The three principal types of rolling mills used for the rolling of steel are referred to as two-high, three-high, and four-high mills. This classification is based on the arrangement the rolls in the housings. Major features of those stands are listed below:

For rolling thin product, such as sheet, smaller rolls are preferred. At one hand, smaller rolls require smaller roll load. On the other hand, smaller rolls have lower elastic deformation and make it possible to roll the sheet to a thinner gauge. If the sheet thickness is equal to near to the roll elastic deformation, the sheet thickness cannot be reduced any more - this is the minimal thickness that can be rolled.

However, a small roll has big roll deflection which makes the things worse if no backup roll works with it. In practice, there are also six-roll, twelve-roll, and twenty-roll arrangements, beside four-roll mills. The more the rolls, the smaller the possible work roll diameter, and consequently, the high the cost. Fig. 1 shows the major roll arrangements nentioned above. While four-high mill can be used for both hot and cold rolling, the six-high, twelve-high and twenty-high mills are primarily used to roll very thin cold sheet.

Fig. 1: Major roll arrangements for flat rolling

Fig. 2 illustrates a three-high reversing mill, with guides and guards displayed. For reversing mill, both sides of the mill need to be provided with guides and guard. Guides are employed in order to prevent collaring and to insure that the piece enters and leaves the pass in the correct position, while guards are used mainly on the delivery side of the mill to control the direction of the piece after leaving the pass.

Fig. 2: A three-high reversing mill with guides and guards

There are also several other arrangements of rolls. One example is the Sendzimir planetary mill (Fig. 3) and Taylor mill. In the planetary mill, a great number of small rolls, which in turn serve as work rolls, are mounted on the surface of two large backup rolls. Since multiple sets of rolls work on the strip simultaneously, the pass reduction can be very high. In Fig. 3, due to the high complicity, a pair of feed rolls are installed.

Other roll arrangements, such as universal mill and three-roll mill (e.g. Kocks mill), are discussed in a separate paper on shape rolling rolls and mill arrangements.

Fig. 3: Planetary mill [21]

A continuous mill consists of several stands of rolls arranged in a straight line (in tandem), with each succeeding stand operating with roll surface speed greater than its predecessor. This type of mill is in very common usage for rolling strip, sheet, billets, bars, rods, etc. Any part of the workpiece, after pass through the roughing, intermediate and finish stands, is rolled from initial shape into the finish one, and emerges from the last roll stand. A semi-continuous mill comprises also a reversing roughing stand for reducing the piece prior to entering the continuous mill for reduction to the finished shape. This arrangement gives moderately high production with lower first cost than a continuous mill.