Main Features of a Modern Wire Rod Mill
Main Features of a Modern Wire Rod Mill
The objective of a wire rod mill (WRM) is to reheat and roll steel billets into wire rods. Wire rods are normally rolled in a high-speed rolling mill, where steel temperature is above 1,000 deg C, maximum speed of rolling can go up to 140 metres per second (m/s) and coils of wire rod produced can be up to 2.5 tons of weight and up to 10 kilometers (km) long. During rolling of wire rods, normally 25 passes to 30 passes are taken in a continuous rolling mill.
Wire rods are used as the material of gears, bolts, springs, bearings, cables, electrodes and other basic components of safety-related parts typically such as automobile engines, drive train systems, and chassis. Wire rods are seldom used as hot rolled for final products, but they are manufactured into machine parts after undergoing one or more stages of post-processing such as heat treatment, forging, and wire drawing at specialist plants. For this reason, every product of wire rods is developed with due attention to its behaviour at the post-processing stages. The necessary requirement from wire rod products is good processibility and fulfillment of required properties after the processing. Also, since the costs of the post-processing is sometimes several times the price of the hot-rolled steel material, it is increasingly important to reduce the total integrated manufacturing cost from the steel material to final product.
Wire rod products, in the present global scenario, are characterized by (i) wire rod product grades which are widely varied from standardized ordinary grades for general applications to high grades for high-end applications according to specifications individually defined for each customer, mainly for the automobile and other manufacturing industries, (ii) the high-end wire rod products which are for safety-related applications typically such as automobile engines, drive train systems, and chassis, are used as functional materials and as such, are required to have highly demanding properties such as high strength and long fatigue life, (iii) wire rod products are semi-finished products, which are seldom used as hot rolled, and they are manufactured into final products after secondary and even ternary processing such as forging, heat treatment, and machining, (iv) The final industrial products for which wire rods are used are subject to tough competition in the market, and (v) the industrial products for which wire rods are used are required to minimize the load on global environment throughout the life cycle.
These days, the user of wire rods demands closer dimensional tolerances and improved metallurgical properties in wire rod products besides the three basic requirements which are (i) to eliminate or simplify one or more of the secondary or tertiary processing to improve productivity and reduce processing cost, (ii) to improve the functionality, i.e., to extend the service life, to reduce the weight, or to bring other advantages to the final product for which the wire rod is used, and (iii) to eliminate the use of environmentally harmful substances during post processing processes.
Wire rods are rolled in wire rod mill. Fig 1 shows rolling of wire rod in a wire rod mill. The production of wire rods in wire rod mills is subject to constant change. Since the beginning of the 2000s, the technologies and methods used to produce wire rod have undergone significant changes. The growing demands on the quality of the wire rods as well as on the flexibility and cost effectiveness of the wire rod mills have necessitated the development of new and innovative technologies and processes. innovative solutions have been introduced to produce increased quality of the wire rod products with considerable energy savings and improved performance.
Fig 1 Rolling of wire rod in a wire rod mill
Modern wire rod mills are high speed mills capable of rolling of smaller dimensions at high production rates, while at the same time keeping investments and operating costs at the reasonable levels. As a rule, wire rod mills are designed for an annual output of between 300,000 tons and over 800,000 tons (two-strand mills). These days four-strand wire rod mills are no longer being built because of various limitations these mills bring into production of the wire rods. The mills are capable of rolling at speeds ranging from 50 meters per second to 140 metres per seconds. Typical product sizes are within the 5.0 mm to 20 mm range. The range of materials comprises low to high carbon steels, cold heading steels, wire drawing steels, alloy steels, spring steels, ball bearing steels, electrode quality steels, reinforcement rods and tool steels. Fig 2 shows typical layout of a two-strand 400,000 tons per annum wire rod mill.
Fig 2 Typical layout of a 400,000 tons per annum wire rod mill
For meeting the increasingly rigorous requirement of the consumer, the wire rod mill needs to have the overall capabilities of the rolling wire rods from the various standpoints namely (i) improving the dimensional accuracy, (ii) implementing size-free rolling (i.e., rolling products of varying sizes without changing rolls), (iii) improving the productivity by speeding up the roll-changing work (shortening the roll-changing time) and (iv) improving the product surface quality by preventing the occurrence of surface defects during transportation in the rolling process.
