Rolling Mill and its Technological Equipments
Rolling Mill and its Technological Equipments
Rolling is a continuous process, during which the input material is deformed between the rotating work rolls. Rolling mill is the place where rolling takes place. The roll gap (a gap between the work rolls) is smaller than the material entry height. As per the direction of passing the rolling stock between the work rolls, the position of rolls axes in relation to the rolling stock, and the course of rolling, a rolling mill can be categorized into longitudinal rolling mill, cross- rolling mill, and diagonal rolling mill.
In the longitudinal rolling, rolls are driven and they draw the material in-between and press it through the height. Due to it, the rolled material is considerably extended (elongated) and spread to a lesser extent. The longitudinal rolling is most popular method of rolling. Longitudinal rolling with plain rolls is used to roll flat rolled products (sheets, plates, strips), while longitudinal rolling in grooved rolls is used for shaped rolled products (rails, sheet piles, bars, sections, and wire rod).
The cross rolling mode is characterized by the fact that the axis of the rolled product is parallel with the rolls axes. The rolls are rotating in the same direction. The rolling stock is rotating in the direction of acting the resulting friction forces, i.e. in the opposite direction than is the direction of work rolls. This mode of rolling is used for the production of shafts or for rolling of grinding balls etc.
Diagonal rolling is a special case of cross rolling. The mechanism of plastic deformation is here a similar one. However, work rolls axes are not parallel, but skew. So, the rolling stock not only rotates, but also feeds forward in the direction of its longitudinal axis, because of the skew axes of the rolls. This mode of rolling is used for the rolling of seamless tubes. It is the popular method for the production of hollow semi-finished products.
Rolling mills are classified as per (i) type of rolling mill stands and number of rolls, (ii) type of rolled products and work roll diameter, (iii) method of rolls rotation, and (iv) layout of mill stands
Type of rolling mill stands and number of rolls
Two-high rolling mills – These are the most commonly used rolling mills. These mills have two horizontally mounted rolls. The rolling mill motor drives either both rolls (top and bottom) or only one roll (normally the bottom roll with the top roll rotating due to the friction between the roll and the rolling stock). As per the rolls rotation direction the mill can be non-reversing (unidirectional) mill or reversing mill.
Three-high rolling mills – These types of rolling mills are with three horizontally mounted rolls. Rolls in these mills rotate permanently in one direction. These mills make it possible rolling with increased number of grooves than in case of two-high mill stands. The rolled stock is rolled in one direction between the bottom and intermediate roll and in the opposite direction between the intermediate and top roll. The fix-fitted intermediate roll is directly driven. The bottom and top roll are driven via the gearbox and they are usually adjustable. These mills are used for the rolling of the shaped rolled products in grooved rolls.
Lauth type three-high mills are used for rolling of plates. This rolling mill differs from normal three-high rolling mills. In these rolling mills, the lower roll is fixed. The middle roll, which has smaller diameter than the upper and lower rolls is raised and lowered by a power-operated lever alternately as the rolling stock passed under or over it. The draft is achieved by adjusting the upper roll using a screw gear. The small middle roll is an idler roll, and it is set in rotation by the friction developed by the upper and lower rolls. It is not subject to bending stress, as it is always in contact with one or other of the large rolls. The advantage of smaller diameter of the intermediate roll consists in supporting the rolled stock elongation. The disadvantage is more rapid wear and tear of the intermediate roll.
Four-high rolling mills – These mills have four horizontal rolls, mounted in a single vertical plane. Two rolls (inner) are work rolls and two rolls (outer) are back-up rolls. Significance of the back-up rolls consists in a chance of using higher roll forces and decrease in bending (deflection) of work rolls. Small diameters of work rolls also permit (except for greater elongation of the rolling stock) a possibility of achieving of more favourable dimensional thickness deviations. The work rolls of the four-high mill are driven while the back-up rolls are friction driven. The four-high mill is used for rolling of plates and for hot and cold rolling of steel strip. It is used both as non-reversing or reversing rolling mill.
