Continuous galvanizing could also be known as Pre-galvanized zinc, mill galvanized, hot dip mill galvanized or continuous hot dip galvanized. Continuous galvanizing is a form of hot-dip galvanizing. Continuous galvanizing is used for coating sheet. 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.
Steelmakers galvanize huge sheets of steel by passing them continuously through the zinc bath and coiling them up afterward. This method of galvanizing forms coatings that are about 3/1,000 inch (0.076 millimeter) thick. Sheet steel galvanized in this way may later be stamped or pressed to form such items as automobile body panels and steel roofing and siding.
ASTM B852 covers grades of zinc alloys, commonly known as continuous galvanizing grade (CGG) alloys, that contain aluminum, or aluminum and lead and that are used in continuous hot-dip galvanizing of steel sheet. CGG alloys are tested and conform to the chemical composition requirements as determined by chemical analysis on samples taken. CGG alloy castings should be free of undue surface oxide, adhering foreign matter, and any flash that would interfere with handling and use. Samples obtained during casting, drilling or sawing are analyzed individually and the average of the individual samples used to determine the analysis values for the lot.
In this process, the steel sheet is passed through a 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.
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.
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.
Continuous Hot Dip Galvanizing / Sheet Metal Coating
The most common application in continuous hot-dip galvanizing (HOG) is the coating of steel strip where industrial gases are used widely to achieve required properties. The basic process requirement is to achieve an oxide free surface so that the molten zinc starts to diffuse into the surface to form a robust bond so that coating will last for years to come.
A typical process starts with steel coil being fed to the pre-heating zone of a continuous furnace and then passing through high heat zones where an annealing process takes place. The temperature of the strip is then reduced to the temperature of the molten zinc bath. The strip then goes through a snout which is submerged inside a molten metal bath. The strip is pulled upwards passing through an air/gas knife arrangement to wipe the surplus molten metal from the surface. The strip is then cooled in air and re-coiled to complete the production cycle.
Steel becomes strip by passing through a series of processes called rolling to achieve its final shape. The rolling process starts with hot rolling where a slab, billet, or ingot is squeezed between rolls revolving at the same speed, but in opposite directions. The hot rolled strip is then cleaned to remove any surface oxide and further processed with cold rolling process with intermediate annealing processes to produce a smooth bright surface. Intermediate annealing plays a key role in the production process. Cold working steel accumulates stress so can only be reduced in thickness to a level where it must be re-annealed to relieve stress before further rolling. The final cold rolled material will therefore be in hardened state due its prior mechanical processing. This hardness and stress have to be relieved to enable post processing, mainly shaping to produce body panels for cars or external structures for home appliances.
The strip is annealed in furnaces where it is conveyed on rollers inside the furnace through several zones to achieve expected properties. This is called a continuous strip annealing furnace and can be very large -seven stories high and 500 m long.
While a furnace plays its part in heating and cooling the strip, it is necessary to achieve an oxide-free surface for both annealed-only and galvanized steel strip.
Bright annealing is carried out in a mixture of nitrogen and hydrogen using the hydrogen as the reducing agent. The surface oxides are cleaned off and after cooling in a protective atmosphere, the strip surface is bright and ready for use for further processing.
In the case of galvanising, the oxide-free surface plays a key role in product performance as the coating metal can only diffuse onto the strip surface a bit when the oxide is gone. It is therefore vital to make sure the reduction of oxides is completed before the strip goes into the molten metal bath.
Most of the continuous galvanising and annealing lines are significantly larger in size than many other heat treatment furnaces. However they tend not to employ furnace circulating fans hence reducing their ability to provide homogeneous medium to transfer heat and furnace atmosphere so that the production is at the highest possible standard.
High speed gas injection technology is used to improve circulation in furnaces where gas movement is poor. This is important especially when it is not practically possible to install and maintain circulating fans. This is particularly the case in continuous annealing and galvanising lines.
When coating metal is wiped with compressed air it is subject to oxidation. The surface of the coated steel becomes rough with oxide residues due to high levels of local oxidation of molten metal. Nitrogen has been proven to improve surface roughness by eliminating excessive oxidation.
Whilst the main driver is still the high quality expectation of automotive manufacturers, there is growing trend of using different alloys to improve the overall quality of the strip product.
This leads to use of nitrogen as a wiping medium since these new alloys requires more inert medium to avoid excessive oxidation so that the expected levels of surface roughness quality can be achieved. Thus nitrogen is becoming an industry standard to achieve a high surface quality in the production of coated steel strip.
Pregalvanized steel is produced by coating coils of sheet steel with zinc by continuously rolling the material through molten zinc at the mills. These coils are then slit to size and fabricated by roll forming, shearing, punching, or forming to produce our pre-galvanized strut products. The G90 specification calls for a coating of .90 ounces of zinc per square foot of steel. This results in a coating of .45 ounces per square foot on each side of the sheet. This is important when comparing this finish to hot dip galvanized after fabrication. During fabrication, cut edges and welded areas are not normally zinc coated; however, the zinc near the uncoated metal becomes a sacrificial anode to protect the bare areas after a short period of time.