12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

The hot-stamping (HS) process has been widely used to produce automobile components, because of the great demand for higher strength steel sheets. Moreover, coated steel sheets, including galvannealed steel sheets (GA), are being applied as HS material [1,2]. The structural change of GA coating during the HS heating process has been researched [2,3]. However, the mechanical properties of this Fe-Zn solid solution has not yet been clearly understood. In this study, the hardness and the compressive deformation property of Fe-Zn solid-solution were investigated.
Hot stamped GA samples were prepared by heating them in an air furnace at 900°C for 4 minutes and then cooling them rapidly by pressing with flat mold tools. The hardness was measured by the micro-Vickers hardness test. The compressive deformation property was evaluated in the micropillar compression test. These micropillar specimens of Fe-Zn solid-solution single crystal were prepared with FIB. The crystal structure was measured by using XRD.
The hardness of Fe-Zn solid-solution was about 300 Hv. This is about six tenths of that substrate after hot-stamping (martensite), and about one-and-a-half times as hard as the substrate before hot-stamping (ferrite). In the compression test, the yield stress of Fe-Zn solid-solution single crystal was about 800 MPa, about half of that of the substrate (martensite). Furthermore, Fe-Zn solid-solution could be deformed by 9 percent. In these results, it was revealed that a Fe-Zn solid-solution is softer than the substrate of martensite, and that Fe-Zn solid-solution has a certain amount of plastic deformation capacity of compression. By analyzing the lattice spacing with XRD pattern, it was revealed that Zn atoms are located randomly in the Fe-Zn solid-solution lattice. Therefore it is presumed the mechanical properties of Fe-Zn solid solution are derived from those of ferrite.

References
[1] K. Imai et al.: CAMP-ISIJ, 18 (2005), 557
[2] J. Faderl et al.: Proc. GALVATECH 2011 Conf., Genova, Italy, (2011), 335.
[3] A. Sengoku et al.: Proc. CHS2 2015 Conf., Toronto, (2015), 363.