Main Features of a Modern Bar and Light Section Mill
Main Features of a Modern Bar and Light Section Mill
The objective of a bar and light section mill is to reheat and roll steel billets into bars and light sections. Bars and light sections come under the category of long products. Long product is a common name for (i) reinforcement steel bars, (ii) shaped steel bar products such as rounds, flats, squares, and hexagon etc., (iii) sectional products such as angles (equal and unequal), channels, beams, tees, and special profiles etc., and (iv) wire rods. Mills which roll long products are known as long product mills. Based on the product being rolled these mills are called, merchant bar mill, bar and rod mill, bar and light section mill, rebar (reinforcement bar) mill, light merchant mill, special bar quality (SBQ) mill, and wire rod mill etc.
The production of bar and light sections in a bar and light section mill is subject to constant change. There is growing demands on the quality of the products as well as on the flexibility and cost effectiveness of the mill. This has necessitated the development of new and innovative technologies and processes. Modern bar and light section mills are high speed mills capable of rolling bars and light sections of special bar quality grades and engineering steels at high production rates, while keeping investments and operating costs at reasonable levels. Rolling in these mills is able to produce a product which is having constant cross section throughout its length.
The capacity of the bar and light section mills can vary widely depending upon the products to be rolled, size and quality of the products, size of the input material (cross section and length), capacity of the reheating furnace, rolling rates, the maximum rolling speed, and the number of shifts / days operations. Mill utilization is a measure of the percentage of time which the mill is rolling steel. The truest measure of performance is as a percentage of calendar time. Factors which influence utilization are maintenance outages, scheduled and unscheduled holiday outages, downtime for cobble clearing, roll and pass changes, excess billet gap, and other factors which create time when a billet is not in the mill.
The bar and light section mills are normally sized for a medium range production up to 500,000 tons per year. In case higher capacity is needed, then the mill is normally designed and built as a multi-strand mill. These mills are normally installed at higher levels (around + 6 metres from the ground level). This is done so that all the facilities such as oil cellars etc. can be installed at ground level for ease of operation and maintenance.
The product range of the bar and light section mill normally consists of those bar and sectional products whose cross-section is smaller than the cross section of the products rolled in medium and heavy section rolling mills. The products of the mill can be round, reinforcement bar, square, hexagon, flat, equal angle, unequal angle, T-section, and channel etc. The qualities of steel billet being rolled in these mills can range from low carbon, mild steel, medium carbon, high carbon, and micro-alloyed and low-alloyed steels. The design of the bar and light section mill needs to provide right solutions for the required performance requirements which include high speed production, microstructure quality of the product, and shortest changeover time from one product to other product etc.
Modern bar and light section mills are expected to meet several requirements such as (i) high mill availability coupled with high productivity and high yields, (ii) meeting the need of low maintenance, (iii) meeting the need of lower energy consumption, (iv) close dimensional tolerances, (v) negative tolerances in sectional weight, (vi) no variation in dimensions throughout the length, (vii) uniform physical properties, and (viii) capability to roll hot billets from continuous casting machine. Further, since the present-day environment is characterized by high competitiveness and tight profit margins, hence, the operational costs of rolling mills is to be as low as possible, while ensuring a final product of the highest quality standards. For this, the mill is required to have reliable equipment, production flexibility, and waste minimization. The mill is to ideally combine the low investment cost and simplicity with high productivity, so that a fast return of investment is ensured.
The lay-out of a bar and light section mill mainly consists of (i) billet charging grid and roller table, (ii) elevator and entry roller table, (iii) reheating furnace, (iv) descaler and mill entry roller table, (v) roughing group of stands (roughing mill), (vi) intermediate group of stands (intermediate mill), (vii) finishing group of stands (finishing mill), (viii) dividing and chopping shears complete with shear blades, blade changing equipment and scrap collection facilities, (ix) water quenching and heat treatment facilities, (vii) cooling bed and sample cutting facilities, and (viii) finishing facilities with straightening machine, cold shear, collecting table with approach roller table, transfer device, piling and bundling system, and tying and / or strapping machines. A dividing shear is provided after the finishing train for dividing the finish-rolled bar to cooling bed length. Roughing, intermediate, and finishing mill stands are to be complete with rolls, chocks, bearings, drive spindles, spindle supports, pinions, reduction gears, and couplings, etc. Smaller section of the rounds and reinforcement bars are normally rolled by slitting the rolling stock after intermediate rolling. This is done to improve the mill productivity for lower section. Fig 1 shows the schematic layout of a bar and light section mill.
