2015 Volume 56 Issue 649 Pages 133-138
In this study, we investigated the coating effect of a piercing punch on the properties of the pierced surface, using 1180-MPa-class ultrahigh-strength steel sheets as workpiece materials. TiN was coated on the piercing punch by physical vapor deposition for this study. The analysis clarified the following facts: i) Holes pierced using noncoated and coated punches present almost the same hole-expansion ratio, ii) the fatigue strength of the holes pierced by the coated punch is superior to those of holes pierced by the noncoated punch. The work hardening, roughness, and residual stress on the pierced surface explain the mechanisms of these results. The application of the coating little increased the work hardening which deteriorates the hole-expansion ratio in general, but this increase was too small to expose the effect on the holeexpansion ratio. On the other hand, the application of the coating clearly improved the roughness by suppressing galling between the punch and the workpiece material. This improvement probably contributes to the superior fatigue properties. The suppression of galling also compressed the residual stress on the pierced surface and contributed to the superior fatigue properties in the case of the coated punch.