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Zn and ZnFe coated 22MnB5 and 34MnB5 steels were subjected to the direct press hardening process in order to investigate the influence of steel composition on the resulting phase structures.Microstructures were characterized using advanced methods of microscopy. In addition, X-ray diffraction, glow discharge optical emission spectroscopy and thermodynamic calculations with Thermo-Calc® were carried out to support the analysis. The results indicate that the steel composition has a clear effect on the phase development within coating and interface regions.Whereas the behavior of the 22MnB5 was comparable to earlier investigations, a clearly nonconventional behavior of the 34MnB5 was observed: the formation of martensitic micro constituents, designated here as α'-Fe(Zn), were identified after die-quenching. The regions of the α'-Fe(Zn) formed mainly in vicinity of steel/coating interface and were emerged into the steel by sharing martensitic morphology with the base steel. The thermodynamic calculations suggest that carbon is effective in stabilizing the γ-Fe(Zn) phase, which enables the formation of the α'-Fe(Zn) in fast cooling. Therefore, the higher initial C content of the 34MnB5 may result in the kinetic stabilization of the γ-Fe(Zn) as the inter-diffusion between Zn and Fe occurs during annealing. Simultaneously occurring carbon partitioning from α-Fe(Zn) to γ-Fe(Zn) could explain a clearly increased C content of the coating/steel interface as well as higher Zn contents in the α'-Fe(Zn) phase compared to 22MnB5. Actually, the present study shows that the same phenomenon occurs also in 22MnB5 steels, but in a much smaller scale. In Zn and ZnFe coated 34MnB5, the thickness of the α'-Fe(Zn) layer was increased with longer annealing times and at higher temperatures. The morphology of the α'-Fe(Zn) layer resembled plate-like martensite and can be assumed to be brittle. Regarding this, the formation of α'-Fe(Zn) interface layer needs to be taken into account in press hardening of 34MnB5 steels.

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Two types of press hardening experiments were carried out to investigate the behavior of ZnFe coated 22MnB5 steel in direct press hardening process. The coating properties were studied using variable process temperatures and times with a flat-die and a forming tool. Coatings were analyzed with optical and scanning electron microscopes. The results indicated that steels that have low coating weights may be processed successfully with short dwell times. For high coating weights a significantly longer dwell time is needed. The behavior of ZnFe coating in hot press forming experiments was in line with literature and the findings of the flat-die experiments. Thus, the feasibility of the experimental press hardening equipment was confirmed.

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.

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