54 巻 (2013) 627 号 p. 348-352
The direct punching of inclined ultra-high strength steel sheets having low ductility was carried out to improve the quality of the sheared edge. In the direct punching of an inclined sheet, the contact between the sheet and punch becomes gradual because of touch from the bottom edge of the punch, and thus the sheared portion tends to bend in the latter half of punching. As the strength of the sheet increased, a burr was formed around the final sheared edge due to the low ductility, and the sheared edge without a burr was not obtained for a 980 MPa grade steel sheet. To prevent the occurrence of burrs, punches having inclined and curved bottoms were developed. In these punches, the contact with the sheet was changed, full contact with the inclined bottom and contact of both edges with the curved bottom. In the punches having the inclined and curved bottoms, small burrs and no burrs were obtained for the 980 MPa grade steel sheet, respectively.