Fig 1 Schematic layout of a bar and light section mill
All stands (roughing mill, intermediate mill, and finishing mill stands) are normally of rigid design and individually driven. Suitable arrangement for quick changing of roll grooves and their quick alignment in the pass line is provided. Roll gap setting is normally by motorized screw down mechanism. Roll balancing for the stands is done either hydraulically or mechanically as per stand design. For accurate adjustment of roll gap, it is preferred to have symmetrical roll separating arrangement in the stands. The connecting and disconnecting arrangement of these stands, rolls, spindle, coupling, and hoses etc. are of quick changing devices.
The changing of stands, rolls etc. is carried out by mechanical means. In order to reduce changing times, the intermediate mill stands and finishing mill stands are normally equipped with a quick-change transfer table. Stands for the next rolling cycle are positioned with the plant crane on a transfer table. Stands to be sent to maintenance are pushed off-line by hydraulic cylinders on the table alongside the new stands. A single shift of the transfer table positions the new stands in front of the mill before they are pulled in-line hydraulically. The changing times of around 20 minutes using this system is being achieved by using a quick-change transfer table.
Rolls are the tools of the rolling mill and are the costliest consumable in the mill. The way the rolls are used to execute their duty of deforming steel is largely determined by the roll pass design. Roll pass design is an important aspect for rolling in the bar and light section mill. The purpose of the roll pass design is (i) production of correct profile within tolerance limits with good surface finish (free from surface defects), (ii) maximum productivity at the lowest cost, (iii) minimum roll wear, (iv) easy working, and (v) optimum energy utilization.
The accuracy and speed of working and roll life are all related to the roll pass design and the choice of the roll material. The rolling sequence of a roll pass design is subject to the limitations applied by the rolling load, the roll strength, and the torque available for rolling. Roll pass design is also to ensure that the physical dimensions and material of the rolls are capable of withstanding the heaviest loads arising during the rolling sequence. The material of the roll is important since it is to be capable of withstanding loads which plastically deforms the rolling stock without itself being plastically deformed.
Rolling tackles which include pre-funnels, static guides, roller guides, guards, boxes and rest bar are to permit rapid assembly / dis-assembly and accurate alignment and they are to be simple in design for manufacturing. Facilities for water cooling of rolls, roll passes and guides are necessary facilities in the bar and light section mill and are to include flow and pressure monitoring systems.
The bar and light section mill also requires pinch rolls, guide tables, guide troughs, bridging troughs, roller tables at appropriate places. Loopers are provided between the stands wherever necessary for tension free rolling. Shears of suitable design are provided in the mill wherever needed for chopping, cropping, and dividing operations. The scrap bucket is provided at the shear and is normally designed to be handled by overhead crane. A scale flume which is running below the mill line carries the mill scale to the centralized scale pit located at suitable place outside the mill. Lubrication and hydraulic systems are needed in the mill to serve the mill equipment are to be provided at suitable locations. Also, all pipes, cable conduits, hoses etc. are to be guarded against damage from cobbles. Fig 2 gives flow of material in a bar and light section mill.
Fig 2 Flow of material in a bar and light section mill
The bar and light section mill also have technological structures and simple parts including safety guards, platforms, handrails, stairs, supports, steel cross-overs, trench and pit covers etc.
The important parameters for rolling in the roughing, intermediate, and finishing group of stands in the bar and light section rolling mill are temperature, percentage of reduction in area, inter-pass time (time between each stand), true strain, and strain rate. Since cross-sectional area is reduced progressively at each set of rolls, the rolling stock moves at different speeds at each stage of the rolling mill.
