Identification and characterization of the iron-zinc intermetallics formed in galvanneal steel.

摘要

The demand to improve the corrosion resistance of steel sheet, particularly for use within the automotive industry, has led to a dramatic increase in the use of coated steels in place of cold-rolled sheet steel. Galvanneal steel results from the post annealing of the zinc-coated steel sheet, in which iron and zinc are interdiffused to form an iron-zinc alloy coating. Within this alloy coating, four main iron-zinc phases, Zeta, Delta, Gamma-1, and Gamma may be present. Manufacture of the most suitable coating requires identifying which phases form during the galvannealing process, an understanding of the properties of each phase and knowing how to control the formation of any particular phase or phases in order to obtain optimum material performance. Positive identification of each phase and the fraction present in a galvanneal coating is very difficult. The primary cause of this difficulty has been the lack of high quality data on the crystal structure and the related microstructure of the separate iron-zinc phases. Therefore, through a detailed investigation of the iron-zinc alloys, we have compiled a database of their microstructural properties and used this information to study commercially produced galvanneal steel coatings.; A series of high purity iron-zinc alloys with iron concentrations in the range 5-30 at.% Fe were prepared and characterized. Bulk iron concentration of the samples were determined by chemical titration and induction coupled plasma spectroscopy. Sample homogeneity was analyzed with an electron microprobe and a scanning transmission electron microscope. Finally, Mossbauer spectroscopy and X-ray diffraction were employed to characterize the microstructural properties of the alloys as a function of iron concentration across each phase.; Next, a new Mossbauer detector capable of analyzing commercial produced galvanneal coatings in-situ was constructed and tested. The detector is able to simultaneously detect the {dollar}gamma{dollar}-rays, X-rays, and conversion electrons which are emitted from the galvanneal coatings following the resonant absorption of a {dollar}gamma{dollar}-ray. The detector probes the full coating depth allowing the complete coating composition to be determined.; Finally, the database of the crystallographic and hyperfine parameters of the iron-zinc intermetallics along with the new detector were used to study several commercially produced galvanneal coatings. The detailed analysis of the coatings has enabled the positive identification of the phases as layers within the coatings. Phase fractions and relative iron concentrations were determined for each coating. Furthermore, the Mossbauer spectral areas showed a linear correlation with the weight of iron in the coatings. Lastly, the effect of aluminum impurity in the galvanneal bath on phase formation was investigated.