Solidification and spangle formation of hot-dip-galvanized zinc coatings
- PDF / 1,928,517 Bytes
- 16 Pages / 612 x 792 pts (letter) Page_size
- 32 Downloads / 283 Views
TRODUCTION
GALVANIZED sheet is manufactured commercially by hot dipping steel sheet in a galvanizing bath to produce a thin zinc-rich protective coating on the steel. The structure and the appearance of the obtained coating layer are influenced by the alloying elements added to the galvanizing bath. Industrially used zinc baths typically contain 0.15 to 0.25 wt pct Al, and Pb up to 0.2 wt pct Al is added to provide the unwelcome reaction of Fe and Zn by building a thin interfacial layer consisting of the intermetallic compound Fe2Al5. Lead is known to produce a structure consisting of very large grains, termed spangles; at concentrations greater than 0.03 to 0.04 wt pct. Until recently, spangle morphology was the only criteria for determining acceptability of hot-dip-galvanized steel, which was mainly used in the construction industry. Nowadays, galvanized sheet steel is extensively used for appliance and automotive applications. Accordingly, the quality requirements for galvanized sheet steel are becoming increasingly stringent because of the higher coating performance demanded, particularly for painted exposed panels.
Because of the more demanding uses of hot-dip-coated products, a better understanding of the galvanizing process, especially of the solidification and spangle formation, is important to control spangle related technological properties such as surface reactivity, cracking behavior, or paint adhesion. Up to now, a lot of investigations[1–6] have been performed, but the formation of the spangle, its origin, and the growth characteristics were widely speculative, and the knowledge of zinc solidification was only fragmentary. The aim of the present work was to supplement previous findings by new observations and to conclude a practicable solidification model for thin zinc layers, which can explain the correlation between crystal orientation, bath chemistry, growth characteristics, surface appearance, and spangle size. The present article is mainly based on a Master’s thesis by one of the authors (JS), which was performed at VOEST-ALPINE Stahl Linz GmbH in cooperation with the Montanuniversita¨t Leoben. II.
J. STRUTZENBERGER, Postgraduate Student, and J. FADERL, Senior Engineer, Metallic Coatings, are with the Research and Development Department, VOEST-ALPINE STAHL LINZ GmbH, A-4031 Linz, Austria. Manuscript submitted February 5, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
EXPERIMENTAL PROCEDURE AND RESULTS
A. Thermal Conditions Concerning zinc solidification, the amount of undercooling occurring in the liquid layer, and the temperature graVOLUME 29A, FEBRUARY 1998—631
Fig. 1—Schematic illustration of the HDG simulator (for better understanding the cooling jets and the wiping jets are drawn in a 90 deg tilted position).
Table I.
Correlation between Cooling Rate, Solidification Period, and Undercooling
Cooling Rate
Atmosphere
Solidification Period*
Undercooling*
2 K/s 4 K/s 15 K/s
air 5 pct H2/N2 5 pct H2/N2
10 s 5s 2s
,1 K ,1 K ,2 K
*Results obtained from two different bath compositio
Data Loading...