Multi-roll mills – These rolling mills consist of six, seven, twelve or twenty horizontally mounted rolls. In all these mills there are only two rolls which are work rolls while all the other rolls are back-up rolls. Normally work rolls are driven and back-up rolls are friction driven. The multi-roll mills are used for rolling of very thin sheets, strips and foils.
Universal mills – These mills have two horizontally mounted rolls and two vertically mounted rolls which are driven via transmission of bevel gear wheels. The vertical rolls act by edging effect on lateral sides of the rolling stock, which leads to creating its lateral ‘walls’, precision angles and sharp edges. The edging rolls used to be mounted from the front of the mill stand, less frequently from the rear side, but sometimes also from both sides of the mill. Universal mills are used for rolling of slabs, universal plates and steel shapes. To enable rolling of wide-flange beams, the vertical rolls are mounted in the same plane with axes of rolls placed horizontally. Only the horizontal rolls are driven.
Special structure mills – They have skew axes of rolls and are used for the production of seamless tubes.
Specialty mills – These mills are used for the rolling of wheel, tyre, thread, and grinding balls etc.
Technological equipments for rolling mills
The basic equipment of the rolling mill, which directly ensures the origin of plastic deformation, is the roll stand. The components of a roll stand are shown in Fig 1 and described below.
Fig 1 Components of a roll stand
Housing – Housing creates a framework of the rolling mill stand and for absorbing the total metal pressure on rolls during the process of rolling. Hence, the housing is to be solid and its structure is to enable easy and fast roll changing. Also, there need to be easy access to all parts of the housing and other details of the roll stand. Each of the roll stands has two housings, in which rolls are placed with chocks (bearings). In the upper part of the housing, there are adjusting screws and the roll counterbalancing device along with their drives. From the structural viewpoint, the housings can be classified into three types. These are (i) enclosed housing where the whole housing is made of one piece and which is more beneficial from the strength point of view, (ii) open housing which has the separated cap, connected to the housing by screws for easier rolls changing, and (iii) housing-less roll stand which has rigid chocks connected by solid and pre-stressed joints. The housing-less roll stand has limited stress relaxation (spring-back) of rolls and has smaller and lighter structure.
Rolls – Rolls are the basic part of a roll stand and are normally the most vital and very costly part of a rolling mill. The rolling stock is plastically deformed between the rolls. Rolls ensure the required shape, dimension and surface quality of the rolled product. They transfer the force and torque load. The deformation of rolling stock is directly accomplished by the rolls.
Rolls are required to carry out the heavy work of reduction of the cross section of the steel being rolled. Rolls have to take all kind of stresses, loads from normal and abnormal rolling and which are changing with the roll wear during a rolling campaign. Roll should never break, spall or wear. They are expected to give excellent performance without causing any problems. Under the conditions of rolling, the contact area of the roll which comes in contact with the rolling stock suffers wear, while other parts of the roll body and roll necks does not experience plastic deformation or fatigue but are under high loads.
The deformation of rolling stock is directly accomplished by the rolls. The rolling stresses are first of all applied on rolls and after that transmitted to other sections of a mill. Consequently, the rolls had to be harder and more resistive to deformation than the metal under processing. Whether the rolls of a particular material or other material are to be used in a particular roll stand depends on the specific duty they have to perform. The important properties to be considered for the selection of rolls include toughness, resistance to thermal cracking, shock loading, or hard wearing. The selection of any particular roll depends on issues such as production demands, initial cost, and specific qualities required etc. Close collaboration with the roll manufacturer is desirable to ensure that these requirements are satisfied as far as possible.
The main parameters which control the properties of a roll are (i) mono or compound roll (roll design), (ii) chemical composition of the material, (iii) casting (mould design, temperatures, weights, inoculation, and down cooling), and (iv) heat treatment. According to the microstructure of the roll materials the groups of grades in which the roll materials can be categorized are (i) hypo eutectoid steel, (ii) hyper eutectoid steel, ADAMITE, (iii) graphitic hyper eutectoid steel, (iv) high alloyed materials like high chrome and high strength steels etc., (v) nodular iron, (vi) indefinite chill double pour (ICDP) cast iron, and (vii) special materials such as sintered carbides (tungsten carbide) and ceramics etc.
