Rolling of steel and major equipment in a Cross-country Rolling Mill
Rolling of steel and major equipment in a Cross-country Rolling Mill
Most of the steel products are rolled from the cast products from continuous casting machine through a series of rolling and finishing operations. The process is called simple rolling, when two rolls of equal diameter and with axis lying in same plane rotate in opposite direction with same rotational speed, and the material being rolled is homogeneous in its mechanical properties and is acted upon only by the forces from the rolls.
Rolling is the most important metal forming process. More than 95 % of ferrous and non-ferrous metals and alloys are processed to their usable shapes by rolling. Usable shapes of rolled metals are plate, sheet, strip, foil, and different sections like rail, beam, channel, angle, bar, rod, and seamless pipe etc. Two common rolling processes are hot rolling and cold rolling.
The primary function of the hot rolling mill is to reheat ingot/ billet/bloom/slab (steel rolling stock) close to soaking temperature point, and then roll it to thinner and longer through successive rolling mill stands driven by electric motors. The steel rolling stock heated up to around 1,250 deg C in reheating furnace, using a solid/liquid/gaseous fuel as the primary energy source. The heated steel rolling stock is rolled in a roughing mill in number of passes where its size is reduced and its length is increased while its shape is modified. This process continues in intermediate and finishing mills also in a number of passes in each of the mill. As rolling proceeds, the length of the product increases, the size of the material reduces and the speed increases after every stand and is the speed is the highest at the end.
In hot rolling, the material is rolled at a temperature higher than its recrystallization temperature. The advantage of hot rolling is two-fold. First, at elevated temperature the strength of the material to be rolled is reduced. Thus the compressive force required for deformation is comparatively less and hence smaller capacity rolling stand can be used for the rolling operation. The second advantage of rolling a material at a temperature higher than its recrystallization temperature is that a large amount of plastic deformation can be imparted without getting it strain hardened. With strain hardening, the deformation stress increases as more and more deformation takes place rendering the material hard and brittle. As a result, the material becomes more and more difficult to be deformed, and beyond limit, deformation leads to various faults or defects.
The rolling process, in general, includes the mechanical forces which are applied to the metal surface through a series of rolls to produce specific shapes and sizes by reducing the size (width and thicknesses). The ingot or continuous cast product of billets, blooms or slabs are the basic materials for the production of a wide range of manufactured forms through hot rolling. Many of these products are the starting material for the subsequent manufacturing operations, such as forging, sheet metal working, wire drawing, extrusion, and machining etc..
A rolling technology is not only a rolling theory, and but it consists as synthesis technology such as hardware techniques of rolling mills or rolling rolls, measurement techniques to observe the rolling state, metallurgy-based software techniques to elaborate materials, control techniques to get highly precise thickness and shape of rolled products, and lubrication techniques to realize extension of roll life and reduce rolling load. The best rolling technology is not realized only with the rolling theory but it is also stimulated with the advancement of the neighbouring techniques.
Torque and power are the two important component of rolling. Torque is the measure of the force applied to the rolls to produce rotational motion while power is applied to a rolling mill by applying a torque to the rolls and by means of work piece tension. In a rolling mill the power is spent principally in the four ways namely (i) the energy needed to deform the steel, (ii) the energy needed to overcome the frictional force, (iii) the power lost in the pinions and power transmission system, and (iv) electrical losses in the various motors.
It is necessary to decide a pass schedule (including a draft schedule and the pass number) to get the aimed shape and thickness from a certain initial material. There are two methods to do so. The one is a method to decide by looking for the pass schedules from the past data and another one is a method to decide by calculating a pass schedule with the rolling theory. In the second method, a pass schedule is calculated near the capacity limit of a rolling mill by using rolling load and torque, and it is decided to adjust the calculated pass schedule so that the rolled product attains the required shape and dimensions when it is rolled in the last pass. The first method shows a strength for the rolling condition in the range where it had a past experience at all, but it is not helpful in the case when a totally new steel grade and product and size is to be rolled since the conditions are considerably different from the past experiences.