Wire rod mill is supposed to supply not merely the materials, but is to supply wire rods to meet the requirements for the long subsequent processing which they undergo in the secondary and tertiary processes until the final end products are produced. Furthermore, since the end products are used in several cases as vital parts in various industrial fields, there are requirements for process rationalization in integrated collaborations with customers. Hence, the mill is to be capable of not only to meet the quality requirements of the users but also to meet their productivity, yield, flexibility, and production cost requirements during the subsequent processing. A high mill yield along with the adequate product quality is of utmost importance for the control of the production cost.
The production capacity of a wire rod mill is dependent on either volume production of a few standard products or having a diversified product mix consisting of wire rods with a large number of steel grades and specialized product qualities.
Modern wire rod mills are expected to meet the several requirements consisting of (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 of wire rods in the range of +/- 0.10 mm and ovality in the range of around 0.2 mm, (v) low scale loss in wire rods of around 0.5 % to 0.6 %, (vi) negative tolerances (in sectional weight), (vii) no variation in dimensions throughout the length of the rod, (viii) uniform physical properties., and (ix) uniform weight with nominal variation between middle and back ends.
Quality assurance throughout the whole length of the wire rod after rolling takes time and labour, and for this reason, quality control of billets, the input material for the wire rod rolling is of high importance. Modern wire rod mills have introduced labour saving systems such as a product identification system consisting of a billet number-stamping machine and automatic number reader and automatic magnaflux inspection equipment to mechanize and automate the conditioning work of billets. In addition, in response to increasingly severe quality requirements, the wire rod mills have significantly improved the capacity and accuracy of product inspection systems and established the plan-do-check-action (PDCA) cycle in quality control through measures such as the feedback of billet defect information after breakdown rolling to the continuous casting process.
Modern wire rod mills are normally designed to work with ‘overall equipment effectiveness’ (OEE) philosophy. OEE value of the mill is the value obtained by multiplying mill utilization, mill efficiency, and the mill yield time. The mill utilization is the available operating time divided by the planned production time and expressed in percentage. The mill efficiency is running time divided by the available operating time of the mill and expressed in percentage. The mill yield time is the time to produce the prime product related to the running time. Product yield is expressed as the weight of accepted coil divided by the weight of the billets and expressed in percentage.
In a modern wire rod mill, the main focus is on productivity and quality. High utilization, efficiency, and yield give high productivity. But high productivity itself is not the only thing for the success of a wire rod mill. It has to produce products which give value to the customers and they get the quality wire rods worthy of the payments which they are making for the wire rods.
The productivity and utilization of the mill is dependent on the downtime in the mill caused by several reasons including the mill layout and the product mix. In the mill, the yield from billets to the final product is registered in all steps and the tracking of the material in the mill follows up the losses taking place in the mill. The major categories of the losses which take place in the mill are (i) gap time losses, (ii) speed losses, (iii) losses due to cobble, (iv) losses due to quality deviation, (v) losses due to mill down time, (vi) losses due to diversion / rejection of wire rods during inspection in the finishing side of the mill, and time losses due to value enhancing operations in case of production of specialized wire rod products.
Several wire rod mills have tackled the various issues by the introduction of in-line heat treatment facilities, development of small-diameter wire rods, and controlled rolling with the objective of omitting certain processing steps, improving productivity, and energy saving. In recent years, amid the ever-growing intense competition and with the intention of establishing stronger competitiveness in wire rod product quality by improving dimensional accuracy and product metallurgical quality, efforts to improve equipment and operations are being promoted.
In a wire rod mill, steel is heated above its temperature of recrystallization and passed through several grooves of the rolls in the rolling mill. Common roll pass designs used in wire rod mills are diamond-square, diamond-diamond, box passes, square-oval, false round-oval, and round-oval. In the wire rod mills normally size free rolling is being done, which means wire rods of any size can be rolled without the limitations of the pass sizes of the rolls held in the mill. This is also being known as flexibility of the series. Furthermore, the simplification of the pass schedule of an upstream rolling mill enables the frequency of the roll changing to be substantially lowered. The wider the range of the product size capable of being rolled with the same rolls (size free range), the greater is the effect of reduced frequency of roll changing. The size free rolling is not limited to the finishing mill. It can also be applied to pre-finishing mill.
