We at Nam Kim steel joint stock company, are constantly trying to upgrade our products and improve our services to suit our customers' requirements. We have got the ISO 9001:2008 and ISO 14001:2004 Certificate. The following products standards we produce are JIS G3141/G3302/G3312/G3321/G3322 and the equivalent ones, we are pleased to introduce our Galvanized Steel Coil, Hot dip Zinc Coated Steel - GI & HGI, PPGI, GL, PPGL.
Continuous sheet galvanizing
is also a hot-dip process, but is only applied to steel sheet, strip,
and wire. A coil-to-coil process, steel sheets from 0.010 to 1.70
inches (0.25 mm to 4.30 mm) thick and up to 72 inches (1,830 mm) wide is
passed as a continuous ribbon through cleaning baths and molten zinc at
speeds up to 600 feet per minute. Preparing
the steel for the continuous galvanized coating begins with cleaning in
an alkaline liquid combined with brushing, rinsing, and drying. Then
the steel passes into the heating or annealing furnace to soften it and
impart the desired strength and formability. In this annealing furnace,
the steel is maintained under a reducing gas atmosphere, composed of
hydrogen and nitrogen, to remove any oxide that may be on the surface.
Just as in the batch hot-dip process, the steel must be completely clean
of oxides and contaminants for a successful coating.
As
the steel exits the furnace, it enters into a vacuum chamber, or snout,
before entering the molten zinc bath to prevent and air from
reoxidizing the heated steel product. The steel is then sent around a
submerged roll in the molten bath to create the bonded coating and
removed in a vertical direction. As the product is withdrawn from the
bath, precisely regulated, high-pressure air (air knife) is used to
remove any excess zinc to create a closely controlled coating thickness.
The steel is then allowed to cool and solidify before contacting
another roll to avoid transferring or damaging the coating.
How is the continuous process is used to make seven different types
galvanized (zinc)
galvannealed (90-92% zinc/8-10% iron alloy)
two alloys of zinc and aluminum (55% aluminum/45% zinc alloy and 95% zinc/5% aluminum alloy)
two aluminum based alloys (100% aluminum, 89-95% sluminum/5-11% silicon alloy)
terne coating (85-97% lead/3-15% tin alloy)
As
the name states, this process is only applied to sheet steels. The
most common applications are in car bodies, appliances, corrugated
roofing and siding, duct work, and culvert pipe. The smooth coating
does allow it to be treated for painting, which will increase service
life. Because of the relatively thin coating, unpainted continuous
sheet galvanizing is recommended for interior applications or where
exposure to corrosive elements is mild. ASTM A653 / A653M Standard Specification for Steel Sheet, Zinc-Coated (Galvanized) or Zinc-Iron Alloy-Coated (Galvannealed) by the Hot-Dip Process governs the coating of steel via the continuous process.
Coating Characteristics
Because
both are hot-dip processes, continuous sheet and batch hot-dip
galvanizing are often confused. One major difference in the two
coatings is the thickness. The continuous sheet galvanizing process has
greater control and preciseness when it comes to zinc thickness as the
air knife used after galvanizing ensures a uniform thickness across the
steel sheet. The coating is mostly unalloyed zinc, though minimal alloy
layers are present, and is ductile and able to withstand deep drawing
or bending without damage. This is important as the coating is applied
prior to final fabrication such as punching, bending, and cutting. Because
of the precise control of coating thickness, continuous sheet is
stocked in a variety of coating weights. One of the most common zinc
coatings is Class G90, which has 0.9 oz/ft2 of zinc (total both sides) or about 0.80 mils (20 μm) per side. Service life for all zinc coatings is linear to zinc th ickness;
however, because the continuous sheet coating is applied
pre-fabrication, final forming and placement often includes punching
holes, bending, cutting, etc. which creates uncoated areas. Like batch
hot-dip galvanizing, the surrounding zinc will provide cathodic
protection to these uncoated areas, but as there is much less zinc
present, best practice is to touch-up any exposed areas after
fabrication to extend service life.
