The behavior of silicon in the solidification of Zn-55Al-1.6Si coating on steel
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NTRODUCTION
THE Zn-55Al-1.6Si coated steel products (trademark GALVALUME* or ZINCALUME† and various other brand names) have been increasingly replacing the traditional gal*GALVALUME is a trademark of B1EC International Inc. †ZINCALUME is a trademark of BlueScope Steel Limited.
vanized products in the building industries. Compared to galvanized steel, Zn-55Al-1.6Si coated steel is superior in several aspects. First, the uniform and fine spangle appearance (Figure 1) of the Zn-55Al-1.6Si coating provides an aesthetic effect to the buildings constructed with unpainted products, and second, because of its unique composition and microstructure, it combines the good durability of an aluminum coating and the sacrificial characteristics of a zinc coating. Extensive monitoring of the performance of the Zn-55Al-1.6Si coated steel products under various environments in the United States proves that the durability of the Zn-55Al-1.6Si coating on steel sheets is more than twice that of an equal-thickness zinc coating.[1] Under most of the residential environments in Australia, the expected life of Zn-55Al-1.6Si coated steel is in excess of 100 years, or 4 to 12 times longer than its galvanized counterparts.[2] Since the first production in 1972, the volume of Zn55Al-1.6Si coated steel produced in the world has been increasing each year. Currently, 45 steel companies from 23 countries in the world are licensed to produce Zn-55Al-1.6Si coated steel, with the annual output volume of 5 million tons,[3] or a total surface area of nearly 1000 km2. The Zn-55Al-1.6Si coating contains, in weight percent, 55Al, 43.4Zn, and 1.6Si, or, in atomic percent, 74Al, 24Zn, and 2Si. In most cases throughout this article, the coating Dr. R.Y. CHEN, Senior Research Scientist, is with BlueScope Steel Research, Australia. Contact e-mail: [email protected] Dr. D.J. WILLIS, formerly Manager, Metallic Coatings Research, is Manager, Knowledge Sharing and Information Management, with BlueScope Steel Research. Manuscript submitted December 22, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A
composition will be presented in weight percent (wt pct or pct), unless otherwise indicated. The coating on steel strip is applied by a continuous hot dip coating process similar to that of continuous galvanizing.[4] Because of the higher melting point of the Zn-55Al-1.6Si alloy than that of zinc for galvanizing, the coating bath temperature is normally set at about 595 °C to 610 °C. After hot dipping, the excess molten alloy on the strip surface is wiped down and smoothed using a pair of air knives on both sides of the strip, which also cool the strip slightly and control the coating thickness. Typically, the coating thickness retained on the surface is about 20 m. The molten coating layer then gradually solidifies as the strip travels upward in a cooling tower above the molten Al-Zn alloy bath, and sufficient cooling is provided to ensure that the solidification process is complete before it reaches the top of the tower. Early studies thought that[4,5] it
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