Abstract: Microstructure and surface oxide distribution of the galvanized layer of 22MnB5 hot formed steel were systematically characterized by scanning electron microscope (SEM) and energy dispersive spectroscope (EDS) at different austenitizing temperatures and holding time. The results show that when the austenitization temperature exceeds 850 ℃, the thickness of the galvanized layer increases, and a microstructure transformation occurs from pure Zn phase to α-Fe(Zn), and the interface with the matrix is blurred. As the austenitizing temperature rises from 850 ℃ to 900 ℃, the thickness of the zinc layer grows with the increase of temperature, and the zinc oxide particles become denser, interconnecting and agglomerating. When the austenitization temperature exceeds 900 ℃, large-sized Mn-rich oxides begin to emerge within the galvanized layer. When the austenitization time extends from 4 min to 8 min (austenitization temperature of 920 ℃), the degree of surface oxidation increases, the oxides increase in size and connect to each other in layers, resulting the α-Fe(Zn) layer being exposed and reducing the protective effect on the zinc layer, causing oxygen atoms to diffuse into the interior of the zinc layer to form Mn-rich oxides.

Key words: hot formed steel, zinc coating, austenitizing, oxide, microstructure