Coining of Pierced Hole with Aid of Scrap

Abstract

The tensile residual stress near pierced holes deteriorates the fatigue properties and hydrogen embrittlement resistance of automotive parts. Consequently, a simple shearing method that can reduce this type of stress is highly desired in the automotive industry. In this study, the effect of a coining method using scrap material on the stress near a 10 mm diameter pierced hole was investigated. The fracture surfaces of the pierced scrap material and pierced hole presented similar shapes because these parts were produced by the separation of a single sheet via 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 the coining stroke, the test was conducted at two levels: a coining stroke in which the product and scrap were at the same level and a coining stroke in which the scrap passed through a hole in the product. The tensile residual stress on the sheared surface significantly reduced as the length of the coining stroke increased. The results of finite element method simulations indicated that the above effects were mainly caused by the change in the shape of the pierced hole and the stress generated by the contact between the pierced scrap material and the hole in the product.

 

This Paper was Originally Published in Japanese in J. JSTP 60 (2019) 301–306. The Abstract is slightly modified.

Fig. 1 Schematic of proposed method.