Home
/ Sheet / Coated
Sheet / Production
Methods / Hot Dip
Process
Coated Sheet
Production Methods
Hot-Dip Process
| 1. |
Entry Uncoilers |
7. |
Heating Zone |
| 2. |
Double Cut Shear |
8. |
Soaking Zone |
| 3. |
Lap Seam Welder |
9. |
Slow/Rapid Cool Zone |
| 4. |
Pre Cleaner |
10. |
Controlled Holding Zone |
| 5. |
Entry Accumulator |
11. |
Zinc Pot & Air Knife |
| 6. |
Cleaning Section |
12. |
Galvaneal Furnace |
| 13. |
Water Quench Tank |
18. |
Inspection Area |
| 14. |
Temper Mill |
19. |
Electrostatic Oiler |
| 15. |
Tension Leveler |
20. |
Rotary Shear |
| 16. |
Chemical
Treatment |
21. |
Exit Coilers |
| 17. |
Exit Accumulator |
|
|
U. S. Steel produces coated steel sheet by the continuous hot dip process at five
locations: Fairfield (Alabama) Works; Gary (Indiana) Works; Mon Valley (Pennsylvania)
Works; Fairless (Pennsylvania) Plant; and PRO-TEC, where we have a joint-venture
facility with two state-of-the-art coating lines. The hot-dip coating process
is used to produce U. S. Steel Hot Dip Galvanized, ACRYZINC®, GALVALUME®,
ACRYLUME®, Galvannealed, Phosphatized, Galvanized Culvert Sheet, Terne and
NI-TERNE Sheet. For U. S. Steel Terne and NI-TERNE Sheet, the hot-dip
coating process is substantially different than the process used for the
other products.
Typically, the process begins with an as-cold-reduced or pre-annealed
steel sheet especially selected for its final attributes (strength and formability)
after processing through the hot-dip coating line.
As the sheet is uncoiled, it is cleaned, because a clean surface is essential
to develop a good bond between the steel and the coating. On many lines, a
U. S. Steel-developed electrolytic cleaning unit subjects the sheet to a
combination of fast-flowing alkaline solution and electrical current for
efficient removal of oils, lubricants and other surface contaminants. Brushes
remove any vestige of remaining soil as the sheet passes through scrubber units. After
hot water rinsing, the sheet is completely dried by passing through rubber squeegee
rolls and a hot air dryer.
The cleaned sheet then goes through a continuous-annealing furnace where it is
given either a high-temperature or pre-heat anneal to prepare it for coating. When
a significant change in mechanical properties is desired, a high-temperature cycle
is used. In this cycle, the steel is heat-treated in a precision-controlled,
oxygen-free atmosphere to provide the different strength and formability levels
required by customer applications. The furnace is divided into zones to allow
exact control of temperatures for heating, holding and cooling the sheet to
attain the desired mechanical properties. When either minimal or no changes
in mechanical properties are desired, the steel is subjected to only a pre-heating
cycle. When the sheet leaves the continuous annealing furnace, the surface is
fully prepared to accept a smooth, even coating.
Immediately after the furnace, the sheet enters the coating bath where it is
immersed in a bath of molten metal. The composition and temperature of the
molten coating metal is rigorously controlled to achieve the optimum bond between
coating and steel. A unique feature of the hot-dip process is that the coating forms an adherent,
intermetallic bond with the steel substrate (similar to welding and/or soldering),
so that the coating and the steel behave as one during fabrication and service.
As the steel exits the coating bath, a molten layer of the coating metal adheres
to both sides of the sheet. The sheet passes between a set of air knives, located
on opposite sides of the strip, where air is directed under pressure against the
strip to regulate the thickness of the molten coating and achieve a closely
controlled, uniform coating thickness on each side of the sheet. In effect, the
air knives wipe off excess metal leaving a controlled amount of molten coating
on each side.
The molten coating solidifies as the sheet moves vertically through the cooling
tower. The solidification process may be controlled to impart selective surface
finishes, such as minimized spangle, to the coating.
After the coating has solidified and the sheet has cooled, it is often passed
through a "temper mill" to achieve a uniform smooth finish on the surface of
the coating. This processing step is most often applied when the coated sheet
product requires a high quality finish after painting. After temper rolling,
the sheet passes through a leveler which is used to improve the flatness of the sheet.
Prior to being recoiled for shipment, a surface treatment is often applied to
protect the coating during storage and shipment to the customer. This surface
treatment may be an inorganic chemical solution, a thin organic film that
serves as a lubricant plus a corrosion-inhibitor, or a rust-preventative oil. These
thin protective films resist storage stain that might occur if the coating gets wet
during storage or shipment.
Also, ahead of the recoiler, the product is inspected to assure that the surface quality
meets the requirements of the intended application.
As noted above, U. S. Steel Terne Sheet and NI-TERNE Sheet are produced by a modified
hot dip process. This process eliminates passage through a continuous-annealing furnace
but includes cleaning, pickling and applying a flux to the surface of the steel prior to
immersion in the coating bath.
|
