Hardening Behavior Characterization of Dual Phase Steels

Abstract

The requirements for higher passenger safety, improved fuel economy and weight reduction in 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 become increasingly popular, since they provide a combination of sufficient formability at room temperature and tensile strength over 1000 MPa. The current standards for DP steels only specifies yield and tensile strength. Steels from various producers have considerably different composition and microstructure; however they still have the same grade name. Combined with the inherited heterogeneous microstructure, those steels exhibit different strain hardening behavior. 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 rolling and transverse directions. The microstructure has been characterized with optical and scanning electron microscopes. The martensite fraction, grain size of ferrite and chemical composition has been correlated to the strain hardening behavior. The results show that the steel with more micro-alloying addition has finer ferritic grain size, which cause higher initial 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.

Keywords

• dual phase steels
• hardening behavior
• alloying design