1470 MPa級超高強度鋼板のピアス加工における金型損傷に及ぼすPVD皮膜特性の影響

Abstract

We investigated the punch damage in the piercing of a 1470-MPa-class ultrahigh-strength steel sheet and discussed the effects of PVD coating properties on the punch damage behavior. Sequential piercing experiments were conducted using SKD11 punches with various types of PVD coating. A maximum of 240 shots of piercing were performed with a small clearance (5% of the pierced material thickness). The friction coefficient μ between the coatings and 1470 MPa steel was evaluated using a plate drawing tester at a high contact load without lubrication. For 120 shots of piercing, the higher the μ of coatings, the larger the damage area. For 240 shots, the effects of the heat resistance and adhesion of coatings additionally appeared. For the coatings with lower heat resistance, such as TiN and TiCN, the damage was greater and the effect of μ was more significant than for the coatings with high heat resistance, such as CrN, TiAlN, and TiCrAlN. In addition, among the low-heat-resistance coatings, the damage was smaller for the high-adhesion type. It was concluded that the frictional property, heat resistance, and coating adhesion are the major factors affecting punch damage in the piercing of ultrahigh-strength steel sheets.