During the process of rolling, permanent deformation is achieved by subjecting the material to high compressive stress by allowing the material to pass through the gap between two rotating cylindrical rolls. The rolls can be flat or grooved, and are kept at a fixed distance apart from each other. The rolls are rotated in opposite direction by means of electrical drive system (motor, gearbox, spindle and couplings). Depending on the direction of rotation of the rolls, the input material enters the gap between the rolls from one end and comes out from the other end with a reduced cross-section, the roll gap area being kept less than the cross-sectional area of the input material (rolling stock). For obtaining the desired final shape of rolled material, it is generally necessary to pass the material through several set of rotating rolls. During each of the passes, the steel rolling stock pass through different set of roll gaps with diminishing cross sectional area.
Long products are normally rolled in several passes, whose numbers are determined by the ratio of the cross-section of the initial input steel material and final cross-section of finished product. The cross-sectional area is reduced in each rolling pass and form and size of the steel material being rolled gradually approach to the desired profile.
The entire assembly of the rolls mounted on bearings is held in bearing blocks (called chocks), which in turn are held between the gaps of two cast frames (called housings), complete with roll gap adjustment facilities and roll driving arrangement. The entire set up is called a rolling mill stand. One or more number of rolling stands in combination with other necessary and related equipment to obtain finished rolled products from one or similar group of input materials is called a rolling mill.
Cross country rolling mill
In a broader sense, a rolling mill consists of a set of roll stands along with a series of equipments which performs both rolling and auxiliary operations. The heated steel material from the reheating furnace is conveyed to the rolling mill where the different operations are being carried out. These operations consists of (i) rolling of the heated steel material in the mill, (ii) transferring of the material under rolling from one roll stand to another, (iii) turning or twisting and shearing of the material in case of some mills, (iv) transporting the steel product after rolling, (v) cooling the rolled material on cooling bed in some mills, (vi) cutting, marking or stamping of the rolled product, and (vii) trimming, packing, and conveyance to the stock of finished product.
The rolling mill is called cross-country rolling mill because of arrangement of the roll stands. In these rolling mills, the centre lines of the roll stands are parallel to each other and the material being rolled is shifted perpendicular to the rolling directions. Transfer and skid tables are used in these mills to reverse the direction of travel of the work piece and convey it from one set of roll stand to other. One of the characteristics of the cross country mills is that the work pieces are to be short enough so that one piece can leave a mill stand before another is transferred to it.
In the cross-country mills, the roll stands are located in a scattered manner. These mills are based on the concept of continuous rolling but the stands are placed so far apart that the piece must leave one set of rolls before entering the next. Such mills are useful for rolling sections which due to size or shape are not adaptable to loop rolling.
There are usually two types of rolling stands which are commonly used in the cross country rolling mills. These stands are 2-high stands, and 3-high stands. This classification of stands is based on the mode of arranging rolls in the housings. Typically, a 2-high stand consists of 2 rolls, arranged one above the other. Similarly, a 3-high mill has 3 rolls arranged one above the other. A 2-high stand has two rolls in it. One which is on top is known as top roll while the other is known as bottom roll. In mills with 2-high stands the rolling is only in one direction. In case of rolling in reverse direction, the mill is to be reversing. In case mill is not reversed then a pullover type two high stand is used. In this case the steel material after rolled in a pass is transferred to feeding side generally over the top of the rolls for further rolling in next pass.
In a 3-high roll stand there are three rolls consisting of top roll, middle roll, and bottom roll. The steel material is fed in one direction through two of the rolls and then reversed through the other pair. The middle roll is common in each feeding. 3-high roll stands are used to reverse the direction of the steel bar being rolled without reversing the direction of motor and gear drive rotation. One gap (between the bottom and middle rolls) takes the bar in one direction while the other gap (between the top and middle rolls) takes the bar in the other direction. To move the bar from the elevation of the lower gap to the upper gap a tilt table can be used. Other methods of moving the bar are also being used, such as, a lift table that moves the whole table up and down.
In 2-high roll stands either one of the roll (top or bottom) or both the rolls are driven. In case of 3-high roll stands, either one or two rolls are driven while the balance rolls rotates by friction. In case of two rolls being driven in a 3-high stand, usually the top and bottom rolls are driven, while the middle roll is friction driven.