Modern bar and light section mills aim to maximize productivity through process optimization. For this purpose, in these mills (i) time needed for each process step and temperature profiles are optimized, (ii) transfer areas are made more efficient, (iii) complex lay-out of the mill is optimized and there is seamless interlinking of production stages, (iv) short processing time and fast production changeover from one product to another are the key criteria for meeting the customer’s requirements, (v) high precision is achieved with rolling stands designed to withstand high loads and changing temperature requirements, (vi) mill capability coupled with sophisticated control technology allow a wide range of process conditions to achieve the desired product qualities, (vii) production cost optimization is achieved through process controlled yield optimization, down-time reduction, and high degree of standardization for the maintenance cost control, (viii) mill components are designed for high durability and high uptime, and (ix) specific consumptions (fuel, water, energy, and rolls etc.) are minimized. Technologies and equipment of these rolling mills are designed for improved quality and features of rolled products, enhanced performances, and operational consistencies. Also mill design is to take care the environmental emissions and discharge of effluents.
The economic efficiency of metal rolling processes in a bar and light section mill is strongly correlated to the quality level of the end-rolled products. Rolling of bar and light section steel products is a complex process where the quality of the product is influenced by a range of factors such as incoming material, mechanical and electrical equipment, operating parameters, lubrication, and automation and control strategies etc.
Mill electric system 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.
For optimized cost-efficiency and to maximize material usage, tight tolerance control for the thickness is necessary for enabling the product to be rolled down as closely as possible to the minimum permissible thickness. Product quality can only be effectively optimized if the mechanical, electrical, and instrumentation equipment as well as the control strategy solution combine together well.
Besides the process control, all the processes for the bar and light section mill are required to be monitored and controlled by an integrated automation system. The mill incorporates automation systems which are integrated with the technological and mechatronical aspects. There are no manual controls and human intervention in the rolling process is also minimized.
The main functions to be performed by the basic automation system for the reheating furnace normally cover at least (i) temperature control for each zone, (ii) gas flow control for each zone, (iii) gas-combustion air ratio control for each zone, and (iv) furnace pressure control. The main functions to be performed by the basic automation system for the mill normally cover at least (i) material tracking, (ii) automatic sequencing, (iii) speed control, (iv) referencing and set up of drives, (v) tension / loop control, and (vi) temperature control. 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, directly related to rolling fundamentals, 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 normally 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 and blocking the furnace from sending another billet.
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. Some mills are having automation at Level 3. 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 through field bus and local area networks (LAN).
The ultimate throughput which can be achieved in a bar and light section mill is limited by the capabilities of the mechanical and electrical hardware. To achieve throughputs consistently close to the mill limit needs high-quality control and automation. At high throughputs, three or more work pieces can be in the rolling mill at different stages of processing at the same time. To avoid catastrophic collisions in the mill, accurate tracking is necessary. The tracking system uses signals from mill instrumentation and process information (for example, as a piece is rolled, so its length increases) to maintain a dynamic map of the mill. It is, of course, to be robust against the loss of individual mill instruments.
Throughput control looks ahead at the rolling schedule and determines which part of the mill installation, furnace, roughing mill, finishing mill, or coiler, can limit throughput. The limiting process is then controlled to achieve maximum throughput and other parts of the process are controlled to match this throughput. This results in an improvement in energy efficiency and a reduction in wear and tear on the equipment, thus reducing costs.
Throughput and quality also interact. As throughput increases, control becomes more difficult, and maintaining of the required level of quality and yield needs careful design of the control system. Quality and throughput control also interact in positive ways.
Some of the bar and light section mills also incorporates thermo-mechanical rolling. Thermo-mechanical rolling is also known as low temperature rolling and is basically a method for on line control of the final material properties during the rolling process. It involves material deformation applied at the last passes of the mill, within the temperature ranges corresponding to partial recrystallization or to suppression of recrystallization. Because of the thermo-mechanical rolling, superior quality product with improved metallurgical and mechanical properties can be obtained directly at the mill itself just by operating at lower rolling temperature. As soon as recrystallization is suppressed, grain refining phenomena occurs, resulting in improved technological properties of the final product. In addition, the surface quality improves considerably. The advantages of thermo-mechanical rolling are fine grain size, avoidance of off line normalizing, improved low temperature toughness, better properties after heat treatment for case hardening steels, shorter annealing time for spring steel, improved fatigue strength on the final component, higher tensile strength for micro-alloyed steels achieved directly in-line, and reduced decarburizing depth etc.
