Continuous Hot Dip Sheet Coating Processes

In continuous hot dip coating of steel sheet, coils of steel are welded end-to-end and are coated at speeds of up to about 200 m/min (650 ft/min). In general, continuous hot dip coating facilities are classified as either hot or cold, as described below.

Hot Lines. In hot processing, steel strip is cleaned and heated in a reducing gas atmosphere, typically a mixture of hydrogen and nitrogen, in order to prepare the surface for coating. Annealing of cold-rolled steel by heating above its recrystallization temperature of about 700 °C (1300 °F) is usually a part of this heating process. Combining the steps of cleaning, heat treating, and bringing the sheet to coating temperature into a continuous operation contributes to making the hot process economical. Hot processing at lower temperatures without recrystallization is also done to produce full-hard, coated, high-strength sheet. Lower temperatures are also used to coat hot-rolled sheet, or to coat cold-rolled sheet that has been previously annealed.

A state-of-the-art hot line capable of producing 450,000 metric tons per year and coating sheet up to 1830 mm (72 in.) wide is shown schematically in Fig. 2. Coils of steel sheet are first loaded onto reels at the entry end on the left. The leading end of each coil is resistance seam welded to the tail end of the preceding coil. This is made possible by the looping tower, which acts as a reservoir for sheet. Sheet previously stored in the looping tower is released downstream so that sheet at the entry point can be stationary long enough for a new coil to be loaded and welded without interrupting the continuity of the coating process.

Fig. 2 Schematic diagram of a continuous galvanizing line. An example of a "hot" line with in-line annealing capability. See text for details.

After welding, the strip enters the first cleaning stage, an alkaline bath that removes oils, dirt, and residual iron fines from the rolling process. Upon exiting the looping tower, the strip surface is further cleaned by mechanical brushing and electrolytic alkaline cleaning. Older lines may rely solely on direct firing of gas burners on the strip surface in order to burn off residual oils.

Following cleaning, the sheet passes into a radiant tube furnace containing a mixture of hydrogen and nitrogen that reduces surface iron oxides. This atmosphere produces a very clean surface that can be easily wet by the coating metal. Heating of the steel also takes place in the furnace. Annealing of cold-rolled sheet is achieved by heating usually just above the temperature for subcritical recrystallization (about 720 °C, or 1330 °F). Following recrystallization, it is generally desirable to cool the sheet down to near the bath temperature (about 460 °C, or 860 °F in the case of zinc coatings) in order to avoid overheating of the metal bath. Most modern lines have jet coolers to convectively cool the strip before bath entry.

The steel strip next enters a pot containing a bath of the molten coating metal. Shown schematically in Fig. 3, the pot region is where the actual hot dip coating process takes place. The first coating pots were little more than steel tubs heated by gas flames. Modern pots are steel vessels lined with ceramic refractory and heated by electric induction. Any hardware in the bath, such as the rolls, bearings, and support members, is usually made of 316 stainless steel or a similar alloy in order to resist attack by the molten metal.

As the strip exits the pot, a film of molten coating adheres to the surface. The thickness of the molten film is controlled by passing the strip between gas wiping dies, which remove excess coating metal with a stream of gas. Coating thickness is determined by the geometry of the wipers, the velocity of the wiping gas, and the distance between the wiping dies and the sheet. The wiping gas may be steam, air, or nitrogen. On-line x-ray fluorescence or isotope gages continuously

Hot Dip Coating Block Diagram
Fig. 3 Schematic of pot region in a typical continuous hot dip coating line

monitor the actual coating thickness and enable rapid adjustments to be made. Figure 4 shows the region of a typical hot dip coating line where the coated strip exits the pot.

Fig. 4 Coated sheet exiting pot at a galvanizing line

In some coating facilities, the region above the pot and around the wipers is enclosed. The enclosed region is purged with the nitrogen wiping gas in order to reduce oxidation of the bath by air. This helps to reduce the incidence of surface defects and give a more uniform coating. In order to prevent fuming and copious clouds of zinc dust, it is also necessary to inject steam into the enclosure to form an oxide barrier on the surface of the molten metal.

