Investigation of Streaky Marks Formed on the Coating Surface of Hot-Dip Galvannealed Steel Sheets

Abstract

Abstract Interstitial-free High strength (IF-HS) steels, the tensile strength ranging from 340MPa to 440MPa, are widely used in automobile industry. The major strengthening mechanism employed to increase the tensile strength of ultra-low carbon steels is solution hardening by the additional elements of P, Mn, Si, Cr and Cu. It has been well known that the surface condition of steel sheet substrates before dipping into a molten zinc pot significantly affects the final products quality of galvannealed (GA) steel sheets, i. e., the occurrence of the visual defects so-called “streaky mark” and others in GA process are strongly related to the surface enrichment of oxides in terms of the chemical composition of the steel sheet substrate. In the present study, high strength interstitial-free steel sheets with a relatively high amount of Mn and P were galvannealed in an industrial continuous galvanizing line and streaky-marks formed on the coating surface were characterized by optical microscopy, scanning electron microscopy, electron probe micro analysis and transmission electron microscopy. Effects of streaky marks on the mechanical properties and paintability of GA coating were also investigated.