Influence of Microstructure on Ductile Damage Behavior of Dual Phase Steel

High tensile strength steel is used to save weight of the automotive body for reduction of CO2 emissions. Because of its high tensile strength, the deformation of high tensile strength steel is difficult. Many kinds of high strength steel are developed to achieve a good balance between high strength and formability. Dual phase steel is one of these steels. It has ferrite and martensite phase as a soft phase and hard phase respectively. In Dual Phase steel, ferrite phase has formability and martensite phase has strength part. However, influences of their volume fraction, chemical composition and heat treatment conditions on material properties are not clear. Therefore, in this study, mechanical properties and micro void behavior on ductile fracture of dual phase steel are investigated. The specimens with difference chemical composition and heat treatment condition are provided. Basic material test is carried out to obtain these mechanical properties. Additionally, the change of void volume fraction during forming is measured and void nucleation site is observed by using laser microscope and image analysis. In the result, dual phase steel containing large volume fraction of ferrite phase shows high uniform elongation and that of martensite phase shows high local elongation. Furthermore, the gradient of void volume fraction of the steel which shows high equivalent fracture strain is lower than that of the steel which shows low equivalent fracture strain.