Uniformity of coating thickness along and across the sheet surface is an important factor affecting the quality and performance of hot-dip-coated sheet. For this reason, it is critical to avoid fluctuations in the distance between sheet and wiping die that may result from poor sheet shape or vibrations. Shape-related difficulties are minimized by use of an adjustable deflector roll (see Fig. 3) that is submerged in the bath and pressed against the strip to reduce crossbow (i.e., excessive curvature in the plane of the sheet). The stationary corrector roll serves to keep the strip in alignment with the wiping dies. Variations in sheet-to-die distance resulting from sheet vibrations are minimized by use of the touch rolls located above the wipers. These reduce the amplitude of sheet vibration at the wiping dies by decreasing the unsupported length of sheet above the pot.

After coating, forced-air cooling is used to reduce sheet temperature. This prevents coating damage due to contact with the turnaround roll at the top of the upleg run. The sheet may be subjected to one or more post-treatments before being rewound into coil form, or sheared into cut lengths, at the exit end of the line.

Cold Lines. In cold processing, steel strip is cleaned, pickled, and fluxed in-line with no heating beyond that required to dry an aqueous flux solution on the sheet surface prior to entering the molten metal bath. Cold lines are also sometimes referred to as flux or Cook-Norteman lines. Because cold lines have limited heat-treating capability, incoming cold-rolled steel sheet requiring heat treatment will have been previously box annealed, or annealed on a separate continuous heat-treating line.

A typical cold line is shown schematically in Fig. 5. As with the hot line described above, this line is capable of producing 450,000 metric tons per year and coating sheet up to 1830 mm (72 in.) wide.

Galvanizing Lines

Fig. 5 Schematic diagram of a continuous galvanizing line. An example of a "cold" line without in-line annealing capability. See text for details.

Fig. 5 Schematic diagram of a continuous galvanizing line. An example of a "cold" line without in-line annealing capability. See text for details.

Preparation of the sheet surface for coating includes alkaline and electrolytic cleaning, pickling in hydrochloric acid, and coating with flux, an aqueous solution of ammonium chloride and zinc chloride. After hot-air drying of the flux solution on the sheet surface, the strip enters the molten metal bath. Exhaust systems are essential to capture the fumes evolved when the fluxed sheet is submerged in the bath.

Because the sheet enters the bath at a relatively low temperature, considerable heat is transferred to the sheet by the bath. Consequently, cold lines generally have pots with a greater heating capacity in order to achieve the same throughput as an equivalent hot line. Downstream from the zinc pot, there is virtually no difference between hot and cold lines.

Post-Treatments. After the coating has been applied, several options are available for post-treatment of the strip. One possibility is galvannealing to produce zinc-iron alloy coatings, as discussed in the section "Zinc-Iron Alloy Coatings" in this article. Another is spangle minimizing, as described in the section "Zinc Coatings" in this article. Additional processing steps may be used to improve mechanical behavior, shape, corrosion resistance, and other properties as follows:

• Tension leveling or roller leveling to improve flatness

• Skin passing to make the surface smoother and to minimize yield-point behavior

• Overaging heat treatment to improve mechanical properties

• Slitting to narrower widths, shearing to produce cut lengths of sheet, and side trimming to remove nonuniform edges

• Chromate passivating to provide temporary protection against corrosion during shipment and storage. Passivation is done by spray or dip application of an acid chromate solution, removing excess solution by use of squeegee rolls, and drying the residue in hot air. Given the increasing environmental concerns about chromium, particularly in the hexavalent form, it is generally felt that other types of chemical passivation will eventually replace chromate.

• Oiling to provide lubricity during forming

• Oiling to provide temporary corrosion protection during shipment and storage. Inhibited oil is used in addition to chromate passivation to provide extra protection, or without chromate for applications in which chromate is inappropriate, such as those involving painting or contact with foodstuffs.

• Phosphating to improve formability

• Flash electroplating with iron to improve weldability

Some of these post-treatments are shown schematically in Fig. 2 and 5. Many could be done offline as separate operations, but in general they can be done more economically online as part of the continuous process, provided the necessary equipment is built into the line initially. Except in rare instances where unused space was deliberately left in a line, retrofit of the equipment for post-treating is problematic.

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