Steel tube starts out as flat steel strip that has been slit to the correct width for the end tube size. It is then passed through a series of rolls that form it into a round tube. After the steel has been formed it passes through a weld box. The weld box for ERW tube uses electrical resistance of the steel tube to create heat. This heat is what welds the two steel edges together. Using this method will produce a strong weld that is just as strong as or stronger than the original steel.
After welding the surface left behind, known as flash is removed if there is a requirement to do so. After the flash control process the tube travels through a cooling box that is filled with special coolant. Once the tube leaves the cooling box it now goes through a group of finishing rolls. These finishing rolls are used to change the shape of the tube and make the final dimensions. Common shapes include: round, square, rectangle, oval, & octagonal. Following the finishing process the tube is generally coated with an anti-rust liquid to protect the steel tube.
Finally the tube passes through a cutoff that cuts the tube to the required lengths. The steel tube generally moves through the mill at fast speeds. The time it takes from entering into the mill to the time it passes through the cutoff is only a few seconds.
After the tube is manufactured there are a number of secondary processes that are performed depending on the end use. Some of the common items are Deburring, Dedimpling, & Coating.
Deburring is a process where the tube passes through a machine with spinning wire brushes. These wire brushes smooth out and remove the jagged edges normally left after the tube is cut. This is done when the ends of the tube will be exposed in the end product.
Dedimpling can be done at the same time as cutting but also may be a secondary process. When certain shapes are cut the knife dents in the tube slightly. The dedimple process is done by a machine that inserts plugs into the end of each tube to remove the dent.
The material used to coat the tube depends on the end application. A tube may be coated with a metallic material such as zinc to protect it, or chrome to give it a gleaming finish. The tube may also be powder coated or painted.