Study on wrinkling limit of interstitial free steel sheets of different thickness when drawn through Conical and Tractrix dies

摘要

This paper deals with the wrinkling limit diagrams of cold rolled and close annealed of different grades of IF Steel sheets, which are used in Automotive Industries. The study pertains to deep drawing into cylindrical cups through Conical and Tractrix dies using a flat bottom punch. It is well established that, the use of either a Conical die or a Tractrix die can enhance the limiting draw ratio compared with that obtainable in a conventional drawing operation. When a Conical or a Tractrix die is employed the need for a hold down or clamping ring is eliminated. However, this enhances the propensity of the blank to fail by wrinkling or buckling, particularly in the early stages of a drawing process in which thin sheet blanks are used. It is proved by these researchers and others that the onset of wrinkling takes place when the ratio of strain increments (d+(εr)/d_(ε0)) reaches a critical value during the drawing process. This value, which could be, determined experimentally over which the wrinkling takes place has been shown in the form of wrinkling limit diagrams for the above different grades of IF Steel sheet. An attempt is also made to rank the IF Steel sheet grade that appears to suppress the wrinkling based on results obtained in the form of wrinkling limit diagrams, established for the above different IF Steel sheet grades, in terms of the strain increments ratio and the stress ratio parameters. Further, it is noticed that as a thickness of IF Steel sheet increases the safe region also increases. This means that higher is the thickness better is the resistance against wrinkling. Drawing through the Tractrix die shows better resistance against wrinkling comparing with the Conical die.