The final dimensional quality of the rolled product is determined by the rolling stands within the finishing mill. The dimensional accuracy in the final product depends on several factors including the initial stock dimensions, roll pass sequence, temperature, microstructure, roll surface quality, roll and stand stiffness and the stock / roll friction condition.Bar and light steel sections are normally rolled in several passes, whose number is determined by the ratio of the sectional weight of the initial input material (billet) and the sectional weight of the final cross section of the finished product. The cross-section area is reduced in each pass and form and the size of the rolling stock gradually approach to the desired profile. The layout of the mill is very important since the mill performance is very much dependent on its layout. There is to be minimum distance between the two equipments. However, it is necessary to meet the requirements of the technological processes.
For meeting the demanding requirements, several important features are incorporated in the modern bar and light section mills. Some of the special features of these mills are described below.
Reheating furnace – Modern bar and light section mills are equipped with energy efficient walking beam furnaces which are normally computerized controlled. These furnaces can have end discharge or side discharge of the heated billets depending upon the mill configuration. These reheating furnaces uniformly heat the billets to the target temperatures at the required production rates and without skid marks and without cold spots. These furnaces are capable of receiving cold or hot billets as the charge material in the furnace. These reheating furnaces have the features of (i) superior quality of heated billets with better temperature control across billet cross-section, (ii) better heating efficiency, (iii) very low fuel consumption, (iv) minimum scale loss, contributing to achieving high material yield, (v) low decarburization and hence suitable for higher quality steel grade, and (vi) maximum operation flexibility and good working conditions even at low productivity.
Housing-less roll stand – Housing-less mill stands are a pre-stressed stand which is more rigid than the conventional stands. These stands have rigid roll chocks held together by solid and pre-stressed joints. The stand sizes differ, depending on the necessary dimensions of the rolls and roll journals, pass schedule, pass form as well as the gearbox and motor characteristics. The main features of the housing-less stands are compactness and rigidity of components, low roll bending modulus, durable multi row roller bearing with self-aligning chocks under load, backlash free balancing of chocks, roller beams designed for simple and exact adjustment of guides and guards etc.
The housing-less stand has limited stress relaxation (spring-back) of rolls. The stand housings are smaller in size and have lighter structure. The roll chocks are free floating on two tension screws on each side of the mill stand, one is right hand threaded and another one is left hand threaded. This mechanism ensures mathematical opening and closing of the roll gap related to the pass line. The roll changing is easy and much quicker with a roll changing device which pulls out the complete roll changing assembly and replaces the cartridge. The housing-less stands are normally arranged in horizontal-vertical no twist arrangement which allows no twist rolling which permits larger reduction and smoother rolling as it eliminates twisting oval into round passes as in the conventional oval-to-round sequence which results in less guide wear and simpler guide design.
The housing-less roll stands are used normally in roughing and intermediate group of stands in modern bar and light section mills. The modular design permits the use of housing-less stand cassettes in all possible configurations such as horizontal, vertical, tiltable and universal configuration.
The main features of the housing-less stand design are (i) conservative component sizing criteria and general philosophy in order to achieve rigidity and compactness of each unit, (ii) low roll deflection modulus (favourable ratio of roll neck to roll working diameter), (iii) long life multi-roller bearings with chocks self-alignment under load, (iv) balancing of backlash between chocks, (v) rest bars designed to allow easy and fine adjustment of guiding devices.
Housing-less mill stands have major benefits during operation which include (i) finished product matching needed tolerance on geometry and size, hence stricter weight control, (ii) time saving during stand changing with off-line roll replacement, (iii) flexibility of operation, since same stand unit used in horizontal, vertical, convertible and universal configurations, hence minimization of spare parts inventory, (iv) highly reduced maintenance time and costs, because of the reduced number of components and easy access, (v) automated gap adjustment, (vi) integration in fully automatic control, and (vii) saving in the depth and size of the foundation.