Rolls are also categorized according to (i) the function such as work rolls, and back-up rolls, (ii) the material and process technology such as cast iron, cast steel, forged steel, alloy steel, and compound rolls etc., (iii) the rolling technology such as plain rolls for rolling of flat products, grooved for rolling of shaped products, and special rolls for special products and specialty mills, and (iv) the placement in the roll stand such as horizontal rolls and vertical rolls.
The work roll has three parts consisting of (i) working part which is the roll body also being called roll barrel and is normally characterized by diameter and length and where the actual rolling of the rolling stock takes place, (ii) supporting part comprising of the roll neck (journal), which holds the roll in the housing of the roll stand, and (iii) connecting part known as wobbler which connects the roll through further equipment with the main shaft of the electric motor. Some rolling mill rolls are shown in Fig 2.
Fig 2 Rolling mill rolls
The roll body is the most important part of the work roll, because it works on the most difficult conditions consisting of high temperatures and pressures. It is also subjected to shocks. Hence, the roll body needs to have high strength, hard surface to resist considerable impacts and pressures from the rolling stock, and resistance to wear and tear as well as to the high temperatures of the rolling stock. The roll neck is to be resistant to wear and tear, high temperature and torsional and bending strain while the wobbler is to be resistant to torsion and impacts.
Chocks and bearings – The chocks houses the bearings which serve for accurate mounting of roll necks, in both horizontal and vertical plane. The load on the rolls gets transferred to the bearings and their assembly (chocks). Roll bearings typically encounter very high radial loads and varying degrees of axial load while running at slow to high speed. The bearings need to have enhanced contact surfaces, material strength properties, and internal geometry and cage characteristics for accommodating these operating conditions.
The basic requirements of roll bearing include (i) high continuous load capacity, (ii) low coefficient of friction, (iii)) a design permitting a large and strong roll neck, (iv) minimum wear, (v) simplicity and ease of maintenance, (vi) high reliability, (vii) a design permitting quick and easy interchangeability from roll to roll, (viii) stability of operation, and (ix) precision design and manufacture. The bearings of the rolls are to provide high performance so that the downtime of the rolling mill can be reduced.
The common bearings used in the rolling mills are (i) sliding bearings in which the shells are of plastics/organic fabric and achieve friction coefficient from 0.02 to 0.1, (ii) bearings with rolling friction which are characterized by small wear and tear and hence high accuracy of the rolled material and in these bearings there is low friction coefficient ( from 0.001 to 0.003), (iii) bush bearings with fluid friction also known as hydrodynamic bearing in which the friction coefficient is similar to antifriction bearings and the bearings are characterized by their smaller dimensions, very low wear and tear, and negligible elastic deformations.
Sliding bearing can be further classified into two categories. Slider bearings with metallic bush have high coefficient of friction and comparatively low life. They are used when high temperatures and pressures prevent the use of other bearings. The non-metallic bush bearings have all the advantages of sliding bearings. In addition, they are low cost and provide good bearing for rolls when the speed can vary considerably or can even reverse. Further, the coefficient of friction is also very low. These are the most commonly used bearing in a low capacity cross-country mill.
Bearings with rolling friction are also called the anti-friction bearings. These bearings include all types of bearings with rolling contact. However, only taper roller bearings are used in rolling mills in multiple row series. The principle advantage of anti-friction bearings is low friction and their ability to work at low speeds.
The hydrodynamic bearings completely enclose the roll neck and bearing surfaces are separated by a liquid film. They have a low coefficient of friction at high speeds. Also they have a very long life, and low space requirement. This has led to their extensive application as a substitute for anti-friction bearing in many non-reversing stands. However, their use is restricted to applications where speeds are relatively high and almost constant. These types of bearings are used where the loads are very high due to high reductions such as flat mills, and wire rod mills etc..
