Prediction of hardening, localization and fracture of multiphase microstructure in boron alloyed steel

Abstract: Over recent years the demand of press hardened ultra-high strength steel for safety structures in automobiles has increased and continuation of this trend is expected.Components with tailored material properties can be manufactured using a thermo-mechanical process with heated and cooled areas in a tool. In industry the cooling rate of the blank is controlled this utilizes the formation of different microstructures with varying mechanical properties within a single component.The structural response in a crash situation can be altered by the design of the component with formation of different material grades based on the microstructure in designated areas of the component.Material models used in finite element analysis are required by the automotive industry and its suppliers. These models contribute to the improvement and quality of press hardened components.In this work a set of tensile test specimens with different volume fractions of phases are produced. As material is the boron steel 22MnB5 chosen which is a common material used in press hardening due to its good hardenability. The specimens are austenitized before starting the heat treatment at different temperatures and holding times. In total fourteen different microstructures are produced. Reference material grades for pure phases are ferrite, bainite and martensite. The produced samples consist of ferrite-bainite, ferrite-martensite and bainite-martensite with different volume fractions, additionally a microstructure consisting of three phases, ferrite-bainite and martensite, is available. Using measured mechanical properties of pure phases and the volume fraction of formed phases different homogenization methods are compared in their ability to represent the mechanical response of mixed microstructures. The homogenization methods account for elastic deformation and the hardening of the material. The onset of necking is seen as the last valid point for all homogenization methods. After this point a localization and damage function for the prediction of softening and fracture is applied. Strain localization and fracture are mesh dependent, therefore an analysis length scale is introduced to account for different element sizes. A weakest link criterion is used for fracture. This means failure of the composite is assumed to occur if one phase fails. The material model for homogenization of mixed microstructures including damage is implemented in the commercial available finite element code LS-Dyna and validated by comparison to experimental results.