Effect of Process Atmosphere Dew Point and Tin Addition on Oxide Morphology and Growth for a Medium-Mn Third Generation
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THE increasing demand for lighter, safer, and more fuel-efficient vehicles legislated by energy and environmental agencies has resulted in third generation advanced high-strength steels (3G-AHSS) receiving significant interest from leading auto steel researchers and OEMs as candidate materials for the manufacturing of reduced mass automotive structural components.[1–5] In particular, the 3G-AHSSs are being designed to possess a superior combination of high strength and ductility vs the first generation AHSSs at a lower cost and a leaner chemistry compared to the highly alloyed second generation AHSS, e.g., twinning induced plasticity (TWIP) and austenitic stainless steels.[1–4]
MAEDEH POURMAJIDIAN and JOSEPH R. MCDERMID are with the McMaster Steel Research Centre, McMaster University, 1280 Main Street West, Hamilton, ON L8S 4L7, Canada. Contact e-mail: [email protected] BRIAN LANGELIER is with the Canadian Centre for Electron Microscopy, McMaster University, 1280 Main Street West, Hamilton, ON L8S 4M1, Canada. Manuscript submitted February 1, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
However, the use of these next generation materials requires innovative solutions for maintaining the structural integrity of the vehicle against corrosion, particularly since the desired weight reduction is generally brought about by the use of thinner material cross-sections and more complex geometries for manufactured parts. Continuous hot-dip galvanizing is a widely practiced, cost-effective industrial process for this purpose, where a zinc alloy coating is applied to sheet products after being heat treated in a dew-point controlled, reducing N2-H2 atmosphere. However, the annealing atmospheres commonly used in industrial practice are not reducing with respect to the commonly used alloying elements such as Mn, Si, Cr, and Al and, as a result, selective oxidation of these elements can occur at the surface and subsurface of the steel, possibly rendering the sheet surface incompatible for reactive wetting by the continuous galvanizing Zn-Al-Fe bath. For example, it has been shown that the presence of an integral, relatively thick external oxide layer can prevent the essential reactive wetting reactions[6–8] from taking place at the steel/zinc interface during immersion in the Zn(Al,Fe) bath, resulting in poor coating adhesion and unacceptable final product quality.
A variety of solutions have been proposed to address this issue, where several authors have suggested tailoring the process atmosphere conditions such that external selective oxidation of the alloying elements is either hindered or transferred from the surface to the subsurface. This can be achieved through different routes, such as increasing the oxygen partial pressure (pO2) of the process atmosphere,[9–14] increasing the hydrogen content of the gas mixture,[15] or implementing a pre-oxidation treatment prior to annealing.[16,17] More recently, the known effect of surface segregation of minor alloying additions such as Sn and Sb on retarding the rates of surface re
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