Effect of Sn Addition, Process Atmosphere p O 2 , and Annealing Time on the Selective Oxidation of a C-2Mn-1.7Si (Wt Pct
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ONE of the most intense fields of study by the steel industry since the mid-1990s has been the galvanizing of advanced high-strength steels (AHSS), which can be challenging due to the selective oxidation of the alloying elements required to achieve the desired microstructures and properties during annealing prior to galvanizing in the commonly employed continuous galvanizing line (CGL) process atmosphere of N2-(5 to 20 vol pct) H2-xH2O.[1–5] The morphology, spatial distribution, and chemistry of these oxides on the substrate surface indicate whether the desired g-Fe2Al5Znx interfacial G. SEYED MOUSAVI and J.R. MCDERMID are with the Steel Research Centre, McMaster University, Hamilton, ON L8S 4L8, Canada. Contact e-mail: [email protected] B. LANGELIER is with the Department of Materials Science and Engineering, McMaster University, Hamilton, ON L8S 4L8, Canada. Manuscript submitted October 18, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
layer can form and whether the finished coating is acceptable.[1–4]. For example, it has been established by several authors[4–12] that the presence of film-like external oxides can block the reaction between the substrate Fe and the bath dissolved Al to form the required g-Fe2Al5ZnX interfacial layer, characteristic of reactive wetting having occurred. However, the presence of either thin film-like or widely distributed nodule-like external oxides can promote contact between the substrate Fe and bath Al to facilitate the formation of the g-Fe2Al5ZnX interfacial layer through mechanisms such as aluminothermic reduction, oxide lift-off, and bath infiltration between the oxide and substrate, thereby leading to high-quality Zn coatings during continuous galvanizing. Several authors have proposed processing methods to alter the external oxide chemistry and morphology to facilitate reactive wetting of AHSS surfaces by the continuous galvanizing bath. For example, annealing under higher process atmosphere oxygen partial pressures (pO2) (i.e., higher dew points)[13–16] and applying the oxidation-reduction process[17,18] have been reported
to favorably modify surface oxide characteristics and, consequently, improve the reactive wetting of AHSSs. Another recently advocated approach to modifying the external oxide morphology is the addition of surface-active elements such as Bi, Sn, and Sb to the steel composition. These elements have a strong tendency to segregate to the steel surface and grain boundaries in order to decrease the local elastic lattice strain energy, attributed to their larger atomic radius compared to the substrate.[19,20] This segregation can also decrease the substrate surface energy, as shown by Seah and Hondros[21] in the case of adding Sn to pure iron. Several studies have examined the effect of Sn[22–25] and Sb[26–30] on the surface and subsurface oxide characteristics arising from annealing. Lyudkovsky[26] added up to 0.08 wt pct Sb to a 0.02C-4Mn-1Si-2Al (wt pct) steel and showed that the most significant drop in the depth of internal oxidation could be achieved
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