In a cross country type rolling mill, the roll stands are so arranged that the work piece is never in more than one roll stand at the same time. Since the roll stands are located side-by side, the work piece is transferred laterally to the roll bites of the various stands. In many of the cross country mills, the rolling of work piece takes place in both the directions. In such mills since the direction of rolling is changed after each pass, it has a positive effect on the quality of the rolled product.
At each pass, there is a reduction in section and a corresponding increase in the length. This means that there need to be facilities available to take the longest piece of steel at the each stand and naturally this applies to both the sides of the mill.
There are limitations to the maximum reduction which is possible to be achieved in a cross country mill using a single drive, especially when it is designed so that there is more than one piece of steel materials being rolled in the rolling mill at a time. Hence, the size of the ingoing steel material is normally fairly small. Further in such mills, the 2-high stands can be set in a manner so that the upper roll of the first stand corresponds in height to the lower roll of the second stand and so on allowing the piece to be processed backwards and forwards alternately along the stand line. If there is a combination of 2-high and 3-high stands in the same cross country assembly, the centre roll of the 3-high stand corresponds to the driven roll of the associated 2-high stand.
An alternative arrangement is for the cross-country stands to be preceded by a single roughing stand in which only an odd number of passes (say 3 to 5) is taken and then the steel can be passed into the cross-country stands. This layout can also be used to roll input material of higher cross section.
In cross-country rolling mills, like in any other hot rolling mills, rolling is done above the recrystallization temperature of the steel material. During rolling in these mills the grains, which deform during the process of rolling, recrystallize, maintain an equiaxed microstructure and prevent the steel material from work hardening. In this type of rolling, hot rolled steel product has very little directionality in the mechanical properties and deformation induced residual stresses.
Cross country type of mill lay out is used for rolling mills having low capacities. This lay-out is generally adopted because of the limited space available for the mill. Cross country type of mill lay-out limits the maximum length of the steel product which can be rolled from the mill.
Cross country mills are normally hot rolling mills which are used to roll shaped steel products such as rounds, reinforcement bars, squares, flats, or sections etc. The shaped steel products are usually known by a common name which is the ‘long products’. Cutting machines, trimming machines, and tools are used in rolling mills. Most of the rolling mills are open train, 2-high or 3-high type. The mechanical coupling between the mill motor and rolling stands is either v-belt and pulley type or speed reduction gear type. The system has flywheel arrangements to guard against the load fluctuation.
Major equipments – The following are the major equipment in a cross country rolling mill.
Reheating furnace is the major consumer of thermal energy. The operational characteristic of the furnace plays a vital role in overall rolling mill process. The reheating furnace is equipped with combustion equipments, such as burners and waste heat recovery systems. Other associated equipment includes charging and discharging system, such as pusher and extractors etc. A typical reheating furnace has preheating, heating, and soaking zones to gradually increase and maintain desired temperature profile of the rolling stock for the process of rolling.
The reheating furnace can be classified in a number of ways. The classification is based on (i) the method of heating in which reheating furnaces can be combustion type or electric with the combustion furnace can be coal, oil or gas fired, (ii) the heat recovery mechanism in which reheating furnaces can be classified as regenerative or recuperative with recuperative type of reheating furnaces are more commonly used, and (iii) the method of charging in which reheating furnaces can be either batch type or continuous type. In the batch type reheating furnaces, the charged material remains in a fixed position on the hearth until heated to rolling temperature. In continuous type reheating furnaces, the charged material moves through the furnace and is heated to rolling temperature as it progresses inside.
Continuous reheating furnaces can be further classified based on the movement of steel stock in heating zones. Most popular continuous type of furnaces includes pusher, rotary hearth, walking beam, walking hearth, or roller hearth type. Most of the rolling mills are equipped with the continuous type of reheating furnaces.
In continuous reheating furnace, the material to be rolled is introduced at one end (feeding end or charging end), which moves through the furnace and is discharged at the other end (discharge end).
There exists a temperature gradient in the length of the furnace. In general, the material and combustion gases move opposite to each other. On the basis of the temperature gradient, the continuous furnace is divided into three zones namely (i) preheating zone, (ii) heating zone, and (iii) soaking zone (Fig 1).
