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A0007 Hot rolling
by Terry Smith
After steel has been cast into blooms, billets, rounds, or slabs it is generally sent to the hot roll mills for further processing into the final shape for sale to the customer. It may be rolled into plate, strip, structural shapes (such as angle, ‘I’ beam, channel etc.) or rods. These processes are all done in mills that have a set of working rolls that are machined and ground into the final shape of the product. Some of these mills may have up to four sets of working rolls stacked on top of one another. Each set of rolls will have the shape of the product machined into them but each section serves a particular purpose. They will have a section that is just a basic shape for beginning the rough shape of the product. Other sections will each be a more refined shape of the one previous to it until the last section which will be the exact shape and dimension of the finished product. This is called the finisher. These mills are reversible and the steel is passed back and forth through each section of the rolls until the final pass through the finisher. Some of these mills will have one other mill just before them that is called the rougher. Steel from the castor is generally only cast in basic sizes and shapes in order to reduce the number of dies needed for casting. The roughing mills are used to reduce the billets or slabs from the caster to a dimension that is required to pass through the rolling mills. The slabs or billets are heated in a furnace that feeds the roughing mill. They are fed in through the back of the furnace at timed intervals and work their way through to the front of the furnace. As one billet is fed in the back, one drops out through the front onto a roller bed which feeds the roughing mill. The mill rolls the billet or slab to the required dimensions for the finishing mill, and it then travels along another roller line to the finishing mill which completes the finish rolling. Everything is timed so that the steel is fed in such a manner that it passes from the back of the furnace and on through the mills and out to the cooling beds from the finishing mill without stopping.
Water sprays are used in all the mills to keep the working rolls cool and to help de-scale the steel as it is being worked. The billets are heated to about 2300 deg. F prior to rolling and the pressures used in the mills coupled with the reduction in dimension of the steel keep the temperature of the steel above its critical temperature. This allows the grains of the steel that have been deformed in the rolling process to recrystalize to an even microstructure and thus will not be work hardened upon cooling.
Coils of steel called strip are rolled in a series of mill stands. Slabs of steel are re-heated in furnaces in the same manner as for other hot rolling mills. The slabs exit the furnace and are first processed in a large roughing mill. This mill is capable of turning the slabs up on edge in order to roll them to the correct width required for the strip being produced. It reduces the overall thickness of the slab down to about 2 inches thick as well. From the roughing mill the slab now continues down a roller line to the first of a series of reducing mills. There may be up to eight sets of mill stands all in a row. These stands each have a set of working rolls with a very large backup roll on top and beneath each of the working rolls. These backup rolls can be as large as 8 ft in dia. and are used to apply the tremendous pressures required for the working rolls to reduce the thickness of the steel passing through them. These mills are non – reversing and the steel passes through once.
Staring at the first mill stand the steel is reduced in thickness as it passes through each successive stand. The steel enters the first stand at a relatively slow rate of speed but as it is reduced and becomes thinner it also becomes longer. As it is becoming longer as it passes through each mill it is also moving at a faster rate. This means that each successive set of rolls in the next stand must turn at a faster rate. Upon leaving the final stand the strip can be moving at speeds up to thirty miles per hour.
At the end of the mill stands there is a large mandrel called a coiler. This coiler is like a large roll but is made up of segments that can be expanded or collapsed by the coiler operator. When collapsed there is a gap between the segments. As the strip is started through the mills it is slowed down considerably until the end reaches the coiler. The coiler operator aligns one of the gaps in the segments with the end of the strip coming from the mill and jogs the mills until the end of the strip gets in between the segments of the coiler. He then expands it, trapping the end of the strip in the closed segment and rotates the coiler until the strip is wrapped around it. He then starts the mill and it runs up to speed and the coiler spins rapidly, coiling the strip up as it comes from the mill. A computer controls all the speeds of the mills and coiler to ensure that the strip is coiled evenly and that there is not to much tension on the strip as it is coiled to prevent it from breaking. It also slows down the mills and coiler when it gets to the end of the strip to prevent a major whiplash of the end as it comes onto the coiler. As the strip comes out from the end mill it passes over an x-ray unit that precisely measures the thickness of the strip to ensure it is the proper thickness and adjusting the mill pressures as required, to maintain the proper thickness. After the strip is coiled a ‘saddle’ mounted on a movable conveyor rises up to support the coil. The coiler operator then collapses the mandrel releasing the end of the strip and the conveyor moves the coil off to the cooling area to be strapped. Some of the strip is sold to customers as is, while others are sent to the cold rolling and finishing dept. for further processing. Generally hot roll strip is rolled from 0.047 to 0.625 inches thick. Thinner sections are usually rolled in the reduction mill in the cold roll dept. and thicker sections are usually rolled on the plate rolling line. Some hot rolled products are made with alloy steels and their cooling rates are carefully controlled on special cooling beds in order to obtain the specific properties required by the customer. Some steel mills also have large heat treating furnaces capable of handling some of the products including plate and certain structural components.
This was just a very basic run down on the hot rolling process. There are a great many different types of rolling mills to produce an almost endless variety of finished product.
The next article will be on the cold rolling and finishing process.
A0007 Hot rolling