Types of Steel Rolling Mills are either hot rolled or cold rolled. These may be either reversing type or continuous type. The motors used for reversing mills need operation in both the directions of rotation. A four quadrant operation of the motor may be required. In the continuous type the motor rotates only in one direction.
Types of Steel Rolling Mills, where the cross section of steel is transformed to required sizes, are classified depending upon the end product required. The choice of the motor to meet the requirements and the choice of the mill-stand depends upon the products required. In blooming mills the end product is slabbing mills produce slabs which are rolled metals of rectangulg section. The blooms of reduced cross section are called billets which can further be rolled into bars, squares and angles. The mills that produce these materials of different sizes are called merchant mills. Slabs are converted to plates in plate rolling mills. These plates have less thickness than width. Strip mills convert these plates into strips which are transformed to sheets ih a sheet mill. Cold rolling is normally used for producing sheets of good quality of uniform gauge. Hot rolling is used to make blooms, slabs and billets.
In reversing mills the steel is passed in the mill-stand in both directions alternately, till it reduces to the required size. In continuous mills, steel is passed in one direction through several stands which press the sheet simultaneously.
The finished sheets from cold rolling reversing mills may have a thickness ranging from 0.15 to 2 mm and more. Black plate produced in these mills may be of the order of 0.07 to 1.3 mm. The bands have a thickness of 0.0015 m and width l m. The production of sheet steel in a cold rolled mill is limited by low sheet speed because of the forward acceleration and retardation and adjustments of gaps in the mill-stand as the sheets are pressed. The production rate can be increased by passing the sheet in one direction.
The billets, strips, and the products of merchant mills are produced in continuous mills. Blooming and slabbing mills are of reversing type.
Reversing Hot Rolling Mills
Hot ingots from the soak pits or from steel making shops are rolled in these mills. These are transported to mill body by means of a car. A crane is used to load the car with these ingots. The mill receives these ingots and processes them. The mill bed comprises a series of rolling mills.
The ingots are fed on the receiving table of the mill bed wherein they are weighed. Ingots travel on rolling mills. They reach the main mill-stand after passing through several tables, such as approach table, intermediate table, and front work tables. After the mill-stand there are tables such as backwork table, intermediate table and runout table. The finally finished steel is cut in a spear table to standard sizes. The length of the mill bed is decided by the length of the product.
The ingots are passed in the mill-stand in both the directions till they are pressed to desired thickness. As the thickness decreases, automatic adjustment of the gap is required, which is carried out by screwing down machanism. This adjustment is made when the mill is made ready for reverse motion.
The metal at entry positions of the mills is aligned by means of manipulator slide guides.
Based on the process discussed above the nature of the drive is as follows:
- A wide range of speeds of operation is required. The duty cycle of the load has frequent starts and (stops) speed reversal. The motor and its control must be selected taking this into consideration. To increase the production rate the dynamic behaviour during speed reversal must be fast.
- The direction of rotation must be reversible without causing serious disturbance to power handling circuits. The method employed should be such that the starting and speed reversal take place without any large dip in the terminal voltage.
- Reliability and accuracy are imperative.
The transport of ingots from the hot chamber to the car, conveying of the finished blooms or slabs, and the mechanical adjustments of the mill-stand are also carried out by several drive motors. These may be integrally controlled with the above mill-stand.
A motor selected should meet the above requirements. Its speed must be controlled over a wide range. The kW rating must be sufficient to drive the intermittant continuous load having the definite duty cycle with frequent starts and reversal. Braking may be required to stop the mill bed if required. Regenerative braking may not be advantageously employed. Plugging may not be suitable here due to peaks of current during speed, reversal. These peaks cause voltage dips and hence must be avoided. Accurate speed control using principles of automatic control is also possible and is a reliable method.
Ward Leonard control of dc motors is very much suitable here. The regenerative speed reversal is possible. Armature current control can be employed for fast retardation. Armature voltage variation in a smooth manner enables a wide range of speed control. Flux weakening of the motor increases this range on the upper side. Load equalisation is possible by means of a flywheel. The speeds can be very accurately set and the system has a very high reliability. This allows closed loop automatic speed control.
Ac motors with conventional methods of speed control are not suitable. An ac commutator motor may be used. But braking may have to be employed using the methods of plugging; dc dynamic braking has been done for normal three-phase motors. This may result in dips of supply voltage.
The advent of thyristor power converters has made the speed control of both induction and synchronous motors very simple. Ac motors employing a variable frequency supply for speed control may be employed. These are becoming competitors to ,dc motors. The cycloconverters have advantages at very low speeds over the dc link converters. So, cycloconverter fed synchronous motors are used very commonly for driving steel mills. The converters facilitate four quadrant operation. These drives meet all the requirements mentioned above.