Modern rolling mills normally use tapered roller bearings for rolls since these bearings can support both radial and axial forces due to the inclined position of the rollers. Four-row and double-row tapered roller bearings are used in rolling mills.
Adjusting and balancing equipment – The rolling process of various rolled products requires changes of mutual position of rolls, either during rolling mills adjustment or in the course of actual adjusting the rolls before individual passes. The mutual rolls position changing can be reached by means of adjusting and balancing equipment. There are several methods for adjusting the rolls. Adjustment of rolls in the vertical direction determines the gap between rolls and hence also the draft (thickness reduction in the pass). Axial adjustment of rolls takes up the lateral clearance. The adjustment is to be accurate and quick, with as small time loss as possible. The top roll is to remain in position and not drop after passing of the rolling stock between the rolls. Hence it is needed to be counter-balanced together with chocks. The balancing is a prerequisite to prevent impacts and wear and tear of the adjusting screw with the nut. Fig 3 gives two popular methods of adjusting and balancing of rolls.
Fig 3 Methods of adjusting and balancing of rolls
Pinion stand – Pinion stands transfer torque of the driving shaft to the work roll. Pinions of various kinds of roll stands differ only in dimensions and sort of teeth. Pinions can have spur teeth, spur offset, helical teeth, or herringbone teeth. By their shape they are similar to the work rolls. However, pinions are not common types of gears (toothed wheels) but have the journal and the wobbler, like the work rolls.
Reducers – The reducers change the number of revolutions of the electric motor to the required revolutions of the rolls. They increase the torque. They can be one-stage, two-stage and three-stage type. The gear ratio of each stage can range from 4 to 6.
Couplings – The couplings serve for transfer of torque of the driving train to the work rolls. Their function consists in absorbing roll force impacts (shocks) and prevention of the transfer these shocks to the electric motor. The common types of coupling used in the rolling mills are cardan shaft coupling, gear coupling, and universal coupling.
Mill motors and auxiliary drives – Rolling is a continuous process and main mill stand drive motors are exposed to high stresses. Any unscheduled stoppage or failure of equipment and drive leads to significant loss of energy, production, and time. Hence, the drive system for main and auxiliary equipment is one of the critical utilities to undertake periodic operational and maintenance practices. Drive used for these are known as primary auxiliary drives. Secondary auxiliary drives are used for mill motors and auxiliary drives for driving fans (furnace combustion system), cooling water pumps, and lubrication system. The roll stand drive system consists of the main electric motor (master drive) with the control system, reduction gears and connecting elements.
The motors used in rolling mills can be broadly classified into two types, AC (alternating current) motors and DC (direct current) motors. AC motors are generally used where the stand is to operate at constant speed in one direction, whereas for variable speeds and reversible drives, DC motors are generally used. AC motors used are further classified into (i) synchronous, (ii) squirrel cage and (iii) wound rotor motors. Similarly, DC motors can be classified into three categories (i) shunt wound, (ii) series wound and (iii) compound wound motors. Each of these types of motors has characteristics which make it suitable for a specific application.
Spindles and wobblers – The spindles consists of steel forged shafts which connect work roll necks with journals of pinions, thus making it possible the transfer of torque. In addition to, they accommodate the axial misalignment as per the position of the roll. The wobblers can be either plain or stepped (with shoulders). The wobbler, except for connecting spindles, functions as a safety device (breaking piece) in case of extremely high load of the roll stand.
Guide equipments for rolling
Guide equipments are the auxiliary equipment placed directly at the roll stand, serving for correct entry the rolled material into the rolls and its exit from the rolls, or for automatic rolling stock guidance in its transfer from one groove to another groove of the same or adjacent roll stand. Guide equipment consists of (i) entry guide equipments for rolling consisting of guides, guide benches, guide bushing, guide borders, open and closed guide boxes, and twisting guides etc. (ii) exit guide equipments for rolling consisting of simple and shaped guards, and (iii) repeaters and looping channels. The aim of guide equipments is to ensure guidance and thus make the correct rolling stock entry into the groove and exit out of the groove easier, and to make it possible at high rolling speeds.
