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(EDS). X-ray diffraction patterns were obtained from the coating samples using a PHILIPS*
Phase Transition in Galvanneal Coatings
*PHILIPS is a trademark of Philips Electronic Instruments Corp., Mahwah, NJ.
N.-Y. TANG, X.B YU, AND F.N. COADY An unexpected discovery of phase transition in galvanneal coatings is herein reported. X-ray diffraction studies of commercially acquired galvanneal coatings indicated that d phase (cP555) in the coatings transformed into T phase (cF408) during the in-line annealing treatment when the holding temperature was abnormally high. Current knowledge of phase equilibria in the relevant Zn-Fe and Zn-FeAl systems failed to predict such a phase transition. In recent years, applications of galvanneal steel have increased due to its excellent corrosion resistance, good weldability, and paintability. Galvanneal production requires strict control of processing conditions in order to reach an optimal phase structure in the coating. To produce galvanneal, the strip passes through a molten Zn bath typically containing 0.135 pct Al. The Zn coating is converted to a ZnFe alloy layer consisting of ⌫, ⌫1, d, and z phases during the in-line annealing treatment immediately following the hot-dipping process. Automakers prefer the so-called “alld-galvanneal” coating in which the z-Zn13Fe crystallites in the alloy layer all transform into d phase with the (⌫ ⫹ ⌫1) layer at the interface of the coating and steel substrate limited to less than 1 mm in thickness. All-d-galvanneal has good friction characteristics in the press forming operation as well as good resistance to powdering and flaking. There is a correlation between the coating phase structure and coating Fe content. The overall Fe content of an all-dgalvanneal coating is within the range of 10 to 12 pct. When there are still some residual z crystallites on the top layer of the coating, the Fe content of the coating is below 10 pct. The Fe content of galvanneal coatings is monitored on- or off-line by producers using various techniques. The desirable Fe content, hence the desirable phase structure of the coatings, is achieved by controlling the processing parameters, including the bath effective Al level and the holding temperature of the annealing treatment. However, an optimal phase structure is not readily achievable. For example, a change in the steel substrate composition may require changes to several processing parameters in order to produce an all-d galvanneal coating. Consequently, the phase structure of galvanneal coatings varies from one coil to the next and from one producer to another. Recently, a number of galvanneal coating samples were industrially obtained by Teck Cominco’s Product Technology Centre. Cross sections of these coating samples were prepared and examined using a JSM-5800 scanning electron microscope (SEM) with an energy dispersive spectrometer
N.-Y. TANG, R&D Manager, Continuous Galvanizing, X.B. YU, Ph. D., and F.N. COADY, Senior Metallurgical Technologist, are with the Product Technology Centre, Teck Cominco Meta
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