Effect of Steel Composition and Processing Parameters on the Penetration Depth of Microcracks in ZnFe-Coated Boron Steels

Abstract

Liquid metal assisted cracking (LMAC) and so-called microcracking are limiting the application of hot-dip galvanized boron steels in the direct press hardening process. This study addresses the role of steel hardenability on the microcracking behavior of ZnFe-coated (galvannealed) boron steels 22MnB5 and 22MnMoB8. Several soaking times and forming start temperatures in the range of 800–520 °C are examined using a laboratory press hardening equipment with a hat-profiled forming tool. The results indicate that the penetration depth of microcracks can be reduced by improving the hardenability of steel, which enables hot forming in austenitic state at exceptionally low temperatures even without accelerated cooling procedures. The austenite decomposition of 22MnB5 leads easily to heterogeneous microstructure (ferrite + austenite/martensite) below the coating/steel interface, which promotes the penetration of microcracks. The crack depth is generally reduced with a conversion-delayed 22MnMoB8 steel; however, a crucial reduction is attained only at lowest hot forming temperatures of 550 and 520 °C. The results of 22MnMoB8 uncouple the effect of high-temperature ferrite formation from the microcracking mechanisms and suggest that the embrittling effect from zinc or zinc-rich intermetallic phases plays a crucial role at conventional hot forming temperatures of 800–600 °C.