Transformations
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I.
INTRODUCTION
O N a continuous galvanizing line (CGL), hot-dip galvanized sheet is annealed immediately after coating to produce galvanneal, a nominal 10 wt pet iron Fe-Zn alloyed coating. The alloyed coating has been shown to have improved weldability, paintability, and cosmetic corrosion resistance over that of a pure zinc galvanized coating. However, the annealing process promotes the interdiffusion of iron and zinc, which leads to the formation of brittle Fe-Zn intermetallic phases. During stamping or forming of the coated sheet, the coating can flake off or powder because of the presence of these brittle intermetallic phases. The powdering can reduce coating quality and hinder further stamping operations owing to a buildup of coating particles on the surface of the coating as well as on the dies themselves. ]~,2]The phases that form during annealing are represented in the zincrich comer of the Fe-Zn equilibrium phase diagram in Figure 1. ]3] To improve the understanding of coating alloy layer formation, simulation of the galvannealing process was performed. Galvannealing has many process parameters which are related to the hot-dip galvanizing and subsequent annealing of the coated sheet. In order to simplify the analysis, all of the process variables were held constant except for the postcoating annealing temperature and hold time. These two parameters were varied and studied for their effects on coating structure development. Powdering performance of these simulated galvanneal coatings is discussed elsewhere, t4] C.E. JORDAN, Doctoral Student, and A.R. MARDER, Professor, are with the Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18015. Manuscript submitted March 29, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
II.
EXPERIMENTAL PROCEDURE
To study the influence of substrate chemistry on galvanizing and subsequent annealing, two steels were chosen for analysis: (1) drawing quality special killed (DQSK) and (2) titanium stabilized interstitial free (IF) steel. The chemical composition of each substrate steel is listed in Table I. The steels were hot-dip galvanized on the Armco Research Center pilot line in Middletown, OH. The sheet speed through the line was 6.4 m/rain. In the first section of the continuous pilot line, the cold-rolled sheet was recrystallization annealed between 650 ~ and 760 ~ in a 75 pet N2-25 pet H2 reducing atmosphere, which had a dew point of -23 ~ The sheet was then hot-dip galvanized at a bath temperature of 460 ~ The galvanized coating was allowed to cool, and the resuiting coating weight averaged between 52 and 55 g / m z on each side of the sheet. The as-galvanized steel served as the starting material for this study. In addition to substrate chemistry effects, the role of bath chemistry was also examined. Originally, two levels of aluminum in the bath in the range of 0.1 to 0.2 wt pet were chosen for study. The total aluminum content of the baths used on the pilot line (as determined by chemical analysis) were 0.11 and 0.22 wt pet. The to
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