THE ROLE OF FRICTION IN FRACTURES OF COATED STEEL SHEETS DURING PRESS FORMING

摘要

Coated steel sheets are broadly used in fabrication of automobile bodies to improve corrosion resistance. Coatings are zinc or zinc based alloys, such as Zn-Ni or Zn-Fe The use of these sheets introduce some manufacturing difficulties during press forming, associated with powder contamination of dies from the coating, surface damage and, in some cases, with fracture of the sheet. The onset of fractures in some parts made from zinc electrocoated steels is quite usual in press shops, and this work has been carried out on samples that failed in actual press practice. In all the cases, the cause of fractures has been associated with local, and unpredicted, high values of friction with the dies, and with the onset of galling between dies and the coated sheet. It was found that all materials that failed in service presented higher values of friction and were prone to galling in sliding friction tests The use of these tests is suggested to predict friction phenomena and to allow a more complete and reliable characterisation of the forming behaviour of precoated steel sheets.