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

Abstract

Liquid metal assisted cracking (LMAC) and so‐called micro cracking, 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 micro cracking behavior of ZnFe coated (galvannealed) boron steels 22MnB5 and 22MnMoB8. Several soaking times and forming start temperatures in the range of 800‐520 °C were examined using a laboratory press hardening equipment with a hat‐profiled forming tool. The results indicate that the penetration depth of micro cracks 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 heterogenous microstructure (ferrite + austenite/martensite) below the coating/steel interface, which promotes the penetration of micro cracks. The crack depth is generally reduced with a conversion‐delayed 22MnMoB8 steel, however, a crucial reduction is attained only at lowest hot forming temperatures 550 °C and 520 °C. The results of 22MnMoB8 uncouple the effect of high temperature ferrite formation from the micro cracking mechanisms and suggests that the embrittling effect from zinc or zinc‐rich intermetallic phases plays a crucial role at conventional hot forming temperatures 800‐600 °C.