Strain Hardening Behavior Characterization of Dual Phase Steels

Abstract

The requirements for higher passenger safety, improved fuel economy and weight reductionin automobile industry necessitates the usage of advanced high strength steel (AHSS)grades. Dual phase (DP) steels are the most widely used one among AHSS. DP steels becomeincreasingly popular, since they provide a combination of sufficient formability at room temperatureand tensile strength over 1000 MPa. The current standards for DP steels only specifiesyield and tensile strength. Steels from various producers have considerably different compositionand microstructure; however they still have the same grade name. Combined withthe inherited heterogeneous microstructure, those steels exhibit different strain hardeningbehavior. The aim of this study is to evaluate the strain hardening behavior of DP800 steels,obtained from different vendors and thus having different compositions and microstructures.The strain hardening behavior was characterized with tensile tests performed along rollingand transverse directions. The microstructure has been characterized with optical andscanning electron microscopes. The martensite fraction, grain size of ferrite and chemicalcomposition has been correlated to the strain hardening behavior. The results show thatthe steel with more micro-alloying addition has finer ferritic grain size, which cause higherinitial strain hardening rate. The steel with higher Mn and Cr has higher martensite fraction,which cause strain hardening rate to be higher at higher strain levels.