In the wire rod mills, it is normal to use rolls with grooves. Common groove sequences in the intermediate mills are ‘square oval’ and ‘false round (round) oval’. It is also widespread to use ‘diamond-square’, diamond-diamond, and ‘box groove’ sequences upstream in the roughing mill. There are other types of the pass designs also. The patented oval-round-round-round roll pass sequence of Morgan is designed for high-reduction rolling and for normalized and thermo-mechanical processing while enhancing surface quality and extending roll life. The roll pass design is to ensure a high yield and adequate quality of the wire rods which is of utmost importance for keeping the cost of production under control.
A wire rod mill basically needs (i) heating facilities for heating the initial material (billet) to rolling temperatures, (ii) rolling facilities consisting of rolling stands with rolls, chocks, guides and guards, (iii) laying, heat treatment, and coiling equipment, and (iv) conveying and handling equipment. All the equipments are to work in close co-ordination with a control on rolling temperatures, gap time loss, speed loss, cobbles, non-conforming product, and quality deviations leading to diversion or even rejection. For achieving the demanding requirements, several important features are incorporated in the modern wire rod mills. Some of these are described below.
Reheating furnace – Modern wire rod mills are equipped with energy efficient walking beam furnaces or walking hearth furnace which are normally computerized controlled. 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, warm, and / or hot billets as the charge material in the furnace.
For example, the design and construction of a billet reheating furnace in the two-strand wire rod mill had been conceived to charge cold or warm billets. It was then expanded to include hot charging, which means billets can be handled and charged into the reheating furnace at a starting temperature of 800 deg C. The furnace was designed with a high level of automation, resulting in a considerable drop in gas consumption and, consequently, scale content. Various software programmes used in both design and management made it possible (i) to reach excellent recorded values of 30 N cum/t of gas consumption in cold charge mode, and 0.6 % scale generation, (ii) to simulate the furnace profile, and to optimize the various reheating curves of the charging temperature, the required discharging temperature, and the types of materials charged in terms of section and quality, and (iii) by means of FEA (finite element analysis) and FVA (finite volume analysis) analyses, the critical construction points of the furnace and all its components. Further, the particular design and construction of the furnace entails low temperature dispersion, which keeps its operating efficiency high.
Descaler – In order to feed the mill with a billet characterized by a proper surface quality, apart from the billet conditioning and reheating practices, descaling is needed. This is a very important requirement. A perfectly clean surface is to be ensured in order to avoid irreversible surface defects in the following rolling stages (rolled-in scale). For this reason, primary scale removal is performed at the furnace exit by a high-water pressure descaler in very short times (elevated billets speeds), to avoid detrimental surface overcooling.
Rolling mill stands and shears – The rolling mill stands represent the core of the rolling process and their configuration has to be proper to suit the dimensioning technological parameters such as steel grades product mix, rolled sizes, minimum and maximum productivity, minimum and maximum rolling time, required biting speed, required shears configuration, available upstream and downstream facilities, and the media availability etc.
It is easy to understand to which extent an inappropriate mill configuration can affect the entire rolling process. In the present-day environment, for the wire rod mills in general and in a broader sense for the wire rod mills having capabilities to roll special steel products, there is the need of more and more process flexibility in terms of steel grade to be processed, rolling strategy to apply, and size changing quickness. This is becoming more and more stringent with the passing time. In modern wire rod mills, in fact, it is not unusual to have more than 250 size changes in a multi-strand rolling mill. For this reason, the possibility to reduce the size changing time and to simplify roll pass design is a priority.
The shear after the descaler is required to have the cutting force, especially when low temperature rolling is adopted in the wire rod mill. The rolling stands are to ensure the appropriate stiffness, with high axial and radial rigidity, to support the high rolling loads. They are to ensure quick change and reduced risk of damaging the hoses in case of a cobble.
