Spangle formation in galvanized sheet steel coatings
- PDF / 2,510,179 Bytes
- 10 Pages / 603.28 x 783.28 pts Page_size
- 92 Downloads / 261 Views
INTRODUCTION
G A L V A N I Z E D sheet is produced commercially by hot dipping steel sheet in a galvanized bath to produce a thin zinc-rich protective coating on the steel. The galvanized layer often has a structure consisting of very large grains, termed "spangles." The spangles are markedly dendritic and have relatively rough surfaces. In some applications, galvanized sheet with smooth surfaces is required, particularly for painted exposed panels in the automotive and appliance industries. A better understanding of the factors related to the growth, size, and surface roughness of spangles could contribute to more control of the surface quality of the galvanized sheet. The presence of large spangles in the galvanized layer has been associated with thermal undercooling of the melt. Cameron and Harvey [1] have reported undercooling in the galvanized layer of between 6 ~ and 10 ~ prior to solidification; Kim and Patil t2] reported undercooling of 23 ~ Examining high-purity zinc liquid droplets, White t3] observed undercooling of less than 1 ~ prior to solidification. It is not clear why the undercooling values reported differ so markedly. Large spangles are associated with solute additions to the galvanizing bath. Lead is the solute generally added to the zinc to produce spangles at concentrations greater than 0.05 wt pct Pb. It has been proposed by Cameron and Harvey t~j that the lead addition markedly reduces the number of effective nuclei in the melt, resulting in much larger grains. Additions of 0.15 wt pct Sn or 1.25 wt pct Cd, without Pb, did not produce spangles in the galvanized layer. E4]At present, the galvanizing industry is trying to eliminate the use of lead in their galvanizing baths for environmental reasons. Spangles are also produced with Sb bath additions of 0.58 wt pct Sb and with Bi additions. Es] Aluminum (0.1 to 0.3 wt pct) is added to the galvaF.A. FASOYINU, Poctdoctoral Fellow, on leave from Obafemi Awolowo University, Ue Ife, Oyo State, Nigeria, and F. WEINBERG, Professor, are with the Department of Metals and Materials Engineering, University o f British Columbia, Vancouver, BC V6T lW5, Canada. Manuscript submitted August 1, 1989. METALLURGICAL TRANSACTIONS B
nizing bath primarily to suppress growth of the Zn-Fe phases at the steel sheet surface. In general, it does not influence spangle growth. Spangle size is influenced by the cooling conditions during solidification. In general, smaller spangles are produced with higher cooling rates. Two processes are used industrially to obtain small grain sizes. In one case, in the Heurtey process, 5-/~m-diameter zinc powder is blown against the galvanized layer during cooling, giving spangles of 0.1- to 1.5-mm diameter. In the second, the "atomized steam process," high pressure steam is blown on the surface of the solidifying coating, producing spangles of 0.1- to 2.5-mm diameter. Galvanized sheet steel with spangles has been temper roiled to produce a smooth surface. This can remove small surface projections but not the large depressions at the gra
Data Loading...