Guides – The purpose of the guides is to correctly guide the rolling stock into the groove of work rolls at the entry side of the roll stand and its safe exit to the run out table of the roll stand. Guide equipments guide the rolling stock at the entry and the exit of the roll pass so as to have smooth rolling of the rolling stock. The guiding equipments are to be sturdy, accurate and stable. They play a major role in ensuring the surface quality of the rolled product. The guides are to be designed for the wide variety of sizes and shapes of rolling stock which are normally encountered in the long product rolling.
Guards are used for preventing the bending of the rolling stock upwards or downwards during its exit from the rolls and/or collaring on the roll. The placing of the guards is important depending on whether the mill stand works with top or bottom pressure, i.e. whether the rolling stock shows a tendency towards collaring on the bottom or top roll.
Guides can be roller guides or static guides. In modern rolling mills roller guides are normally used both for entry and exit guides. The design of roller guides is based on rolling frictions and hence these guides have a number of advantages over static guides whose design is based on sliding friction. Roller guides ensures faster and accurate adjustment of the guiding elements when compared with the static guides. Since the contact of the rolling stock with the guide is carried through the rollers working on rolling friction, it becomes possible to considerably increase the wear resistance of the working elements (rollers) and to reduce the possibility of such rolling defects like scratches, laps, and score marks etc.
Repeaters and looping channels – Repeaters are devices used to receive the work piece as it emerges out from one stand and loop it through 180 degree into an adjacent stand automatically. This consists of grooved channels or troughs which guide the leading end of the rolling stock through 180 degree or in some cases through an S-shaped path in forward running repeaters. The front end of the stock is driven round the repeater by the succeeding stock until it is gripped by the next stand. The speed matching between the adjacent stands is usually such that the succeeding stand runs slightly slower than the balancing speed which causes the loop to grow in size. The repeating channels are designed to allow the stock to kick out on to a flat table under these conditions. Sometimes the repeaters function as twist guides as well.
Auxiliary equipment of rolling mills
Auxiliary equipments in rolling mills include descaling equipment after the reheating furnace, pinch rolls, roll cooling equipment, and roll/stand changing facilities, shearing, handling and tilting devices, shearing, product straightening, and piling and bundling etc. Some of the auxiliary equipments are described here.
Descaling equipment – It is placed before the mill after the reheating furnace. High pressure water is sprayed onto the surface of the hot rolling stock steel to remove the scale. The rolling stock needs to be scale-free before entering the mill stand to ensure that it has perfect surface definition and quality. Removing the scale also helps to reduce the wear of the mill stand’s rolls. Pickup on the rolls, if not properly cleaned, can easily be transferred onto the rolled product. The equipment required for this process basically consists of a high pressure pump, powered by an AC motor, which can be controlled by a variable speed drive. High pressure pump supplies water through specifically designed header and nozzles, which are directed at the hot steel. The removal of scale takes place due to the combined action of the water pressure and the rapid cooling of the steel.
Devices for longitudinal transport/handling – The common equipment are roller tables, feeding rolls, bumpers, stops, pushers, and extractors. Roller tables consist of a series of rollers either driven by line shafting and bevel gears from a common drive or by individual motors. In some improved designs, the bevel gears have been replaced with spur gears. The roller tables serve to feed the material being rolled into the rolls and receive it from the rolls. Hence they operate under severe conditions of mechanical impact, repetitive short-term duty cycles and dynamic transients (acceleration and decelerations). The roller tables connect the separated stands of large and medium sized mills. There are required on majority of the mills for conveying the rolling stock towards as well as away from the rolling stand.