Housing less roll stand – The housing less (HL) roll stands are used normally in roughing and intermediate group of stands in modern wire rod mills. The modular design permits the use of HL stand cassettes in all possible configurations such as horizontal, vertical, tiltable and universal configuration. 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 HL 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 HL stands have been now modified considerably with both structural and functional changes, resulting increased safety and operating rates. The HL stands are now designed with a higher safety factor and protected on all sides to contain any material outflow (cobbles, water splashes, etc.). This solution secured operator safety in the rolling area, minimizing the accident risks to zero.
The cartridge design has been optimized and most of the external tubes for media connection have been eliminated (water / air / oil). This means that the assembly time for hoses is more than 50 % shorter. Substantial changes have also been made to the spindle design. The new HL stands are very sturdy, providing the mill operators with 20 % to 30 % additional operating advantages than the earlier HL stands.
The advantages of the HL stands include (i) saving in the depth and size of the foundation (ii) the rolled product meets the required form and dimensional tolerances, (iii) there is time savings for stand changes as the roll changing takes place outside the rolling line, (iv) there is considerable reduction in time for maintenance due to lesser number of components and easier accessibility, (v) automated roll gap adjustment, and (vi) operational flexibility since the same stand unit can be used in any position.
Cantilever roll stands – Cantilever (CL) roll stands are compact stands which are used in a wide range of sizes for a variety of applications. These applications include (i) single strand mills in horizontal and vertical arrangement, (ii) in split intermediate trains of two or more strands mills, and as pre-finisher stands in wire rod delivery sections. The advantages of these stands include (i) smaller foundations, (ii) cassettes of the same stand type are interchangeable even between horizontal and vertical stands, (iii) high load bearing strength even with small diameters hence ideally suited for high-speed wire rod blocks, (iv) optimum accessibility, and (v) fast roll and stand changing.
Cooling and equalizing loop – The material entering the no twist block is required to be intensively cooled for final rolling at low temperatures. This is then to be followed by a sufficiently long equalizing section to allow the metallurgical properties to be achieved uniformly over the cross section of the finished wire rods. Without equalization section, the temperature difference between surface and core can be so large that different micro-structures can be created during the subsequent forming process. On the other hand, there are steel grades which are required to be rolled as hot as possible and for which a long equalizing section leads to deterioration in quality. The loop technology allows these two demands to be perfectly reconciled. The material from the intermediate train can take the short direct route or the route through the loop with additional water boxes and long equalizing sections before being rolled in no-twist blocks.
Pre-finishing block – Pre-finishing block is configured to produce the needed process feed sections to the no-twist block to support the maximum product finishing speed. All process sections are rolled in the pre-finishing block utilizing an oval-round pass design sequence to minimize the investment in rolls, reduce operating cost and enable high productivity, and high quality of the process sections. The pre-finishing block stands are used to produce the sections needed for the no-twist block. The pre-finishing block utilizes 230 mm cantilever roll housings to provide the needed separating force capacity and roll strength needed for the process. Carbide rolls are used for increased pass life, thereby reducing downtime and increasing mill efficiency. The carbide rolls also provide superior surface quality over the life of the groove, further improving the quality of the finished product.
No-twist block – It is also known as no-twist mill. In wire rod mills, it represents one of the key elements. Only through this development, it has become possible to safely roll thin wire rods at speed of over 120 m/sec. The no twist blocks can be of 4, 6, 8, and 10 roll stands for twist free rolling. A primary gearbox drives the roll units through two common shafts. No-twist blocks having two different sizes of roll units are available, with 170/150 mm diameter rolls and 225/200 mm diameter rolls. All roll units are identical and inter-changeable. Fig 3 shows drawing of a six-stand no-twist block.
Fig 3 Drawing of six-stand no-twist block
No-twist blocks are available with reduction ratios varying from 10 % to 25 % per pass, depending on steel grades to be rolled. The block is now even designed for a speed of 150 m/s. No-twist blocks use tungsten carbide rings having a pass life of 600 t to 700 t with super finished surface of the end product. The advantages of the no-twist blocks are (i) ultra heavy-duty housings, (ii) low ring changing time, (iii) negligible spring action, (iv) reduced maintenance, (v) simpler section control, remote adjustments under load, and (vi) flexibility of rolling of different wire rod grades.
