Rolling mills used to produce steel plate generally consists of following types or groups:

• Two-high or three-high single stand mill
• Four-high reversing mills
• Tandem mills
• Semi-continuous and continuous mills
• Universal mills

The universal mill produce a rolled width, while in all other mills, final widths are primarily attained by edge shearing, though some installations use the edging equipment for both edge working and approximate width sizing.

This group consists of types discussed below. Those mills are inefficient or obsolete today.

Two-High Pull-Over Mills - The two-high pull-over plate mills were essentially an adaptation of the then existing sheet mills to plate rolling. Plates rolled on these mills were limited in size and weight, due to the difficulty to return the rolled material manually to the entry side for successive passes. This restricted their utility and soon rendered these mills obsolete for producing finished plate.

Two-High Single-Stand Reversing Mills: Practical limit of application exists particularly for the production of wide, lighter-gage plate. The fact that all passes from slab to finished plate were made on the same set of rolls accelerated roll wear, which is always greatest in the central portion of the roll body and in itself imposes a restriction on the gage that can be finished satisfactorily. Roll deflection also is an important factor and, for the same roll diameter, increases with the body length for like reductions. An increase in the roll diameter to provide more strength and stiffness increases the separating force between the rolls for the same draft. This type of plate mills are obsolete in developed countries such as USA.

Three-High Plate Mill - The three-high mills provided design features which, to a degree, overcame some of the principal limitations of the two-high reversing type. In the three-high mill, the top and the bottom rolls are of large diameter, whereas the middle roll is friction-driven and usually about two-thirds of the diameter of the top and bottom rolls. The top roll can be raised and lowered in the housing by power-operated screws, and the middle roll can be brought into contact alternately with the top and the bottom rolls. In making the bottom pass, the stock passes between the middle and the bottom roll while the top roll serves as a backup roll. The stock is raised on the delivery side by a tilting table for a return pass between the middle and the top roll, while the bottom roll serves as a backup roll. The sequence of alternate passes is continued until the stock is reduced to the desired finished plate thickness.

The middle roll is changed when combined roll wear produces a crowned plate which approaches the permissible tolerance limits. The replacement roll is itself crowned to compensate for the wear which already has taken place on the top and the bottom rolls. Successive replacements in the course of a week's rolling schedule are turned with progressively increased crowns to compensate for the continued wear of the top and the bottom rolls. During the weekly mill-repair shutdown, the top and the bottom rolls are either turned in place to their original contours or are replaced with newly dressed rolls. The cycle of replacement of middle rolls with progressively increased crowns is repeated in the following week [21].

When a single-stand plate mill is supplemented by an additional stand, it forms a tandem plate mill. There are two primary advantages to construct mills in this way. Firstly, when the total work of reducing slab to plate is divided between two mill stands, satisfactory surface finish and shape of rolls can be maintained for longer periods between roll changes. Secondly, since the work is divided between two units operating simultaneously, the required time interval for the reduction of a slab to a plate is reduced and the overall capacity of the unit is increased correspondingly.

The various tandem-mill arrangements represent a wide variety of mill unit combinations, such as:

• a two-high reversing rougher with a three-high finisher,
• a two-high reversing rougher with a four-high finisher,
• a three-high rougher with a three-high finisher,
• a three-high rougher with a four-high finisher.

So many tandem arrangements exist because they represent modifications of original rather than new installations, and because they were accomplished by the maximum utilization to existing equipment, the minimum expenditure for new equipment and a minimum of alteration to auxiliary facilities.

Both semi-continuous and continuous mills constitute multi-pass roughing units, and two or more single-pass continuous units in which the plate is reduced simultaneously to the finish size. The difference between the semi-continuous and continuous mills exists in the roughing section: semi-continuous mills have reversing roughing units and continuous mills come with non-reversing roughing units. In both mill types, two-, three- and four-high stands with or without scale breakers, broadside stands, squeezers, and edgers are used as roughing units, while four-high stands are used as finishing units.

The semi-continuous mill arrangement, although requiring a larger capital investment, has a number of operating advantages over both the single-stand and the tandem-mill types. The total reduction work is divided between individual stands to an even greater extent than in the case of the tandem mills. The roll wear of individual stands is, therefore, less than that of the prior mill types. The total time increment for reduction from slab to plate also is less, and the tonnage capacity per unit of time correspondingly is greater. The reason for the short reduction time and high tonnage exists in that the rolling is performed simultaneously in all or many stands.

Due to short time required to reduce slab, mills of those types are permitted to roll sheets as well as plates.

Universal mills integrates horizontal and the vertical rolls into a single mill unit and work the stock simultaneously. An universal plate mill is a single-stand unit. The purpose of the vertical rolls is not only to work the edges of the stock in the process of reduction, but also to produce a rolled width in conformance with specified standard tolerances. Although universal plate mills producing widths as large as 1524-mm (60-inches) were installed, the bulk of the installations is usually 1219-mm (48-inch) width and under.

Traditionally, large plates produced with universal plate mills were used to fabricate deep wide-flange beams and columns. Since more and more large beams were rolled directly, the number of installations and the capacity of the universal plate mills has decreased rather than increased in the past several decades. The universal plate mills in operation today date back several decades to their installations dates.
 

[21] W.T. Lankford, Jr. et al (ed.): The Making, Shaping and Treating of Steel. United State Steel. 1985. ISBN 0-930767-00-4.