Reducing sizing mill (RSM) – It is a versatile sought after rolling technology. It is also known as precision sizing mill. From existing conventional rolling mills, it is difficult to meet the requirements of close tolerances. This can only be met with difficulty and at the expense of loss of efficiency, especially with regard to the loss of mill utilization time and lower yields. At times this is not feasible or cost prohibitive. In conventional roughing and intermediate mills, the tolerance of the finished product is influenced mainly by the variations in the cross section of the feed material into the finishing section of the mill. RSM takes advantage of the special features of the 3-roll technology, in which the spread during deformation is low and the deformation efficiency is high. There are several advantages of the reducing sizing mill. RSM is installed in the mill line for the purpose of rolling any desired finish size to very close tolerances. It is possible to adjust each roll gap under load and it can be fully automated.
Thermo-mechanical rolling – It is also known as low temperature rolling and is basically a method for on line control of the final material properties during the rolling process. It involves material deformation applied at the last passes of the mill, within the temperature ranges corresponding to partial recrystallization or to suppression of recrystallization. Because of it, superior quality product with improved metallurgical and mechanical properties can be achieved directly at the mill itself just by operating at lower rolling temperature. As soon as recrystallization is suppressed, grain refining phenomena occurs, resulting in improved technological properties of the final product. In addition, the surface quality improves considerably. The advantages of thermo-mechanical rolling are fine grain size, avoidance of off line normalizing, improved low temperature toughness, better properties after heat treatment for case hardening steels, shorter annealing time for spring steel, improved fatigue strength on the final component, higher tensile strength for micro-alloyed steels achieved directly in-line, and reduced decarburizing depth etc.
Walking rake type cooling bed – Cooling bed is used for the uniform air cooling of the rolled material (bars or sections). It transfers the bars one by one to the roller table, on which they are transported to the finishing section of the mill. The rake type cooling bed has a saw tooth pattern which is why it is also known as a rake type cooling bed. 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 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 front ends of the bars and light sections are also leveled at the discharge side and a fixed number of cooled rolled bars are sent for final length cutting by cold shear for subsequent bundling or piling. Cooling beds can also use a chain transfer as the traversing method.
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 normally 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 cooling bed 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. A typical schematicdiagram of a cooling bed cooling bed is shown in Fig 3.
Fig 3 Typical schematic diagram of a cooling bed
A number of solutions are available for the finishing of bars and sections in the finishing area of the mill. A typical bar and light section mill include have a straightening machine feeding to a cold shear with gauge beam. Correct layer preparation is the key to productivity and this is achieved by profile feeding system. Flying type cold shears are also used when the rate of productions are high from the mill. 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 are essential.
The recent improvements in the area of exit side of the cooling bed, straightening machine, and cold shear 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 which can be shifted out of the line for maintenance without stopping mill production. Further in order to pre-align the bar layer on the cooling bed run out roller table, a chain transfer and a carriage type extraction system is normally provided so that the bars are inched out of the bed rakes at the needed centerline distance between the bars and kept this way by gentle depositing on the run-out roller table with the carriages.
The bundling and stacking section of the mill can also have several solutions. A typical solution consists of a simple bundling machine while for sections magnetic stackers are the norm. All the operations are to be mechanized and automated including the removal of the short bars or the labeling in ideal scanning position of the tags. Special care is normally given to the final shape of the bundles, with optimum arrangement of the bars and sections. Stackers can have different designs depending of the requirement. For precise stacking of the light sections the overhead pendulum system is used while for light medium sections, the stacking system with magnets underneath is used.
The bar counting system consists of automatic bar counting which operates on an optical principle and separation system for forming bundles. The separation system is composed of three fixed chain transfer devices between which the counting system is installed. The optical device together with a pulse generator installed on the chain transfer drive performs the counting and recording of Each single bar in transit without overlapping or double reading.
Bar and light section mills have tying and strapping machines for bundles and piles. These machines are designed for continuous operation, Tying machines use commercial size wires cfor tying and the machine head is hydraulically operated. The strapping machines are pneumaticaly operated and use commercial steel straps of different available width. Strapping can be carried out either by clamping or welding.