Structural and Textural Characterization of the Substrate and Coated Layer in an Industrial Galvannealed Interstitial-Fr
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INTRODUCTION
THE modern auto industry demands steels having excellent corrosion resistance, especially in those countries where corrosive calcium or sodium chloride spreads are used to prevent roads from freezing during winter.[1] Several technologies have been developed that employ products that use zinc-based coatings. Among them, the galvannealing process has received the most attention, because galvannealed coated steel exhibits superior corrosion resistance, better paintability, and good weldability.[2] In the galvannealing process, steel is first immersed in an aluminum containing zinc bath and then given a postcoating heat treatment. This heat treatment causes the zinc in the coating to interdiffuse with the substrate iron to form several Fe-Zn intermetallic phases that are stacked on the steel substrate.[3] Industrial galvannealed coatings contain approximately 10 wt pct iron. Though galvannealed layers have the preceding advantages, they suffer from an inherent drawback in the form of poor formability. The higher is the iron content in the coating, the poorer is its formability. The embrittlement of the coating mainly depends on the iron content and distribution of different Fe-Zn intermetallic phases.[4–6] Several efforts have been made to correlate the microstructure of galvannealed coatings to their performance, especially the coating formability during press forming operations.[7–23] Most of the work has been carried out using galvanizing and galvannealing simulators, where the growth of the coating takes place under A. CHAKRABORTY, Researcher, and R.K. RAY, Visiting Scientist, are with the Research and Development Division, Tata Steel, Jamshedpur, PIN-831 007, India. Contact e-mail: ani_chakra@ yahoo.com S. SANGAL, Professor, is with the Indian Institute of Technology, Kanpur PIN-208 016, India. Manuscript submitted October 30, 2007. Article published online July 19, 2008 2416—VOLUME 39A, OCTOBER 2008
fully controlled experimental conditions. The relevance of the results of such investigations to the industrial galvanizing and galvannealing processes, where there is practically very little control of the working parameters during operation, is questionable. That is why the present work has been undertaken to characterize the structure and texture of the substrate and the coated layer and also to study the mechanical behavior of an industrially produced galvannealed coating over interstitial-free (IF) grade steel to yield data that may be useful to the steel industry. The present study involves several characterization techniques such as grazing incidence X-ray diffraction (GIXRD), cross-sectional optical microscopy, crosssectional scanning electron microscopy (SEM) with energy-dispersive spectrometry (EDS), glow discharge optical emission spectroscopy (GDOES), cross-sectional transmission electron microscopy (TEM), and also anodic dissolution. These techniques were employed to characterize the galvannealed coating and to identify the different Fe-Zn intermetallic layers such as the gamma (C), gamma1 (C1),
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