Continuous Galvanizing

The continuous galvanizing process applies a zinc coating to the surface of a continuous ribbon of steel sheet as it passes through a zinc bath. The coated sheet coils are either directly roll formed or fed into stamping presses, or blanked/sheared and then formed into parts. The sheet thickness might be as thin as 0.010 inch [0.25 mm] or less, to as thick as 0.25 inch [6.3 mm]. The facilities in operation worldwide are typically “light-gauge”, “intermediate-gauge” or “heavy-gauge” coating lines. Product from light-gauge lines is used mostly for applications in the construction industry (roofing sheets, building sidewall panels, flashing, etc.) The largest application for product made on intermediate-gauge lines is automotive body panels. Product from heavy-gauge lines is used for culvert, automotive structural parts, grain bins, etc.

In this process, the steel sheet is passed through the molten zinc bath at speeds as high as 650 fpm [200] mpm. As the moving sheet exits the coating bath, it drags out molten zinc. The desired coating thickness is attained by the use of “gas knives”. These knives typically use air as the gas, and are directed at both sides of the sheet to remove excess zinc. The coated steel is then cooled and the zinc solidifies on the surface of the sheet.

Key: 1. Entry Gauge - 2. Double Cut Shear - 3. Welder - 4. Cleaning - 5. Entry Looping Tower - 6. Preheat Furnace - 7. Direct Fired Furnace - 8. Radiant Tube Furnace - 9. Gas Jet Cooling - 10. Controlled Cooling - 11. Zinc Pot - 12. Air/Nitrogen Wiping - 13. Galvannealing Furnace - 14. Tower Roll Clean - 15. - Quench Tank - 16. Water Cooling Dryer - 17. Coating Weight Gauge - 18. Temper Rolling -19. Tension Leveller - 20. Skinpass Mill Dryer - 21. Delivery Looping Tower - 21. Electrostatic Oiler - 23. Flying Shear - 24. Tension Reel - 25. Coil Cars - 26. Scale and Automatic Shipping

The continuous galvanizing process for producing coated steel sheet involves a series of complex steps, one of which is to anneal the steel to soften it and make it more formable. More details about the continuous galvanizing process are given in GalvInfoNote 2.1.

One of the most important features of the continuous galvanizing process is the formation of a strong bond between the steel and its zinc coating. At the processing speeds used on continuous galvanizing lines the strip is only in the zinc bath for 2 to 4 seconds. During this brief time the molten zinc and steel must react to form a strong metallurgical bond by way of diffusion. The bonding region is an intermetallic compound, termed the “alloy layer”.

This thin alloy bonding zone, which is usually only 1 to 2 micrometers thick, is critical because after the coating is applied and the sheet cooled to room temperature, it is recoiled and shipped to customers for forming into the desired shape. For example, the sheet might be deep drawn to form a canister, it might be stamped into a car body panel, or it might be roll-formed into a building roof panel. For the forming operation to be done successfully, the steel and zinc have to be well bonded to each other. If the bond zone is not formed, or not formed correctly, the steel and zinc would not “stick” together during the many critical forming steps that the coated sheet might undergo.

An adherent and formable bond zone requires that the alloy layer be thin and of the correct composition. This is because the intermetallic compound that the bond layer consists of is very hard and brittle, an inherent characteristic of such alloy layers. There is no metallurgical process that will make the bond zone soft and ductile. By producing a thin alloy layer of the correct composition, the coated steel sheet can be formed into many intricate shapes without loss of adhesion between the steel and zinc coating. If the alloy layer becomes too thick, or is of the wrong composition, cracks develop in it during forming and the steel and zinc coating may disbond when formed. A thin alloy layer of the correct composition can be bent and stretched without cracking and disbanding. 

In summary, it is very important for the steel and zinc to form a proper bonding zone, and that this zone is thin. The producers of hot-dip galvanized sheet readily accomplish this by focusing on two primary control points:

1. The addition of a controlled amount of aluminum (approximately 0.15 to 0.20%) to the molten zinc coating bath, and

2. Control of the steel sheet temperature at the point where it enters into the molten zinc and control of the temperature of the zinc coating bath.