Early Time Evolution of Selective Oxidation in a CMnSi AHSS
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INTRODUCTION
ADVANCED high-strength steels (AHSS) are of interest to the automotive industry as a partial, yet important contribution to the light-weighting of cars required to meet increased fuel economy requirements.[1–4] Possessing improved strength and ductility compared to standard high-strength low-alloyed steel (HSLA) commonly used in cars today,[1] AHSS can be formed into thinner gage parts with similar or improved strength. This allows cars to remain crashworthy despite the weight reduction.[5] Properties of AHSSs are derived from their highly engineered microstructures, which in turn are determined by their chemistry and thermomechanical processing parameters.[6] Silicon and manganese are common alloying elements found in many AHSS grades.[7,8] Though critical to mechanical performance, these elements often cause cosmetic problems further down the processing line. During final heat treatments to achieve the desired microstructure, silicon
MARY E. STORY and BRYAN A. WEBLER are with the Department of Materials Science and Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213. Contact e-mail: [email protected] Manuscript submitted September 10, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
and manganese both selectively oxidize under the conditions typically found in an industrial annealing furnace (N2-H2, dew point temperature (DPT) 253 K ( 20 C) to 243 K ( 30 C)).[9–11] While not particularly detrimental to material properties, the presence of silicon and manganese-rich oxides on the steel surface complicates the hot-dip galvanization process.[12–14] The oxides on the surface effectively form a barrier between the zinc and iron, preventing reactive wetting and formation of the Fe2Al5Znx layer. This ultimately results in bare spots on the finished product and leaves it vulnerable to corrosion. While oxidation of silicon and manganese cannot be completely avoided, particularly for more highly alloyed AHSS grades, work has been done to find ways of minimizing external oxidation and controlling its chemistry and morphology.[7,12,15–20] Manganese-rich oxide formation over silicon-rich was found to be somewhat less detrimental to the hot-dip galvanization process, due to their potential to aluminothermically reduce in the zinc bath.[15,21] Several works suggest certain external oxide morphologies can positively influence hot-dip galvanizing results.[22–26] It has been found that reactive wetting is possible when oxide nodules are widely spaced and surrounded by relatively thin internodular manganese-rich films. The precise mechanism of how this works has not been experimentally verified, but it has
been proposed that the molten zinc bath was able to penetrate the oxide/steel interface around oxide islands via cracks which had formed due to differences in thermal expansion coefficients.[27] It was also suspected that aluminothermic reduction of the thin internodular manganese-rich film also contributed to the success.[15,21,26] Given the influence of both oxide chemistry and morphol
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