Coining of Pierced Hole with Aid of Scrap

Abstract

A simple shearing method to reduce tensile residual stress on a pierced hole is highly desired in the automotive industry because this type of stress deteriorates the fatigue property and hydrogen embrittlement characteristics of automotive parts. In this study, the effect of a coining method using scrap material on a 10-mm-diameter pierced hole was investigated. The pierced scrap material has a fracture surface shape similar to that of the pierced hole since these parts are generated by separation of a single plate with crack propagation. Therefore, it is possible to use the pierced scrap material as a precise coining tool. The effectiveness of this method was investigated for 980-MPa-grade steel sheets with a thickness of 1.6 mm. To examine the effect of coining stroke, the test was conducted at two levels: a coining stroke at which the product and scrap were on the same level and a coining stroke at which a scrap passed through the hole of the product. The tensile residual stress on the sheared surface was significantly reduced as the coining stroke increased. The results of FEM simulation indicate that these effects were mainly caused by the shape change of the pierced hole and stress caused by contact between a pierced scrap material and a hole in the product.