Selective Oxidation of a 0.1C-6Mn-2Si Third Generation Advanced High-Strength Steel During Dew-Point Controlled Annealin
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IN recent years, medium-Mn advanced high-strength steels have garnered significant interest from the automotive industry due to their superior combination of specific strength and formability as well as their potential ability to absorb energy during crash events. Use of these alloys may allow manufacturers to design vehicles with significantly increased fuel efficiency without compromising passenger safety. However, due to the relatively short history of material development in this class of alloys, there are a limited number of studies on the other essential engineering properties of these steel grades.[1] For example, corrosion protection of the steel
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] Manuscript submitted September 19, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
is of prime importance for maintaining the structural integrity of manufactured parts, for which the continuous galvanizing process is a cost-effective means of applying protective metallic coatings to sheet steels. In the continuous galvanizing process, a continuous steel sheet passes through an annealing heat treatment in a controlled, reducing atmosphere prior to dipping in a molten Zn(Al,Fe) bath, where the dual purpose of the annealing heat treatment is to control the microstructure of the substrate and to render the substrate surface suitable for reactive wetting by the molten alloy bath. However, obtaining a high-quality Zn coating on the steel surface can be challenging in the case of medium-Mn substrates due to the substantial levels of alloying elements, such as Mn and Si, which are necessary for obtaining the desired mechanical properties.[2–4] This challenge arises from the basic thermodynamics that, under typical continuous galvanizing annealing heat-treatment atmospheres, which are reducing with respect to Fe, alloying elements such as Mn, Si, Cr, and Al will be selectively oxidized. Kinetic factors, as described in a relatively simplistic manner using the
Wagner model,[5] dictate whether or not the selectively oxidized elements will manifest on the external surface and/or in the internal subsurface. In the case of the occurrence of external oxidation, it has been shown that these can prevent reactive wetting and the formation of the desired g-Fe2Al5ZnX interfacial layer during dipping of the steel in the Zn(Al,Fe) bath.[6–9] As a result, the final product can suffer from poor coating adhesion or contain unacceptable coating effects such as bare spots. A number of authors have focused on improving the reactive wetting behavior of low-alloy, high-Si content Transformation-Induced Plasticity (TRIP) and high-Mn Twinning-Induced Plasticity (TWIP) advanced highstrength steels (AHSS) by the continuous galvanizing bath, through tailoring the annealing process atmosphere conditions[7,10,11] as well as by modifying the alloy design.[12–15] These contributions largely sought to deter
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