Hardenability Improvements and Rate-Limiting Reactions During Hot-Dip Galvanizing of High-Mn Dual-Phase Steels

Abstract

<p> Intercritically annealed steels, such as dual-phase steels, have found widespread use in automotive structural components due to their high strength and ductility. Elements such as Mn, Al and Si, added to improve the mechanical properties are selectively oxidized during heat treatment and limit the ability of the alloy to be reactively wet during continuous hot-dip galvanizing. Subsequently, a limit has been placed on the amount of alloy which can be used if the steel is to be subsequently galvanized. The specifics of this limit have not been explored in detail, nor has the mechanism of decreased wettability been well demonstrated in the literature other than to say that the galvanizing reaction is limited by oxides on the surface.</p> <p> Using a force balance, it is shown that the presence of MnO on the surface of steels greatly reduces the wettability with a typical galvanizing bath (Zn-0.2wt%Al, Fe-saturated, 460°C). Furthermore, it was determined that this is caused by the additional and rate-limiting step of aluminothermic reduction of the oxide layer with the bath Al, required for subsequent inhibition layer formation. By using a low pO2 during annealing, the wettability was improved by reducing the thickness of the MnO layer when compared to intermediate and industrially common values of pO2. Using a high pO2 also resulted in improved wettability since the internal oxide which was formed did not reduce the wettability since it was not exposed to the bath alloy.</p> <p> Improvements in hardenability were also explored via dilatometry showing that the formation of bainite is delayed with increasing Mn content, as well as a decrease in transformation temperatures from γ during cooling (i.e. Ms and Bs). At ~5wt% Mn, only the the transformation to αM could be observed. This opens the door to higher strength, galvanized steels - as well as possibly galvanized martensitic steels.</p>