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A0008 Cold Rolling and Finishing, Pickling Steel
Part 1 of 2
by Terry Smith
This article on cold rolling and finishing will be in two parts. This first part will be about the preparation of coiled steel strip for cold rolling. Some strip steel goes through a part of the cold roll process without having the mill scale removed but the vast majority of it is run through a pickle line first.
Pickling steel is a process that removes mill scale from the surface of the steel before it is run through a cold reduction mill process. Pickling is accomplished by running the steel strip through a bath of sulfuric or hydrochloric acid. In some special steel processing plants they may use nitric or hydrofluoric acid to pickle the steel or a blend of acids. Hydrochloric acid leaves a brighter surface on the steel but can cause problems with some high alloy and high carbon steels. Hydrogen embrittlement occurs when the hydrogen in the acid reacts with the surface layer of the steel and cracks can develop, therefore sulfuric acid is most often used.
A pickling line is several hundred feet in length with a un-coiler and other process equipment at one end, a long steel tank, that is rubberized inside, that contains the acid, and an exit end with a re-coiler and associated process equipment. Steel strip is fed through the acid tanks in one long continuous strip, but there are several processes involved in doing this.
The strips of coil from the hot rolling mills are stored in a large bay at the entry end of the pickle line. The coils sit on edge on the floor and a large capacity overhead crane (about 50 ton capacity) lowers a large ‘square’ shaped ‘C’ hook and guides the bottom of the hook through the center opening of the coil. He picks up the coil and sets it in a saddle mounted on a movable conveyor in the floor. This conveyor is lined up with and travels under a large collapsible mandrel on the un-coiler. The saddle can be raised to align the mandrel with the center of the opening of the coil. The coil is aligned and the mandrel is expanded to hold the coil firmly in place. The strap put around the coil in the hot mill is cut and the saddle lowers allowing the loose end of the coil to come free. Then the operator slowly rotates the mandrel until the end of the coil comes in contact with a small set of feeder rolls. There are several sets of small rollers behind the feeder rolls, called leveler rolls that keep the strip laying flat as it comes off the un-coiler. The end of the strip is not square due to the hot rolling process when it comes to the end of the billet. It could be concave in shape or convex but in either case it must be squared off. There is a set of shears at the end of the leveler rolls. The operator advances the strip into the shears to crop off the end and to square it up. This is done both at the start of the roll of strip and at the end of the roll as well. The reason for this is explained in the next step of the process.
After squaring the strip is advanced forward into a large flat bed welder. This welder is two separate sections with a gap in between the sections. The strip advances on the bed of the entry section of the welder until it projects slightly over the edge. Then a series of hydraulic pistons come down, clamping the sheet to the bed of the welder. At the other side of the gap there is a bed similar to the entry side. On this side the tail end of the previous strip is clamped in position in a slight overhang as well. One half of the welder is powered while the other half is grounded. After being clamped the entry end welder begins to move slowly forward and power is applied. As the two edges of the strip come together an arc is created between them and the entry end closes the gap as they weld themselves together. If the customer does not mind a weld joint in the strip of steel a set of trimmer knives pass over the top and bottom side of the weld joint and shaves off the excess metal at the seam leaving it the same thickness as the joined sheets. If, however, the customer does not want a welded joint, or if the sheets are different thicknesses or widths, the seam is not trimmed. After the strips are joined they leave the welder and pass over a large roller and down into a pit that is directly below the pickle tanks. This pit contains another large roller that the strip passes under then travels upward to another large roller at the entry end of the pickling tank.
The large roller down in the pit is mounted on a carriage that travels on a set of rails that run the full length of the pickle tanks. The reason for this arrangement is to keep tension on the strip even at all times and to take up slack or play out strip during the whole process. This is to allow for the exit end of the pickling to steel be drawing strip through the line while the entry end is preparing the next coil to be joined. It will also pick up slack from the entry end while the finishing end processes the end of the coil.
Inside the pickling tank there is a series of arms made of acid proof materials. These arms can be raised and lowered as required. As the strip enters the tank it passes over one set of arms that support it and keep it from rubbing on the bottom of the tank. A second set of arms are above the strip, and they ensure the strip doesn’t rise above the level of the acid. In the event of a breakdown or other delay, these arms are raised, lifting the strip of steel out of the acid. This prevents the acid from causing severe pits or even completely dissolving the strip during the down time. Steam is injected into the acid as well to heat it and improve the pickling process. The acid is diluted with water in a very controlled fashion and injected into the entry end of the tank and weaker waste acid is withdrawn at the exit end at the same rate. The waste acid is pumped into a large holding tank and later to waste acid rail cars for proper treatment and disposal. The tank has a cover on it with large suction pipes to draw off the acid fumes from the tank. These fumes are sent into a large cooling tower that has water flowing down the inside to absorb the waste acid from the fumes. This waste water is also collected and properly treated before it is discharged into the environment.
After passing through the acid bath the steel exits the tank and is subjected to several water jets, both on the top and bottom sides to rinse off the acid. Next it passes by a large air pipe, both top and bottom, that uses high pressure air to blow off the water and dry the strip. Because of the surface having been cleaned with acid, it is subject to rusting very quickly. To prevent this, after the strip is dried it is sprayed with a light coating of oil. This oil is easily washed off, if the steel is to be galvanized at a later stage. After oiling the strip passes through another set of feeder rolls and approaches a set of shears. These shears are used to cut out the welded section of the strip if necessary, or to cut the strip after a sufficient length of strip has been re-coiled. The mandrel on the exit end is the same type used when coiling hot strip coming from the mills. It has separate segments that have a gap when the mandrel is collapsed, but close on the end of the strip when expanded. After coiling the roll is then removed by a saddle conveyor, strapped and moved to either the shipping bay for delivery to the customer, or transferred to the cold roll mills and/or annealing department.
Part two of this article will deal with the cold roll reduction mill, temper mill, annealing, slitting and shearing of the strip.