Flexible reduction sizing (FRS) block – This block has been developed for rolling higher grades and simultaneously improving the metallurgical properties of the rolled product. This is a four-strand block with speed shift gear boxes. It is installed down line of a no-twist wire rod block. On the FRS block all dimensions can be finish rolled with the advantage of one family rolling, which means that only one pass size is used in each stand over the whole size range. Due to the cooling section in between the no-twist block and FRS block, thermo-mechanical rolling becomes feasible. There are several good design features in this block.
Reducing sizing mill (RSM) – It is a versatile sought-after rolling technology. 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 using RSM in a wire rod mill. RSM can be integrated after a conventional finishing block, boosting the mill productivity on small sizes by up to 60 %. The extreme precision of RSM has been proven in several practical applications. The pass design is patented and enables true single family rolling from the first stand after the reheating furnace to the last stand of the block ahead of the reducing sizing mill. Added after a conventional rod finishing block, the patented RSM unit can considerably increase finishing speeds on smaller sizes.
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 the suppression of recrystallization. As soon as recrystallization is suppressed, grain refining phenomena occurs, resulting in improved technological properties of the final wire rod product. In addition, the rod surface quality improves considerably. Fig 4 shows schematic temperature profile for thermo-mechanical rolling with loop in wire rod mill.
Fig 4 Schematic temperature profile for thermo-mechanical rolling with loop in wire rod mill
Thermo-mechanical rolling in the wire rod mill refines the final grain size as a result of dynamic recrystallization. Combined with final in-line water cooling and the superior controlled cooling on control cooling conveyor system, thermo-mechanical rolling plays an important role in determining final product properties. This is particularly beneficial for low- alloyed and medium-alloyed steel products which are subsequently spheroidize-annealed during downstream processing. The ability to strongly control grain size also influences subsequent transformation to hard products such as bainite and martensite by shifting the transformation start time and temperature. Hence, thermo-mechanical rolling can minimize direct downstream cold working and reduce annealing times.
The combination of processing on control cooling conveyor system and low rolling temperatures provides the capability to reduce hardenability in some critical grades of wire rods. Ultimately this promotes ferrite formation and retards the evolution to bainite and martensite. The refined grain size achieved through thermo-mechanical rolling improves diffusion during heat-treating and can result in reduced heat treatment times and temperatures. For those rods which are not heat-treated, the refined and complex structures increase tensile pickup during cold deformation, producing several advantages such as (i) reduced as-rolled tensile strength, (ii) improved downstream response, and (iii) increased work hardenability. The improvements stem from grain refinement and microstructural control. The good control of the cooling process at the control cooling conveyor system combined with the reduced hardenability of the wire rods makes the process very stable and reduces the chance of forming unwanted hard phases.
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 use of two no-twist blocks (normally a six-stands block and a four-stands block) allows all the dimensions of the wire rods to be rolled thermo-mechanically and inexpensively with high production rates. By splitting the no-twist block, it becomes possible to finish roll in four passes maximum.
With sufficient cooling and good temperature equalization over the cross section, thermo-mechanical rolling at high production rate is hence possible. Cooling and equalizing loop before the no-twist block plays an important role during the thermos-mechanical rolling. It allows the in-going temperature into the first no- twist block to be reduced to 750 deg C and that with a temperature profile of less than 50 deg C. This precondition of reaching 750 deg C again before the second no-twist block for the sections to be rolled with ten passes become feasible and hence making thermos-mechanical rolling possible even with small dimensions.
Drive for no-twist block – Normally all the stands of a no-twist block have complex gear box configurations which are subject to wear and maintenance. They are driven jointly by one or more huge motors (up to 7,000 kW) in tandem arrangement through a primary gearbox and two common shafts. No-twist block has such limitation since it is able to roll with fixed reduction ratios which need fix roll ring diameters.
An electronic gear box has been now been developed which controls the motors of a no-twist block relative to one another so that the stands function like a no-twist block and can be more precisely controlled. Due to this drive, the fixed reduction ratios between the stands are eliminated. This helps in roll sizing as a wide range of different area reductions can be rolled in the same stand. The roll ring management also gets simplified and the number of passes needed can also be reduced.
