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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.

This paper presents an experimental study of the effects of dynamic strain aging on the mechanical behavior of selected high carbon and chromium-manganese steels in dynamic loading condition. In ferritic-pearlitic steels, the dynamic strain aging is typically caused by carbon, nitrogen, and possibly some other small solute atoms. Therefore, the thermomechanical treatments affect strongly how strong the dynamic strain aging effect is and at what temperature and strain rate regions the maximum effect is observed. In this work, we present results of the high temperature dynamic compression tests carried out for two different ferritic-pearlitic steels, 16MnCr5 and C60, that were heat treated to produce different microstructure variants of these standard alloys. The microstructures were analyzed using electron microscopy, and the materials were tested with the Split Hopkinson Pressure Bar device at three different strain rates at temperatures ranging from room temperature up to 680 °C to study the effect of the heat treatments and the resulting microstructures on the dynamic behavior of the steels and the dynamic strain aging effect. The results indicate that for both steels, a coarse grain structure has the strongest dynamic strain aging sensitivity at small plastic strains. However, at higher strains, all microstructures show similar strain aging sensitivities.

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