Effect of the Furnace Atmosphere on Galvanizabihty of AHSS and X-AHSS

摘要

Fuel mileage and greenhouse gas emissions requirements have instigated development of light weight steels in automotive parts. To keep up with the demand steelmakers focused their full attention on efforts to develop Advanced High Strength Steels (AHSS) and, by extension, eXtra Advanced High Strength Steels (X-AHSS). With increasing amounts of alloying elements such as Si, Mn and Al in the steels, they were faced with a critical problem which Zn is uncoated due to Si-Mn-Al oxides formed on the steels surface during annealing. In this study, we investigate effects of dew point in the furnace on galvanizibility of the AHSS and X-AHSS. When changing dew point of between -60 °C to 5 °C in an atmosphere having 5% of H_2 as well as N_2 as a remainder, chemical composition and morphology of the oxides of TRIP (Transformation-induced Plasticity) and Duplex steels were analyzed. For TRIP steels, inward flux of oxygen in the atmosphere of high dew point suppress outward diffusion of Si, Mn and oxidation onto the steel surface resulting in improvement of Zn wettability. In the case of Duplex steels, Mn diffusion and formation of thick Mn oxides onto steel surface were not restrained by high dew point furnace atmosphere, while Si and Al were oxidized internally. Zn wetting property of hot-dip galvanized Duplex steels was slightly inferior to that of TRIP steels. On the other hand, both TRIP and duplex steels showed excellent coating adherence due to the internal oxidation induced by increasing dew point.