Tilting or lifting tables – In large 3-high stand, the rolling stock is required to be mechanically lifted from the pass line of the middle and bottom rolls to the higher pass line of middle and top rolls. To achieve this, the tables on either or both sides of the stand can be designed to either tilt or lift.
Devices for cross transport – The commonly used devices are manipulators, transfer tables, transfers, displacement and shifting devices, and running troughs with stops.
Manipulators – They are used for rotating the rolled stock at a specific angle around its longitudinal axis.
Pinch rolls – The basic function of pinch rolls is to push the rolling stock in a particular direction. It does not cause any kind of physical change in the rolling stock and is normally used to move the material over a distance. It is also used for creating tension in the rolling stock. It is installed at various places in a rolling mill. It is driven through an AC/DC motor and consists of 2 high gear boxes, solid rolls with carbon shaft.
Shears – Hot shears are used in the rolling mills for front and tail end cropping, cobble cutting and dividing. Crank, rotary and combined shears at different speed ranges are generally employed to optimize front and tail end cropping, cobble cutting and dividing. Depending on the mill requirements the shears can be used along with pinch rolls and auxiliary chopping shears.
Several types of shears can be employed by a mill to cut the rolled product as it rolls, as it exits the finishing stand, and cold shearing before stacking or bundling. Depending on the product shape and material grade, shears can be used to cut the front of the bar as it proceeds through the mill. These are typically flying shears. The blades of this shear move parallel to the bar during the cut. In multi-strand rolling of rebar, there is a requirement of a shear in the mill which provides a clean front end of the bar to avoid cobbles at the slitting stand. Certain grades, such as leaded steels, require front end trimming to prevent cracks at the front end from splitting open and the bar wrapping the rolls.
A drum type shear is generally used for product with a simple shape such as flats or rounds. The blades are mounted on a rotating cylinder (or drum) and are set at a ‘lead’ speed to minimize the ‘kinking’ of the bar.
Cooling bed – A cooling bed is part of a rolling mill located at the end of the rolling mill. Cooling bed is the equipment where rolled products are cooled and at the same time item-by-item transferred to the roller table, on which they are transported to the finishing section. It supports and permits the hot rolled products from the last stand of the rolling mill to cool. Cooling bed naturally cools the material as well as cross transfers towards the discharge end. In a cooling bed the temperature of the entire length of the bar is to cool at the same time. If not, it develops stresses in the bar.
Manual cooling bed has slope for the bar to move forward by sliding action due to gravity. Mechanical cooling beds are rake type. Several types of mechanical cooling beds are used. The rolled bar as it enters the cooling bed slides onto the first notch on the rakes. The initial notches provide continuous support for the bar on a casting called a grid casting. Long plates with notches set at some distance apart, support the bar after it moves beyond the grid castings. The bar moves across the cooling bed by the movement of alternative plates moving in a cycle of lift, move, and retract, by the action of eccentric cams. Repeating of this cycle moves the bars as they are delivered from the mill. The length of the cooling bed is determined by the maximum run-out bar length, optimized by the selling lengths to minimize crop losses. The width of a cooling bed is determined on the basis of mill productivity (tons/hour) and the time required for cooling.
Walking rake type cooling beds are used in modern long product mills. The purpose of the cooling bed of a movable rake design is to uniformly air-cool the rolled bars or light sections and transport the same in a phased manner from the entry of the cooling bed to discharge side. The movable rake type cooling bed is normally of a walking beam design. The mechanism ensures that the bars and light sections are uniformly positioned over the toothed rakes. The cooling bed is usually designed considering the smallest and the maximum size of the bars and light sections being rolled, delivered from the finishing stand of the mill, and the cooling time required for the various sizes of the bars and light sections. Rake type cooling bed design depends on bars cut previously to given lengths, to slow them down, to transport them crosswise over a cooling surface ensuring that the rolled bars or light sections in very wide range of lengths, are kept as straight as possible, to collect the bars or light sections at the end of the cooling surface to predetermined packs matched to the requirements of the cold shear, and to discharge finally same onto a roller table which conveys the packs to the cold shear.
