Hole expansion ratio in intercritically annealed QP 980/1180 steel grades as a function of testing condition

Abstract

United States Department of Transportation fuel economy and safety standards have led to the development of economically viable steels with excellent combinations of strength and ductility, better known as 3rd generation advanced high strength steels (AHSS). Quench and partitioned (QP) steels are of interest due to their excellent mechanical properties, though forming may be a challenge since stamped parts with a sheared edge may experience cracking at low strains. Hole expansion testing (HET) was performed on intercritically annealed QP 980 and QP 1180 steel sheets with two punch geometries (conical and flat bottom), edge conditions (sheared and machined), and complementary microstructural analysis was performed to better understand the effects on hole expansion ratio (HER). X-ray diffraction and electron backscatter diffraction experiments were performed to better understand factors affecting retained austenite (RA) stability as a function of strain. In this study, the QP 980 and QP 1180 steel grades had similar HERs for a majority of the testing conditions. Conical and flat bottom punches resulted in similar HERs despite varying edge conditions for a majority of the testing conditions, and the machined hole samples resulted in a higher HER than sheared hole samples regardless of punch geometry. The RA in QP 980 transformed at a faster rate than QP 1180 as a function of tensile strain. The relative stability of RA in QP 1180 was attributed to both RA morphology and the surrounding microstructure.