Continuous Galvanizing Line

Overview

A continuous galvanizing line turns bare cold-rolled steel coil into zinc-coated strip in one unbroken pass. The strip is welded end to end into an effectively endless ribbon, cleaned, annealed in a reducing atmosphere, dipped through a bath of molten zinc near 460 C, wiped to a precise coating weight by gas jets, cooled, given a surface finish, passivated, oiled and rewound. The line runs continuously at the pot even though coils are finite, which is the reason for the welder and the two accumulators: they let the entry and exit ends stop for coil changes while the central process holds constant speed and temperature.

The coating forms because hot clean steel wets molten zinc and a thin iron-zinc alloy layer grows at the interface, bonding the coat metallurgically rather than mechanically. Everything upstream of the Zinc Pot Section exists to deliver the strip clean, oxide-free and at the right temperature, and everything downstream exists to set the coating weight, freeze it without marking, and protect it for storage.

Entry and continuous feeding

Two coils are loaded in turn by the Coil Loading Car onto the Expanding Mandrel of each Pay-off Reel. While one reel pays off, the next coil is staged on the second reel. When the running coil nears its end, the Strip Joiner takes over: the Crop Shear squares both strip ends, the Strip Welder fuses them with its Seam Weld Wheel electrodes, and the Seam Trimmer dresses the bead flush so it passes through the rolls and pot without trouble. During this weld the entry end is stationary, so the Entry Accumulator pays out its stored loop. Its Accumulator Carriage rides up and down a tall Accumulator Tower, lengthening or shortening the festoon over the Loop Roll rolls to absorb the speed difference. The Entry Bridle sets the tension that carries the strip into the cleaning section.

Cleaning

Cold-rolled strip arrives covered in rolling oil and iron fines, and zinc will not wet a dirty surface. The Cleaning Section removes both. The strip first runs through the Degrease Tank, where heated alkaline cleaner is sprayed and the strip is dipped. The Brush Scrubber then scrubs both faces with rotating Brush Roll rolls to lift loosened soils. A cascade Rinse Tank flushes the alkali away, Wringer Roll wringer rolls strip the carry-over water, and the Strip Dryer leaves the surface dry before it enters the furnace.

Annealing furnace

The Annealing Furnace does two jobs at once: it recrystallizes the cold-worked steel to restore ductility, and it reduces the thin surface oxide so the steel will bond with zinc. Heating is by Radiant Tube tubes fired by Recuperative Burner burners, so the combustion gases stay inside the tubes and never touch the strip. The strip climbs in temperature through the Preheat Zone and Radiant Heating Zone, holds in the Soak Zone, then drops in the Jet Cooling Zone to a few degrees above bath temperature. Throughout, the Atmosphere System holds a reducing mix of hydrogen in nitrogen with a low dew point, watched by the Dew Point Sensor and Oxygen Analyzer. The strip is steered by Hearth Roll rolls and leaves through the Furnace Snout, a sealed throat whose Snout Tip sits just below the zinc surface so the hot strip enters the melt without ever meeting air.

Zinc pot and coating

The Zinc Pot Vessel holds the molten zinc, lined with refractory by the Pot Refractory Liner and kept at temperature by the Channel Induction Heater, which heats the metal by inducing current in a circulating loop of melt. The strip descends through the snout, wraps the Sink Roll at the bottom of the bath and rises again, steadied by the Stabilizer Roll rolls so it leaves the bath without flutter. These submerged rolls run on the Pot Roll Bearing bearings carried by the Pot Roll Support Arm. The bath surface is kept clean by the Dross Skimmer, and zinc level is held by the Zinc Ingot Feeder.

As the strip rises out of the bath it carries far more zinc than the finished spec allows. The Air Knife Section section fixes that: the Air Knife Header nozzles, fed by the Air Knife Blower, blow a thin sheet of gas at the strip and strip the excess zinc back into the pot. Coating weight is set by the gas pressure, the nozzle gap and the line speed, with the Knife Positioner holding the gap and the Coating Weight Gauge reading the result on both faces so the control loop can trim the knives in real time.

Cooling, finishing and exit

The wiped coating is still molten, so the Cooling Tower freezes it with forced air from the Air Cooling Box boxes before any roll touches it. Once solid, the strip runs through the Skin Pass and Leveler section, where the Skin-Pass Mill Stand applies a small elongation to set surface texture and the Tension Leveler takes out residual shape. The Chemical Treatment then applies a passivation film at the Passivation Coater to resist white rust in storage and an oil film at the Electrostatic Oiler for forming and corrosion protection.

At the exit the Exit Accumulator mirrors the entry: it stores strip so the Recoiler can stop while the Exit Shear cuts a finished coil free and the Belt Wrapper starts the next one. The Exit Bridle holds exit tension while the reel winds the coated coil.

Drive and control

The whole line is tied together by the Drive and Control System system. Each reel and bridle runs on a regenerative AC Drive in the cooled Drive Cabinet, so a decelerating reel feeds energy to the accelerating bridles. The Line PLC coordinates speed, tension, bath temperature and furnace atmosphere as one process, taking strip tension from the Tensiometer Roll rolls and presenting recipes and live readings to the operator on the Operator HMI. The coating spec, the steel grade and the line speed all sit in those recipes, and the controller balances them so the strip leaves with the ordered coating weight, finish and flatness.