Rolling of Steels in Mills | Metals | Industries | Metallurgy

The control of the rolling of steel in a hot-strip mill is an important subject and deserves greater attention as the requirements are highly demanding because of the high speed of the process and the close tolerances in the operating conditions.

The steel enters the mill in the form of red slabs about 30 mm thick. The slabs pass through a sequence of stands, or sets of rolls, that squeeze the steel progressively thinner. As the steel leaves the last set of rolls reduced to a thickness of 0.25 mm or less, it can be travelling at 60 km per hour.

A typical hot-strip mill may have a dozen or more stands. The thickness of the steel leaving each stand is governed by the gap between the rolls. The gap is regulated by a controller that positions the upper roll by means of powerful motor-driven screws.

The forces developed in compressing the steel are so large that they stretch frame supporting the rolls and bend the rolls themselves. Accordingly the thickness of the strip leaving a set of rolls will depart significantly from the setting of the gap between the rolls.

For the precise control of the thickness of the strip leaving the final stand of the hot-strip mill a measurement of the strip’s thickness by an X-ray thickness gauge must be fed back so that the roll positions can be adjusted to meet the thickness specification of the product.

In practice, it is necessary to provide this kind of feedback at each mill stand. Elsewhere along the line the thickness of the strip can be estimated with sufficient accuracy from the gap between the rolls, the pressure on the rolls and other operating measurement.

The important point is that the way the reductions of the thickness of the strip are distributed among the sets of rolls affects the quality of the product, the total amount of energy required and the rate of production. The distribution needed for good mill performance is determined by a computer at a higher level in the hierarchy that takes into account the type and grade of steel being rolled, the final thickness of the strip, the condition of the rolls and a variety of other factors.

In addition to controlling the gap for each set of rolls it is necessary to closely regulate the rolls rotational speeds. A mismatch of speeds between two successive sets of rolls would cause the steel strip between them to buckler or tear. Either event would lead to a costly shutdown of the mill.

The problem is complicated by the fact that as the strip is squeezed thinner it gets proportionately longer. As a result each set of rolls has to run faster than the preceding set. Synchronizing the roll speeds, therefore, involves interaction with the roll-gap settings.

One common means of adjusting the settings of the successive speed controllers is to measure the tension (or the degree of slack) in the strip between the two sets of rolls and adjust the speed settings to keep the tension constant.

Another important variable is the temperature of the steel at various points in the mill. Slabs entering the mill must first be heated to bring them up to rolling temperature. Controllers adjust the flow of fuel to the furnace so that each slab achieves its required temperature at the scheduled time for the rolling operation. The heating cycles can be managed so that fuel consumption is held to a minimum.

The temperature of the steel strip during rolling affects the metallurgical properties of the strip, and so it is necessary to monitor the temperature of the strip at many points along the mill. The temperature just before the strip is coiled up at the end of the process is particularly critical. Feedback control is, therefore, provided for varying the spray of coolant applied to the strip in order to maintain the temperature within the permissible range.

Many other kinds of sensors are needed to provide a comprehensive picture of the rolling process. For example, there are position sensors that signal the computer system on the arrival of each new slab so that the appropriate sequence of operations can be initiated. Other formation is gathered to diagnose the condition of the mill equipment.

When something goes wrong, information is needed to determine the source of the problem and to specify what remedial actions can be taken until repairs can be made. Not least, the sensors provide data for evaluating the performance of the mill, for bringing upto date the coefficients in the various control algorithms (procedural rules) and for recording the history of the operation for accounting, analytical and legal purposes.

It would be obvious from above requirements that computer control can be easily justified for control of hot rolling mills. A computer control system has these primary tasks : data acquisition control, and actuation. The value of a process variable (such as temperature, or pressure, etc.) transformed by a transducer into an electrical signal.

The data-acquisition task, implemented in software, is to convert the signal into usable form, check, the resulting quantity to ensure that it is within the expected limits and make the data available to a control task. The control task combines the measured value with other information about the system to compute a value for the control input signal, which is transmitted to the actuator (such as positioning motor drive).

Since measurements of process variables are usually in analogue form, and since the digital computer can process data only in digital form, the analogue data must be converted into binary digits, or bits (0’s and 1’s). The computer’s output must then be reconverted from digital into analogue values so that it can be accepted by the actuators. Such transformations are carried out by special hardware: analogue-to-digital and digital-to-analogue converters.

The frequency with which the process is interrogated by measuring devices and subjected to control actions can vary widely. For example, the temperature of steel slabs in the exit zone of the reheat furnace of a hot-strip mill need to sampled only once very few minutes.

On the other hand, the tension of the steel strip travelling between rolling stands must be measured many times a second. Still other quantities, such as the number and type of slabs to be rolled in the mill, need be summarised and sent to a higher level computer no often than once per work shift.

Unscheduled events that abruptly change the state of the system call for an immediate and specific response from a designated computer in the system. Typical of such events are the malfunction of a piece of equipment, the need to change a set of roll in the mill and the receipt of an urgent order to be scheduled into the production system.

The computer control system is equipped to handle such events by means of an interrupt facility. The program currently being executed is interrupted and the information contained in various controllers at that moment is saved, usually be the operating system.

The program responsible for servicing the external stimulus is activated, and when it is completed, the execution of the interrupted program in resumed. In this way the unscheduled external event is synchronized with the control-system software.

The software of most computer control systems incorporates a means of categorizing unscheduled events, so that priorities can be assigned to them. Typical of these categories are breakdowns in operator communication, power failures, alarms that certain process conditions are seriously in error and routines to co-ordinate the peripheral devices of the computer.

Events in such categories are assigned priorities according to their urgency and are given access to the control system through a multilevel interrupt system. Thus interrupt tasks can themselves be interrupted by events of higher priority.

The timekeeping process in a real-time computer control system is realized connecting the computer to a ‘clock’ or counter, that interrupts the computer at specified intervals, usually every few milliseconds. By keeping a running sum of the clock events in internal computer memory the control program can determine arbitrary intervals and even times of day.

Distributed control is quite popular for control of rolling mills. In such a system for the control of a hot-strip steel mill, each set of rolls is controlled by its own microprocessor. Inputs to the processor report the roll position and pressure. Outputs control the motor drives that determine the position and speed of the rolls.

Other processors are assigned to determine the thickness, temperature and tension of the strip and controlling the flow of the coolant spray. The set points of all the control loops are calculated by a supervisory processor on the basis of algorithms designed to optimize the performance of the mill.