Friction and Die Wear in Stamping Prephospated Advanced High Strength Steels

摘要

Prephosphated steels have been developed by applying the phosphate coating on zinc coated sheet steels to increase the lubricity in the automotive stamping process and adding extra corrosion protection. The prephosphate coating was also found to be able to further absorb the lubricant, which can reduce the oil migration and excessive amount of lubricant dripping on the die surface and the press floor. Due to its enhanced lubricity characteristic, the applications have been expanded to more-recently developed advanced high strength steels (AHSS). Because of the higher strength of AHSS, it is crucial to understand their performance under more extreme forming conditions such as higher die temperature, contact pressure and sliding speed, etc. The intent of this study is to investigate the tribological performance and die wear behavior of prephosphated AHSS in the die tryout and production conditions. In this study a typical hot-dip Galvannealed (GA) coated DP600 with and without prephosphate coatings is selected for the tribological performance study. The bending under tension test and strip draw test were used to evaluate the friction behavior. The Marciniak stretch-forming test and the limiting dome height test were also included to evaluate the formability performance of prephosphated AHSS. In addition to the DP600, a hot-dip Galvanized (GI) coated DP980 with the prephosphate coating was also included in the die wear study. The cyclic bend die wear (CBDW) test system based on the modified bending under tension test was used to investigate the effects of the prephosphate coating on the die wear and zinc coating build-up in the production stamping condition. Results indicated that the prephosphate coating has a stable tribological characteristic in reducing the friction coefficient even at the elevated die temperature, higher contact pressure and higher sliding speed conditions. Also, the prephosphate coating can further reduce the die wear, prevent the zinc coating built-up on the die surface and increase the formability.