High-speed shear – The shear is designed for head and tail trimming of wire rod, at maximum rolling speed of the wire rod mill for both plain and deformed water-quenched / self-tempered wire rod. The high-speed shear features a series of advanced design characteristics. Its particular features and associated benefits are (i) unit compactness, (ii) single-pair blade-holder / single-drive design, enabling cropping and chopping operations to be carried out by the same pair of blade holders, (iii) advanced blade locking / centering system with faster blade changing, and (iv) short-stroke electrically-actuated diverter. The new diverter design has been one of the key elements in developing the second generation of the high-speed shear since it enables (i) reduction of deviation angle amplitude, hence reducing friction and minimizing wear on diverter and conveyors, (ii) shorter deviation cycle, improving operation synchronism and efficiency well beyond the design speed, (iii) considerable reduction in blade width, (iv) narrower blade-holder resulting in better operating efficiency, less friction on guiding elements with lower wear rate, and reduced noise levels at the highest speeds.
Pinch roll arrangement – The new generation pinch roll arrangement consists of a system for automatic servo motor pinch force control. With the intelligent pinch roll there is no need to manually adjust the roll gap in order to set the pinch force. Instead, the roll gap is automatically set to induce the correct quantity of pulling force for tension regulation without slipping on the wire rod or marking the wire rod when closed. The system also allows for faster response for tail end slow-down on small diameter, high speed products, thereby improving the tail end ring control.
Loop laying head – The laying of different wire rod sizes in uniform loops with the loop laying head even at high rolling speed is an important criterion. The loop laying head is to have the very good characteristics in terms of reliability, noise level, and coil formation. Further due to thermo-mechanical rolling for several grades, the laying temperatures for certain grades are greatly reduced because of the metallurgical reasons. This has put higher demands on the laying head particularly on the laying tubes and they are to be made of special material.
The loop laying head is composed by several parts as shown in Fig 5. The input shaft (1) transmits rotation from motor. A guide pipe (3) is located at entrance of laying head device to guide the wire rod into the laying pipe (7). A pair of bevel gears (2 and 4) is assembled on the input shaft and output shaft (5). The output shaft (5) is connected with laying pipe holder (6). There are two bearings supporting the output shaft (5). There is cantilever part of the laying pipe holder (6). Laying pipe (Fig 5b) is the most important part for this laying head device. The diagram of a laying pipe can be seen in Fig 5b.
Fig 5 Loop laying head
The new developments in loop laying technology ensure a superior coil pattern and an optimized laying pipe wear rate with absolute operating stability and vibration-free operation at production speeds. The traditional laying head concept has radically changed with the application of several innovations such as (i) the new generation laying head features a unique oil-film bearing design for the rotor support (instead of traditional roller bearings) which provides absolute stability during operation, lower wear, and extended life of the laying pipe, as well as minimized maintenance, and (ii) a sturdy advanced design which ensures an optimal response even in highly unbalanced conditions, and vibration-free operation at full speed. Hence, the number of balancing operations is drastically reduced. The rotors of the present-day laying heads are optimized and designed for very high speeds with very few vibrations.
Further, because of the use of calculations and three-dimensional checking of the laying heads in the design phase, the loop laying heads can reach unparalleled performances of getting first-rate finished product quality, with very low consumption in the loop laying pipes and less need for spares.
Recently, a pipe less loop layer has been created which offers several advantages for operators. The pipe less rotor eliminates the worn pipes which means reduced maintenance and downtimes. It ensures repetitive loop formation with steady, regular laying on the controlled cooling conveyor and reduced noise level compared to the traditional loop laying head. Further benefits are reduced unbalance vibrations due to axial-symmetrical wearing and predictable and controllable wearing results, with no cobbles due to unexpected pipe wear.
Control cooling conveyor – The controlled cooling conveyor system is one of the important parts of a wire rod mill for achieving the desired properties of the wire rods for a wide range of different steel grades. The optimum combination of speed, fan power, and cover position on the conveyor enables processing in a wide range of conditions, including both fast and slow cooling modes within a single system. This capability enables wire rod mills to produce a broad spectrum of plain carbon and alloy steels, as well as stainless steels and other specialty grades.
