A series of processes are carried out within a plate mill to produce plates. Below is a much simplified illustration of a plate mill. Its feedstock is steel slabs, usually produced by continuous casting.
Slabs are heated in a furnace to a temperature in a typical range of 1150-1270 °C. Reheating reduces the deformation strength of steel sufficiently to allow it to be rolled, and ensure that all alloying elements are in solid solution. The furnace incorporates machinery to transport a stream of slabs through it, and is usually fired by gas burners.
Prior processes and reheating cause an oxide layer (scale) to build up on the outer surfaces of the slab. High pressure water sprays (~200 bar) are used to remove it.
Plate mill stands are usually of 4-high design. They are the largest type of rolling mill, since they roll an exceptionally wide product under high torque conditions.
The simplest configuration is a single stand, though two-stand variants are used for higher outputs. All plate mills are of the reversing type (reducing the gap and hence the stock thickness pass by pass).
Plate mills are unlike other flat rolling mills in that rolling is carried out in two stock orientations. The first dimension developed is the width, by rolling across the casting direction in a practice known as broadsiding. The plate is then turned through 90º and rolled to the desired thickness.
The sequence of drafts applied to the plate is known as the rolling schedule. Different constraints apply at different points in the schedule. Some of these reflect machine limits (such as torque limits when the stock is thick, or roll separating force limits as final thickness is approached). Others are concerned with product condition (such as following a temperature-strain path suitable for the development of the required properties). Internal stresses in the plate must be minimised by compensating for the deflection of the rolls under the action of the rolling forces, and thickness control necessitates compensation for the elastic stretching of the mill housings. A modern plate mill incorporates many control systems and actuators to manage these factors and to assure the quality of the finished product.
Once the required dimensions and thickness has been reached in the mill, the plate undergoes cooling. For many steel grades and applications, the cooling path is as influential in the plates’ metallurgical development as is the strain path in the mill. Systems for this purpose of controlled cooling are known as accelerated cooling systems, and are designed to reduce the plate temperature at high rates, down to a level at which all microstructural transformations are complete. The full-surface uniformity of cooling is critical if buckling is to be avoided.
Some mill-exit cooling systems (known as direct quench systems) are designed to replace the off-line heat treatment process of quenching and tempering, which is used to produce extremely high-strength steels.
Whether or not accelerated cooling is applied, plates are finally air-cooled as they are transferred across a cooling bed. In special cases, stacking of plates to reduce the cooling rate is employed. This is to allow hydrogen to diffuse out of the plate, thereby reducing its embrittling effect in service.
After the cooling bed, plates are inspected. If localised surface damage is found, grinding may be applied to rectify it. For critical applications, ultrasound testing is applied to ensure internal soundness (freedom from segregation).
Shearing serves three purposes. As well as cutting discrete plates to their ordered sizes, it also removes edge and end material which invariably deviates from the required properties because of abnormal heat transfer at the plate extremities. The shearline also extracts test samples which are used to certify the plate properties.
Levelling is a stress-relieving process carried out through low and controlled elongation in a multi-roller machine, the plate surface undergoing a series of reverse bends. Most plate mills incorporate a hot leveller for use on all plates which is positioned before the cooling bed. Many also use a cold leveller on selected products (those needing flatness improvement) in the shearline area.