Continuous Hot Rolling Mills
Billets or strips are produced in these mills. They operate in the forward direction only. The mill-stands are of two kinds here, roughing mills-stands and finishing mill-stands. These stands are also two or four high depending upon the number of rolls the stand has. In a four high stand inner rolls are smaller than the outer ones. The gap of rolling is maintained by the outer ones. The metal is worked simultaneously in the finishing stands. The roughing operation is not simultaneous,
The basics of the process described are
- When a mill has to produce billets of different sizes the gap between working rolls of the mill-stand must be adjustable.
- To ble to reduce the thickness of the metal gradually the motors of consecutive mill-stands must have differing speeds. This requires that the motor must be capable of speed control in the range 1.5:2. Speed control should be accurate.
- The sag of the metal between two stands must be avoided. This sag may occur when there is a slight difference in the operating speed. The speed drop may occur due to sudden application of load, which normally happens when the metal comes into contact with the rolls. A closed loop control must assure quick restoration of the speed of the motor. The motor must have a very fast dynamic response to avoid sag.
Based on the above, a motor to suit the job may be selected. The motor must have a constant speed at a given setting. It must have its speed controlled over a given range. The dc motors controlled by Ward Leonard control, ac commutator motors and ac motors fed from thyristor converters may be advantageously employed here.
Reversing Cold Rolled Mills
Another Types of Steel Rolling Mills is the metal in the form of a reel is used to feed the mill-stand. On one side of the mill-stand there is a delivering reel and on the other side there is a receiving one. The mill-stand may be two or four high. When the receiving mandrel is empty, the threading of the metal on to this empty one is done manually. The speed of the motor should increase with uniform acceleration, ensuring the required tension and pressure. Otherwise the sheet would break. The sheet is allowed through the mill-stand in both forward and backward directions till the metal of desired thickness is obtained.
The drive requirements immediately following the above process are the following:
- The drive must be capable of reverse rotation. A four quadrant operation must be possible.
- One or two individually driven motors may be used. The work rolls may be driven directly. The back up rolls are provided with motion whereas the working rolls move by friction.
- The coiling motors besides the driving toilers ensure the desired tension of the strip between the toilers and mill-stand. This is necessary to prevent looping of the strip and/or breaking.
- The gap adjustment must be made simultaneously with the reversing. The latter is accomplished by screwing down the upper rolls.
- The inertia of the motor must be kept low and lower than that of the
- Torque control as well as speed control must be possible to maintain constant tension of the strip. In a dc motor the torque control is possible both by field control as well as armature current control. As the diameter of the roller decreases the torque must also decrease. This is achieved by field However, field weakening in dc motors is limited by commutation and armature reaction effects. It is also limited by stability conditions of the motor. The armature current control may be employed beyond this limit.
- The acceleration of the drive must be uniform to avoid breaking.
The motor selected for this purpose must have its torque developed, causing a smooth acceleration. It should be capable of four quadrant operation with smooth speed reversal. Torque control at different speeds must be possible. To suit these requirement, a versatile motor is a dc motor controlled by Ward Leonard control with flywheel effect. Static Ward Leonard control may become economical with the availability of thyristors at reasonable rates. Three-phase ac commutator motor or cycloconverter fed synchronous motors are suitable for the job.
Continuous Cold Rolling Mills
These work only in the forward direction and no reversing is required. The metal passes in one direction only in different stands till the product has the required thickness, The mill may be two high or four high. The coiler roller requires accurate torque and speed control. The strip tension must be constant and large. Low speed operation is required while threading the steel into the rolls. Immediately after the threading the speed of the motor must be increased. The speed must be brought down while the metal leaves the mill-stand. A large variation in the speed of the mill drive is required.
Motors for Mill Drive
Dc motors are very versatile as motors for mill drives due to their characteristics of high starting torque, capability for wide range of speed control, precise speed setting, large overload capacity and pull-out torque. Care must be taken to have satisfactory commutation in the complete working range. They can be accelerated, braked and reversed very fast. Further, the inertia of the motor must be very small. The motors for mill operations are normally TEFC motors with a high class of insulation.
The speed control of dc motors in Types of Steel Rolling Mills is accomplished by Ward Leonard control with flywheel effect. Dc dynamic braking may be employed for quick stopping and braking at a controlled rate. Sometimes mechanical brakes are also employed. Conventional Ward Leonard systems may be replaced by thyristorized units.
When smooth speed control is required, ac motors with conventional methods of speed control are not suitable. Ac commutator motor, such as Schrage motor may be employed. The thyristor power converters provide a variable frequency supply which can be used for speed control of ac motors. Both torque control and speed control are possible. For low speeds, cycloconverters can be used to give a smooth speed control. Thyristorized dc drives can be used in the place of Ward Leonard de drives.