Lift-Off of Surface Oxides During Galvanizing of a Dual-Phase Steel in a Galvannealing Bath
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ADVANCED High-Strength Steels (AHSSs), with their improved strength and good formability, are now widely used for body and closure applications in the automotive industry.[1–4] AHSSs are also deemed as ‘‘lightweight’’ metals because utilization of these materials also enables mass reduction and therefore increased fuel efficiency without sacrificing vehicle safety. To improve their corrosion resistance, AHSSs are coated with a thin layer (typically ~10 lm in thickness) of zinc (Zn). Cost-effective processes of continuous galvanizing (GI) and galvannealing (GA) are commonly used in the steelmaking industry.[5,6] To improve the strength of AHSSs, higher concentrations of manganese (Mn) and silicon (Si) are added as alloying elements, which raises concerns as to how these elements will affect the GI and GA processes in
K.F. CHEN and B. LI are with the Department of Chemical and Materials Engineering, University of Nevada, Reno, NV 89557. Contact e-mail: [email protected] I. ASLAM and M.F. HORSTEMEYER are with the Center for Advanced Vehicular Systems, Mississippi State University, Starkville, MS 39759 and also with the Department of Mechanical Engineering, Mississippi State University, Starkville, MS 39759. R.L. MARTENS and J.R. GOODWIN are with the Central Analytical Facility, The University of Alabama, Tuscaloosa, AL 35487. F.E. GOODWIN is with the International Zinc Association, Durham, NC 27713. Manuscript submitted October 29, 2018. Article published online May 17, 2019 3748—VOLUME 50A, AUGUST 2019
manufacturing. During annealing prior to galvanizing, Mn and Si are selectively oxidized and the oxides are difficult to be reduced.[5,7–9]. The surface oxides influence the galvanizability of AHSS in terms of formation of inhibition layer Fe2Al5 or Fe2Al5 xZnx[10,11] (a recent study suggested that the low temperature stable phase is Fe3Al8 rather than Fe2Al5[12]) which in turn influences the quality of the surface coating[13] and subsequent processing such as mechanical forming and welding.[6] Thus, it is necessary to understand the interfacial reactions and to properly characterize the resultant interfacial structure formed during galvanizing of AHSSs. For GI processes of AHSSs, a relatively high concentration of Al (0.14 to 0.2 wt pct. Hereafter material composition and bath Al concentration is referred to weight percentage unless stated otherwise) is dissolved in the Zn bath. This concentration leads to a fully developed inhibition layer, as reported by numerous experimental studies,[10,13–19] even when a layer of surface oxides was formed during heat treating.[15,18] Additionally, research has been conducted to find out how GI process parameters influence the quality of the zinc coating, such as Al concentration in the Zn bath[20,21] and particularly the dew point of annealing atmosphere.[10,22,23] A number of researchers have conducted experimental observations on phase evolution with different annealing times and characterizations on the interfacial structure, to understand how dew point,[10,14,24] steel composition especial
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