Fig 1 Three zone pusher type continuous reheating furnace
Continuous reheating furnaces are further classified according to (i) number of heating zones (one to five, top or top–bottom), (ii) the method of movement of material (pusher, walking beam, walking hearth, rotary hearth, or roller hearth), (ii) based on heat recovery, the reheating furnace can be either regenerative or recuperative.
Pusher reheating furnaces are more commonly used in the cross country type rolling mills. In the pusher these furnaces, the cold steel stock is pushed forward with the help of pushers at the charging side. These furnaces are designed for heating billets/pencil ingots or smaller sections of blooms. The hearths of pusher furnaces are generally short in length and sloped downward longitudinally towards the discharge end in order to permit easy passage of the steel stock through the furnace. However, presently, the pusher furnaces are even longer with hearths up to 30 m (metre) length. The steel stock is moved forward by pushing the last piece charged with a pusher at the charging end. With each pushing of the cold steel stock against the continuous line of material, a heated piece is discharged at the discharge end through an end door upon a roller table feeding the rolling mill, or pushed through a side door to the rolling mill roller table by suitable manual, or by mechanical means, or withdrawn through the end door by a mechanical extractor.
In order to increase the throughput of the furnace, additional combustion zones are introduced by changing the profile of the furnace from single zone to multi-zone and placing the burner at more than one location, for example front-fired, side-fired, bottom or top-fired furnaces.
A reheating furnace with two combustion zones delivers better results for the temperature gradient than a single combustion zone furnace. It consists of two combustion zones, viz. soaking and heating. In these reheating furnaces, burners are arranged front firing in the soaking zone and top and side firing in the heating zone. If the heavy material is required to be heated in the reheating furnace, then 3, 4, or 5 combustion zones can be employed in order to increase the total temperature level and productivity. To cater to such requirements, two or three heating zones are a norm in the furnace, with burners being mounted in each of them. This customized design makes it possible to have a higher temperature at the end of the pre-heating zone, which shortens the length of the zone and increases the total length of high-temperature zones. As a result, heating of the steel stock in the furnace is more intensive.
The advantages of pusher type furnaces are (i) high production per unit capital investment, (ii) high hearth area efficiency and higher specific production per unit of space utilized, (iii) ease of charging and discharging, (iv) gradual rise in temperature permits charging of all grades of cold materials, and (v) more control of the rate of heating at all temperature levels. The disadvantages associated with pusher type furnaces are (i) limits the cross section of the charge since the contacting surface is to be square to avoid piling up inside the furnace, (ii) no flexibility for heating efficiently small quantity or low thicknesses of rolling stock, (iii) it is marginally difficult to maintain water cooled skid and also limits the thickness of rolling stock to a maximum of 300 mm to 350 mm when water cooled skids are used.
Rolling mill equipment
Cross country rolling mills consist of a number of equipments which together contributes to execute the rolling process with ease and efficiency. Some of the equipment is essential to constitute rolling operation while many of them are additional equipment used to improve the productivity and the efficiency of the mill. The main equipments used in these rolling mills are described below.
Mill housings– Mill housings are one of the most important structures of the rolling mill since they hold the mill assembly in position. Housings are elements in a rolling mill which hold chock assemblies, adjusting and other mechanisms, and retain proper positions. Thus their construction and dimensions have to take into account sizes of related elements. The forces which act on the rolls during rolling are completely transferred on to them through the nuts of the adjusting mechanism. The housing of the rolling stand requires high rigidity, sufficient strength for taking the loads, simplicity of design and minimum cost of production. One-piece cast housings of simple form (rectangular section) are used for roughing mills. These are called ‘closed type’ housing. In the some of the mills sometimes the housing has a detachable top for easy removal of the rolls, especially in the linear mills. Such housings are called ‘open type’ housings (top beams connected by bolts to the pillars). These types of housings are used wherein change of rolls are frequent.
Mill bearings – The load on the rolls gets transferred to the bearings and their assembly (chocks). The mill bearings can be classified into three types.
The first type is slider bearing. Slider 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 slider 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.
The second type of bearing is the hydrodynamic bearings. 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, wire rod mills.
The third type of bearings is 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.
Rolls – Rolls are normally the main and very costly consumables in a rolling mill. They are most vital part of a rolling mill. The deformation of metal work piece is directly accomplished by the rolls. The rolling stresses are first of all applied on rolls and after that transmitted to other sections of the mill. Therefore, the rolls had to be harder and more resistive to deformation than the metal under processing.
