Effect of various shearing shape conditions for the scrap-used coining method on tensile residual stress on sheared edge

Abstract

Application of high strength steel for automobiles is required to achieve fulfilment of the CO2 emission regulation and improvement of collision safety. Nevertheless, high strength steel has the problem that hydrogen embrittlement and fatigue cracks are likely to occur especially on sheared edge. The previously proposed scrap-used coining method could reduce tensile residual stress on sheared edge using the scrap generated by shearing as the coining tool with a counter punch. However, the conditions this method could reduce tensile residual stress effectively were not clarified. In this report, the effects of the following shearing shape factors on residual stress on sheared edge were investigated, the size of shearing line curvature using punches with a diameter of 10 and 20 mm, positive and negative sign of curvature by testing both of the product and scrap part, and the flat and dome shape of counter punch head. Residual stress on sheared edge could be reduced in the 10 and 20 mm of punch diameter regardless of the product or scrap part. FEM analyses showed that the change of stress distribution in processing area during the scrap-used coining is caused by the shearing hole expansion and reduction and the sheared edge compression by contact of the product and scrap and these mechanisms are common to both of the parts. The dome shaped counter punch is able to reduce the coining load in addition to almost the same amount of residual stress as the flat shaped one. Thus, the tool cost decreases because the cushion pin required to get the reaction force for coining could be reduced.