Case Studies of Edge Fracture of Dual Phase Steel Stampings

Abstract

With the implementation of Advanced High Strength Steel (AHSS) becoming more common for automotive manufacturers to reduce mass and/or improve performance, special stamping considerations must be made. Certain production parts may split at trimmed edges where strain levels are well below the forming limit curve of the respective grade, which is more applicable to necking fractures/splits. Similar to the presence of hard inclusion stringers (i.e. MnS) that can cause edge fractures in high strength low alloy steels, AHSS steels most susceptible to this phenomenon typically consist of dual phase or multiphase microstructures containing both a hard phase (martensite) and a soft phase (ferrite). Specific examples of these parts will be discussed, including studies to determine the root cause of the edge fracture and to communicate the solutions for consideration in appropriate standards and specifications. The unexpected fractures at low strains were not limited to specific strength level, supplier or stamping plant but typically occurred when an edge was under tension in a stretch forming condition. This paper will provide a review of various production case studies emphasizing the importance of the trimmed edge condition in edge fracture susceptibility. A comparison will be made between good and poor trim quality along with an updated definition for a good trimmed edge based on traditional shear face analysis. In addition, die design recommendations will be made for optimal trim conditions in both blanking and die trimming to help avoid unpredictable edge fractures. Finally, the paper will discuss one case study which emphasized the importance of material selection in eliminating an edge split issue in a component containing a flanged hole.