Shaped products are rolled between grooved rolls. Grooves are cut by a roll turning machine, On mating rolls, these grooves form passes through which the steel material is passed to obtain the aimed cross section. Before getting the final shape, the steel material being rolled goes through many passes. Roll passes are classified as (i) roughing pass, or break down pass, (ii) leader pass, and (iii) finishing pass. Roughing passes are intended to reduce the cross-sectional area. Leader passes gradually brings the cross section near to final shape while the finishing pass provides the steel material its final or the required cross section.
Since the rolls are used to roll steel in the rolling mill, hence their performance depends on many factors which include the materials used and the loads to which they are subjected to during service. The roll design is influenced by the limitations applied by the rolling load, the roll strength and the torque available for rolling. The material of the rolls is to be capable of withstanding loads which plastically deforms the rolling stock without itself being plastically deformed.
The deformation of metal work piece 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 iron rolls or steel rolls 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 rolls can be classified into the following four categories.
Steel rolls – The steel rolls have carbon composition in the range of 0.2 % to 0.3 %. The steel rolls can be cast steel rolls or forged steel rolls. The rolls can be either sand cast or chilled mould cast. In some of the cast steel rolls for critical application, the roll body is chilled cast while other portions are sand cast. Some of the steel rolls have high alloy contents.
Iron rolls – The iron rolls have carbon content in the range of 2.5 % to 3.5 %. The iron rolls can be grey iron rolls and alloy iron rolls. Different types of commonly used iron rolls include (i) clear chill rolls, (ii) indefinite chill rolls, (iii) spheroidal graphite iron rolls, and (iv) double poured rolls.
Tungsten carbide rolls – These rolls are manufactured by pressing and sintering powdered carbide. They are usually fashioned in the form of rings of relatively small diameters which can be used in association with steel arbors. These rolls are normally used in the finishing stands of a wire rod mill.
Composite rolls – These types of rolls consist of arbor and a ring or sleeve-type member which is shrink-fit over the arbor. These rolls have the advantage that if ring or sleeve has been worn down, the arbor can be refitted with new outer member.
Drives – Rolling mills are powered by electric drives and suitable transmission lines are necessary between them and the rolls. The drive consists of (i) electric motor of sufficient capacity, (ii) drive belts, (iii) fly wheel, (iv) set of gears, (v) pinion stand and (vi) couplings. In some mills a shaft connects the motors to gear box connected by couplings at both ends. The gear box provides for the speed reduction from motor speed to the roll speed. The power is then transmitted on to the pinion box where it is distributed to a number of shafts, depending on the number of rolls to be driven. Generally AC (alternating current) motors are used in the cross country mills since speed control is not an important issue for such mills.
Flywheel – A flywheel is a mechanical device with a significant moment of inertia used as a storage device for rotational energy. Flywheels resist changes in their rotational speed, which helps steady the rotation of the shaft. Flywheel acts as a reservoir by storing energy during the period when the supply of energy is more than the requirement and releasing it during the period when the requirement of the energy is more than the supply. Flywheel provides an effective way to smooth out the fluctuation of speed. The stored kinetic energy relies on the mass moment of inertia and rotational speed.
Lead spindle – The lead spindle is used to connect the prime mover with the pinions and can be of universal type, either short-coupled or long with carrier bearings, depending on the position of the motor in layout. If short-coupled, standard flexible couplings can be used. The lead spindle is attached to the bottom pinion of 2-high mills, and to the centre pinion of the 3-high rolling mills.
Mill pinions – The pinions are gears serving to divide the power transmitted by the drive between the 2 or 3 rolls, driving the adjacent rolls in opposite directions. As per the earlier practice, the pinions had either spur teeth or a divided face and staggered spur type teeth but the present practice is to use double helical teeth. Helical gears provide smoother drive, as some parts of the teeth are in contact at all times, making the transmission of power continuous.
Spindles – Spindles are used to connect pinions with rolls of the rolling mill if not a direct driven type. In direct drive case, the spindle is connected directly to the motors. Spindles are made of cast or forged steel and are fitted at each end with wobblers similar to those on the rolls or with the universal couplings, depending upon type of rolling mill.
