The effects of property differences in multiphase sheet steels on local formability

As the automotive industry continues to push towards higher strength materials, lower weight, and reduced costs, high strength ferrous alloys have provided a viable solution. However, as strength levels increase, formability concerns, specifically in regards to local formability at areas of significant geometrical change, have arisen. One of the contributing factors to reduced formability limits has been identified as localized variations in material microstructure. In contrast to single phase steels used in the past, many of the current alloys currently being pursued utilize complex, multiphase microstructures. This presents new and interesting challenges for the current steel producers and consumers. This paper investigates the influences of systematic differences in phase properties on local formability by use of unique testing techniques including nanoindentation to determine phase hardness, and a bending under tension test to determine formability limits. It was found that increased local formability limits were observed with decreased hardness ratio between the hardest and the softest phase in the microstructure.