties evolved. Components with tailored material properties possess different mechanical properties in designated areas. The mechanical properties in a blank are modified by the formation of different microstructures. Martensite is a microstructure with high strength but low ductility, ferrite has lower strength but higher ductility. Using special tooling tough martensite and soft ferrite can be placed in adjacent sections in ablank. Between those sections a transition zone consisting of amixed microstructure exists with mechanical properties between martensite and ferrite. Transition zones possess intennediate cooling rates, hence formation of bainite and composites of bainite and another phase can from. This paper presents an approach of modelling the complete process from austenitized blank to fracture. The method presented relies on the prediction of phases formed during cooling using an austenite decomposition model. In the course of ferrite formation the carbon content in the remaining austenite increases, the carbon content in austenite influences formation of additional daughter phases. The estimated phase composition is used in a homogenization scheme to predict the hardening of the material during plastic deformation. Fracture in the different microstructural phases is predicted using the strain decomposition provided by the homogenization and a fracture criteria. The homogenization scheme and the fracture criteria use measured data from single phase microstructures, i.e. ferrite, bainite and martensite. A heat treatment process for tensile test specimens is used to produce samples with different volume fractions of the microstructures ferrite, bainite and martensite. The pre-cut specimens are austenitized, ferrite is formed in a second furnace with lower temperature, bainite and martensite are formed by the use of a temperature controlled plane tool. Prediction of the phase content in mixed micro structures showed good agreement with microstructural characterization and therefore results can be used as input value for the homogenization. Comparing experimental and numerical results for a variety of different mixed microstructures good agreement in the prediction of hardening and fracture is found. It is concluded that the use of a homogenization method combined with a fracture model can be used to predict the mechanical response of mixed microstructures. The method described in the present work can be applied in the development of hot stamped components.