During ‘forced cooling’, air is blown through the loosened windings with maximum manpower and open covers to cool the wire rods as quickly as possible in order to achieve the laminar pearlite micro-structure. During ‘delayed cooling’, the wire rod loops are transported without fans, with the cover closed and at low conveyor speed in order to keep the temperature in a given range for as long as possible. This enables achievement of a ferritic / pearlitic micro-structure of the wire rods. The results are improved as rolled rod properties. This enables the production of more grades in a directly usable condition, hence reducing or eliminating downstream processes, such as spheroidize annealing.
The control cooling conveyor consists of sections from the entry section up to the coil reforming chamber. These sections have rollers, roller nozzle decks, covers, plenum chambers with air distribution system using the air blowers. An entry tilt section is there to provide the correct interface between the laying head bottom tripper assembly and the conveyor rollers, so that the optimum ring pattern is there onto the conveyor. The entry section also uses close-centered rollers to prevent small diameter head or tail ends from getting caught between the rollers. Front end positioning is provided with automation system to ensure the correct presentation of the head end rings on to the conveyor to further prevent this from occurring. Ductile iron side wall deflectors at the exit of the laying head also prevents turbulence in front of the laying head eliminating the deformation of the small diameter product rings as well as scratch on large sizes. Fig 6 shows control cooling conveyor system.
Fig 6 Control cooling conveyor system
Positioned just below the rollers, are nozzle decks with angled nozzles extending up between the rollers, generating a high velocity air flow for maximum contact time with the rod for fast cooled products. Below the nozzle deck is the air distribution system which gradually provides more air at the edges than that at the centre. This gradual change in air flow provides added improvements with respect to tensile uniformity around the ring.
To improve the tensile uniformity throughout the coil for fast cooled products, the speed between each section is adjustable using VVVF (variable voltage variable frequency) drives. Between each half section a fixed speed increase is provided through the sprocket drives on the rollers. By gradually increasing the speed along the conveyor, the contact points on the overlapping rings are continuously changed and the spacing between the rings increased to help with higher cooling rates. As mentioned above, with the increase in conveyor speed the product is to be slowed prior to entering the coil reforming chamber otherwise the rings are going to bounce off the upstream wall of the coil reforming chamber. Hence, drop zones are provided prior to the exit section to allow the product speed to be reduced.
Prior to the drop section, centering rollers are provided to push the rings to the centre of the conveyor. Bullet rollers are provided at the exit of the drop section to ensure the ring fall squarely onto the next section. Close-centered rollers on the succeeding roller module prevent the rings from getting caught between the rollers when they drop. The exit section is a traversing section, and includes another set of side wall deflectors to preserve coil centering for presentation to the coil reforming chamber. The exit section traverse position is set based on the speed of the product entering the reforming chamber. The position of the exit section ensures that the rings drop squarely over the nose cone and do not dive into the coil reforming chamber, which can result in sloped or slanted coils. The presentation of the rings into the reforming chamber is the starting point for a good coil package.
Coil reforming chamber – For ensuring the optimum coil package, the coil is to be formed in an orderly manner and also completely formed inside the upper portion of the coil reforming chamber. The ordering of the rings coming off the exit section is achieved with a ring distributor, which is a proven design of rotating blade within the upper portion of the chamber which directs how the rings fall onto the forming coil. The reform ring distributor collects rings high in the reforming chamber using a rotating blade for optimal placement. This system shortens the coil package, easing shipping, and storage space concerns, which in turn reduces costs. Improved collection also results in better-shaped coils for fewer tangles and snags at pay-off.
The depth of the reforming chamber is to be kept as short as possible to prevent large drops from the iris fingers to the coil plate and to maintain a fixed distance between the top of the coil and the underside of the ring distributor, so that the benefit of the ring distributor is preserved. The length of the drop from the iris fingers to the coil plate is to be minimized. A mechatronic package controls the reform cycle, using a series of laser sensors to measure the coil height in the coil reforming chamber to adjust the coil plate rate of descent maintaining a constant collection height.
By producing a stable ring pattern from the laying head onto the conveyor and controlling the rings entering the reforming chamber with the ring distributor and automatic coil plate descent, the coil formed is densely packed with a thick wall and constant outer diameter (OD) and inner diameter (ID). With a densely formed coil on the coil conveyor (C hook or vertical pallet) the coil shape and integrity is maintained through further cooling of the coil. As a result, the coil shape is maintained during compacting and the compacted coil does not relax during shipping and hence the ties do not become loose.
