Edge cracking in high strength steels

Abstract

A study has been made of the sheared edge ductility of a series of coldroll gage high strength steels. It is found that inclusion shape control is beneficial, especially for steels with tensile strengths ranging from 345 to 550 MPa (50 to 80 ksi). Edge ductility is increased for all the materials when the quality of the edge is improved; this involves removing the heavily coldworked sheared region either by machining, trimming (repunching), or a heat treatment which results in the recrystallization of the edge. Following a heat treatment, the steels with no inclusion shape control have hole expansion values similar to those observed in materials with inclusion shape control before heat treatment. It is suggested that for the steels with the stringer type inclusions, low edge ductility will be exhibited when fracture initiates in the deformed shear edge region and propagates along the inclusionferrite interface. The elimination of this coldworked region makes crack initiation more difficult and, thus, there is greater ductility. A tempering study of dualphase steels suggests that the hard martensite islands play a similar role to the stringer type inclusions in reducing sheared edge elongation. It was also observed that the load to punch out a disc is proportional to both the thickness and the tensile strength of the material.