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.

The Influence of Precooling on Liquid Metal Embrittlement and Microstructure Properties of Galvanized 22MnB5 Tubes

“…First, the flow stress of the steel escalates with decreasing hydroforging temperature. [ 30 ] When subjected to rapid cooling at lower temperatures, the decomposition of austenite is substantially impeded, resulting in the predominance of an austenitic microstructure within the matrix just before the onset of hydroforging. At this juncture, the austenite exists in a supercooled state, and when deformation is applied, its free energy escalates, thereby amplifying the driving force for the austenite‐to‐ferrite transformation.…”

“…After immersion in a low‐concentration nitric acid–ethanol solution, martensite exhibits a brownish color, ferrite appears white, and bainite displays a layered black pattern. [ 29,30 ] As illustrated in Figure 13a, the microstructure of the quenched steel consists of a fine‐grained martensitic phase with a small fraction of ferrite. This is primarily attributed to the short holding time, which prevents the complete growth of austenite before its transformation into martensite.…”

Experimental Investigation of Hot Forming of Zn‐Coated Hot‐Stamping Steel

“…The content of Al element added is 5 wt pct. Järvinen et al [ 19 ] provided novel hot‐stamping steel of 22MnMoB8 for LME research. It can reduce the crack due to the conversion‐delayed property.…”

Effect of Heat Treatment on the Formability of Zn‐Coated Hot Stamping Steel

“…The method to decrease the friction in the forming process was also proposed to eliminate the crack formation. Järvinen et al [ 17 ] studied the microcrack of boron steel with Zn–Fe coating and demonstrated the measurements to reduce the penetration of microcrack. The experiment results showed that the low hot forming temperatures of 520 and 550 °C can lead to an obvious reduction of crack depth.…”