Coil compactor – Coil compactor is the key machine in the coil finishing system which is used for pressing and binding of the wire rod coils. The new generation horizontal compactor has superior cycle time, higher availability, advanced HMI (human machine interface) for control and very low maintenance. It has energy consumption which is 50 % less compared to previous generations, while continuing to perform with higher pressing force and shorter cycle time. This is achieved by a new advanced power-controlled hydraulic unit system, which also reduces installed power.
Automation and process control – Wire rod mill automation provides a consistent solution for the control high speed wire rod mill from the process control to basic automation systems integrated with a set of special sensors specially designed for quality control and production monitoring. In the design, all automation units and instrumentation are integrated to each other through a local area network to meet mill production and process requirements. The architecture of the automation system is normally based on client-server structure, with a single process control system database assuring consistency of the data used to calculate the equipment set-up and to manage the production.
Material tracking, automatic labelling, consumption accounting, quality monitoring, and equipment life tracking are some of the other important additional functions provided by the process control systems. The large quantity of data acquired from instrumentation and basic automation can be analyzed with a dedicated business intelligence tool. Operator work-stations, based on personal computer hardware, support the operator’s decisions displaying the necessary information about process and equipment status. The equipment control automation system is based on distributed architecture of PLC (programmable logic control) units, where the single unit is dedicated to control single machines or groups of them, simplifying handling and maintenance, making trouble-shooting easier.
There are special sensors and instrumentation which are used for the automation of the wire rod mill. There is a magnetic presence sensor which uses the disturbances caused by the metal material being rolled on the magnetic field induced to determine the wire rod presence. Installed inside the water boxes used for the material thermal treatment, this sensor allows precise material detection and tracking of both hot and quenched wire rod even in presence of water and steam.
There is a modern system for the on-line measurement of cross-section of hot rolled wire rods. A magnetic contact-less sensor forms the core of the system. It is sturdy and compact and is designed to be maintenance and wear free. The working principle of the sensor is based on detection of the eddy currents, generated on the surface of the rolled stock by a variable electromagnetic field. The system is used for applications such as (i) weight /metre and measuring of the section, (ii) control of section, and (iii) optimization to the minimum tolerance. It is typically installed downstream each section of the wire rod mill (intermediate, pre-finishing). The system allows achieving some immediate benefits such as (i) reduction of cobbles, (ii) reduction of wear of roller guides, (iii) reduction of breakdowns of roller guides, (iv) precise gap adjustment for wear compensation and easier set-up of a new production, and (iv) instant warning in case of out of tolerance events.
There is a sensor for the profile of the wire rod. This sensor is installed at finishing block exit. The sensor is equipped with one or two rotating measurement heads. It provides non-contact on-line profile shape inspection and dimension measurement for the hot wire rods. It is integrated with automation system and it is possible to achieve fast 100 % inspection of shape and dimensions, reading and monitoring of the complete profile of the rod. There is additional instrumentation which consists of in-line surface inspection system for the wire rod. It is a no-contact inspection system dedicated to on-line detection of surface defects in rolled wire rods.
There is roller guide calibration and alignment system. It is a calibration system designed to aid the operator during (i) setup of the roller guide in the workshop, (ii) alignment of the roller guide mounting bases with the rolling ring pair groove in the finishing blocks.
There is impact drop compensation for twin module block and multi-drive blocks. A high-speed auto-adaptive speed drop compensation of the drives at biting of the material in each stand (or group of stands) is provided. It allows reducing relative speed variations of between the motors of a multi-drive block down to 0.5 % or even less of the nominal speed. The right tension in the material is maintained along the front-end transient. Hence, the section of the material is kept constant along the whole stock length.
High-speed controls are also provided for high-speed shear rotor-diverter synchronization which allows head and tail cut at the maximum rolling speed of the wire rod mill. Also, the synchronization between the laying head and high-speed shear rotor during at head cut allows a 100 % reliable and precise positioning of the first loop of the wire rod on the control cooling conveyor. This avoids the risk of cobble on the cooling conveyor.