Walking beam cooling bed has a saw tooth pattern which is why it is also known as a rake type cooling bed. When cooling billets, walking beam turn over cooling beds have a feature that continually rotates the billet so that where it touches the bed changes continuously, thus the billet does not distort its shape in the cooling process.
The front ends of the bars and light sections are also leveled at the discharge side and a fixed number of rolled pieces sent for final length cutting by cold shear and bundling or stacking.
Straighteners – Straighteners are used for giving the rolled product certain specified degree of straightness. Various types of straightening machines are used and they are broadly classified in three groups. These are (i) cross-roll straighteners, (ii) section straighteners, and (iii) stretch straighteners. This categorization is based on the basic principles on which they operate. In the first two categories of straightening machines, the rolled product is made to pass through a set of staggered rolls, producing reverse kinematic loading of the rolled product, resulting in redistribution of the plastic strains, which secures the desired straightening.
Multi line straighteners are used at high productivity rates. The concept is to straighten cooling bed lengths in order to have less feeding operations and better utilization of the straightening roll drives. Proper alignment and centering of the bars under the rolls is essential. The recent improvements in this area are (i) use of automatic section feeding to the straighteners, (ii) quick change of roll sets mounted on a stand by carriage, motorized roll gap arrangement, and (iv) the whole unit is mounted on a platform that can be shifted out of the line for maintenance without stopping mill production.
Coiling machines – Coiling machines are used to wound long rolled products into coils. Making coils of the rolled products facilitates handling and improves the yield.
Stacking and bundling machines – After cooling, rolled bars/sections are typically straightened in a roller straightener and cut to sale length by a cold shear and then they are either stacked or bundled. In case of stacking of angles, they are stacked in a two down, one up arrangement.
Tying and strapping machines – Tying and strapping machines are used for coils, bundles, and piles. These machines are designed for continuous operation. Tying machines use commercial size wires for tying and the machine head is hydraulically operated. The strapping machines are pneumatically operated and use commercial steel straps of different available width. Strapping can be carried out either by clamping or welding.
Transport of bundles/packs and coils – Bundles/packs and coils are mostly transported by cranes using magnets or special suspensions/slings.
Mill electric system – It consists of transformers and switch gears, DC (direct current) and AC (alternating current) motors, variable speed drives for the motors, motor control centres, field sensors, instruments, and actuators, control panels, control desks, and control pulpits etc.
The mill automation is provided to carry out the reliable rolling with minimum of human interventions. The mill automation level can be at level 1 or level 2. At level 1 which is the basic level of automaton, the automation includes programmable logic controller (PLCs), human machine interfaces (HMIs) for operation and monitoring, SCADA (supervisory control and data acquisition) systems, as well as process and production control computers, all in centralized or distributed topology, interconnected via field bus and local area networks (LAN).
The mill automation carries out several functions. Some of them are described below.
- Main control desk, with management function mode and rolling speed calculation.
- Regulation cascade speed between stands. Cascade control uses the reduction concept (R-Factor) to calculate the mill cascade speed reference. This parameter is directly related to rolling fundamentals and simplifies the setup and operator control. During production the loop and tension control automatically adjust the R- Factor, ensuring minimum material stress between the stands.
- Impact speed drop compensation. The system speeds up the stand during the head threading, reducing the speed drop when the material impacts the rolls. Once the bar is inside the stand, the control changes back to the mills cascade speed reference.
- Minimum tension/loop control between stands. Tension/loop control between the stands reduces the material stress along the mill and it helps in improving the dimensional accuracy of the product.
- Shear cut control for cropping and cutting processes. The performance and accuracy of the shears in a mill is critical to increase the yield and avoid problems when the bar enters the stand.
- Automatic cobble detection is usually designed to help operators react faster to unexpected events and continuously track the bar. If a cobble occurs, the system automatically reacts to minimize the effects by commanding the upstream shears to chop the existing bars blocking the furnace from sending another billet.