Effect of substrate grain size on iron-zinc reaction kinetics during hot-dip galvanizing
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I.
INTRODUCTION
Interstitial-free (IF) steel grain boundaries are essentially carbon free due to the precipitation of Ti and Nb carbides within the grains. Hisamatsu postulated[1] that the chemical nature of IF steel grain boundaries increases their thermodynamic activity during hot-dip galvanizing, resulting in the preferential nucleation and growth of Fe-Zn phases at these sites. Further, he suggested[1] that a finer grain size substrate is more reactive, because Fe-Zn phases are thought to first nucleate and grow at substrate steel grain boundaries; therefore, as more grain boundary area is available for reaction with the liquid Zn bath on a fine grain size steel surface, more rapid Fe-Zn phase growth results. Recent investigations[2,3,4] have shown that as the grain size of a Ti stabilized IF steel decreases, Fe-Zn reaction rates increase during post-dip annealing. Guttmann et al.[3,4] proposed that the Fe2Al5 inhibition layer developed on the substrate during galvanizing forms epitaxially so that Fe2Al5 crystals developed colonies of the same orientation as the underlying substrate steel grain. The colonies form low-angle boundaries when contained within an underlying substrate grain interior, and therefore, Fe and Zn diffusion follow normal diffusion rates at these low-angle boundaries. However, at the location of a substrate grain boundary, two adjacent Fe2Al5 colonies of different orientation form a random high-angle boundary. This high-angle boundary can become a short circuit diffusion path for Fe and Zn interdiffusion, which leads to Fe-Zn alloy phase outburst formation (rapid Fe-Zn phase growth) at substrate grain boundary sites. Thus, the steel substrate grain boundary is C.E. JORDAN is Physical Metallurgist, Knowles Atomic Power Laboratory, with Lockheed Martin, Schenectady, NY 12301. A.R. MARDER is Professor with the Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18015. Manuscript submitted March 5, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
expected to be indirectly related to outburst formation, because the steel substrate grain boundary fixes the location of the short circuit diffusion paths in the Fe2Al5 inhibition layer.[3,4] The effects of IF steel substrate characteristics, such as grain size, on Fe-Zn reaction kinetics during processing are of great interest to the steel coating industry, as a detailed understanding of their specific effects is a necessary prerequisite to the development of optimally alloyed galvanneal coatings. The objective of the present study is to investigate the effects of steel substrate grain size characteristics on Fe-Zn reaction kinetics and phase formation during galvanizing. II.
EXPERIMENTAL PROCEDURE
The substrate material used to study the effect of substrate grain size on galvanizing reaction kinetics was a lowcarbon steel alloy initially produced in ingot form by BHP Steel (Port Kembla, Australia). The chemistry of this laboratory material is given as alloy LC in Table I. The ingot was rolled to final sheet thickne
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