Reduction gear boxes/reducers – The reduction gear boxes ‘reducers’ are used in the mills where speed of motor is higher than required for rolls. Depending on the required reduction in speed, reducers can be used having 1, 2 or 3 stages.
Guides – Guides assist the steel stock in entering and leaving the rolls. They are termed entry or delivery guides according to their location and are customarily secured to a rest or cramp bar, running in parallel with the rolls and mounted across the housings. Fixed guides are those having no moving parts and are normally made of cast iron, to minimize dangers or preventing scratching of rolling stock. In the simplest form, a fixed entry guide comprises two castings clamped together to form a bell-mouthed box. Roller guides have been developed to overcome the tendency of guides to scratch the rolling stock. Such guides, which are used particularly as entry guides, incorporate one or more pairs of idle rollers profiled to the appropriate shape of the rolling stock.
Repeaters – 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.
Roller tables – The roller tables consist of a series of roller 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 rolled stock towards as well as away from 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 tilt.
Shears – There are different types of shears which are used in a rolling mill. The large hydraulically or electrically-driven shears with up-cutting or down-cutting blades are used to crop the segregated and deformed ends of large sections or for dividing the large sections into shorter lengths for rerolling. Pendulum shears are the shears, coupled close to a stand, with blades supported in a frame free to move in pendulum fashion are used to cut moving stock, such as deformed back-ends disappearing into the following stand. Flying shears are those shears which cut the moving stock. This term is normally used to describe the shears in those mills where the rolling stock must be divided at the emerging speed into several lengths. Crop shears are also known as cobble shear. In rolling mills, crop shears are located ahead of repeating trains to remove the deformed or split front ends of stock after roughing. They are generally arranged to remove the head end of the stock, but they can also be set in continuous motion to divide stock into short lengths for clearance when the front end has cobbled further down the train.
Snap shears are usually located with the automatic repeaters. These shears are generally pneumatically operated to snap closed and stay closed until reset. They are useful for taking back end samples of repeated stock or for preventing the remaining stock feeding a cobble further down the train.
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 driving fans (furnace combustion system), cooling water pumps, and lubrication system. In multi-stand continuous hot rolling mill, the power and speed of motors need to be selected to suit the rolling schedule.
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.
Cooling bed – A cooling bed is part of a rolling mill located at the end of the rolling mill. It supports and permits the hot rolled products from the last stand of the mill to cool. Cooling bed naturally cools the material as well as cross transfers towards the discharge end. Manual cooling bed has slope for the bar to move forward by sliding action due to gravity. Mechanical cooling beds are rake type. In large capacity mills, a walking beam cooling bed is the most common type of cooling bed. In a cooling bed the temperature of the entire length of the bar to cool at the same time. If not, it develops stresses in the bar. If part of the bar stays continually in touch with the metal supports, then it cools at a different rate than the parts of the bar which remains continually open to the ambient air. A walking beam cooling bed consists of moving and stationery skids which lift, traverse and lower the product numerous times so that where it touches continually changes permitting all the bar to cool at the same rate. The bar walks its way to the end of the bed where it is placed on a set of roller tables. 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. Cooling beds may use a chain transfer as the traversing method. Cooling beds are sized so that the product cools within a particular cycle time.
Centralized oil lubrication system – The centralized oil lubrication system is installed in some of the large mills. It helps in automatic lubrication of gears of gear box, pinion box, etc. The lubricating oil is filtered, cooled, and re-circulated in a closed loop.
Cooling water system – Cooling water system helps in cooling of mill stand rolls, bearings, etc. The water is cleaned, cooled to ambient temperature and re-circulated in a closed loop. For the water needed thermo-mechanical treatment (TMT), normally there is a separate water cooling system.
Power supply, distribution, instrumentation, and control system – The electrical power supply and distribution system of the rolling mill mainly includes transformers circuit breakers, high tension capacitor banks, and control panels. Variable Voltage Variable Frequency (VVVF) drives for regulating the speed of AC motors particularly in finishing mills is the latest development in this area. PC (programmable controller) or PLC (programmable logic controller) based instrumentation and control system with valve actuators are used for automation of the mill, front and end cropping shears, TMT water-cooling system, flying shear, etc.