Development of Modern Hot Strip mill and its Main Features
Development of Modern Hot Strip mill and its Main Features
Hot strip mill (HSM) is also known as ‘wide strip mill’ (WSM). It was the most important single advancement in iron and steel technology in the interwar period. In the development of hot strip mills, automobile industry has played a crucial enabling role. The objective of a hot strip mill is to reheat and roll slabs into thin strip with a wide range of thickness. Because of its huge size and large investment, a hot strip mill need to have a life time of several decades. The mill is to be capable of meeting the market demands for a wide range of steel grades, in particular, high strength and advanced high strength steels (AHSS) with good cold formability and with superior strip properties.
Present day hot strip mills are to satisfy several requirements consisting of (i) high mill availability coupled with high productivity and high yields, (ii) meet the need of low maintenance, (iii) meet the need of lower energy consumption, (iv) improved product quality featuring precision thickness and profile tolerances normally as needed by the present day customers which can be much closer than specified in various international standards and which are achieved through high-performance control elements, process models, and monitoring systems, (v) enlarging product mix, e.g. using new cooling strategies on the exit roller table, (vi) more flexible rolling schedules to ensure short delivery times and cost-effective rolling of smaller batch sizes, and (vii) lower operating and energy costs with innovative equipment and monitoring systems.
The objective in a hot strip mill is to roll thick slabs into thin strips. Slabs are the semi-finished steel product which are produced in a continuous casting machine. Reheating furnaces are used to heat slabs. When they reach a temperature of around 1,240 deg C, they are lifted out of the furnace on to the roller table. Roller tables are used throughout the hot strip mill to move slabs and strips. The automation system guides the slab through the first stand, which is called a roughing mill stand. There are different ways to make thin strip in a hot strip mill. In one of the method, the process of thinning the strip is divided between the roughing mill and the Steckel mill.
When the slab exits the roughing mill it has to have a thickness of less than 32 mm to enter the Steckel mill. To be able to go from a thickness of 220 mm down to 32 mm, the slab has to be rolled several times, called passes. The slab is guided by the automation system backward and forward and totally rolled five, seven, or nine times depending on the material and the slab thickness. When the slab exits the roughing mill it is around 80 m (metres) long and is now called a strip. The Steckel mill makes the final thinning of the strip down to desired thickness, which can vary between 2.5 mm to 12.5 mm. In the Steckel mill, it is necessary to guide the strip backwards and forwards through the stand, like in the roughing stand. Since there is only one stand in a Steckel mill, it has a lower establishment cost but has on the other hand a lower production rate.
Two problems arise, if a strip is brought backwards and forwards through a mill stand. As time goes it becomes colder and with that tougher and harder to roll. It also becomes longer, up to a kilometer. To solve these two problems, the Steckel mill has two coilers, one on either side of the stand. To prevent the strip from becoming cool each coiler is placed in a furnace. Further, since the Steckel mill is the finishing stand the exiting strip has to live up to certain quality conditions when the rolling is finished. This means that there are demands on high performance and accuracy in the rolling.
A number of control and compensation modules are used in the mill to be able to manage this. After exiting the Steckel mill the strip is cooled with water and coiled to a roll. In continuous hot strip mill, to make the needed reduction the strip is rolled in several stands in tandem. It enables a high production rate but has high establishment cost.
Fig 1 shows the outline of a typical continuous hot strip mill. Its purpose is to process cast steel slabs into steel strip. The first obvious effect is the large dimensional change of the processed piece. The slabs, of up to 35 t weight, are typically 250 mm thick and 10 m long, and the rolled pieces are typically 2 mm thick and 1,250 m long. This reduction in thickness is achieved by passing the piece through a series of rolling mill stands.
Fig 1 Typical hot strip mill layout showing some key rolling parameters
Typically, at the first stand, the roughing mill, the thickness of the hot slab (1,240 deg C) is reduced by making several passes, forward and reverse, through the mill. At the end of this roughing process, the piece is 35 mm thick and 70 m long and its temperature has dropped to 1,050 deg C. Further reduction in thickness takes place in the six or seven closely coupled finishing roll stands. The strip elongation is so great that the piece can straddle a region from the finishing mill approach tables to the coiler. During this part of the process, piece temperature is important and is to be, typically, 870 deg C after the last rolling stand and 600 deg C at the coiler. The width of piece ranges from 500 mm to 2,000 mm, but width changes in the process are limited to a few percent. Majority of the range is achieved by varying the width of the slabs entering the process.
The hot strip mill changes, not only the work-piece dimensions, but also the microstructure of the steel, which is important in determining its final mechanical properties. Microstructure is primarily determined by the finishing mill deformation, the exit temperature from the finishing mill, and the cooling of the strip on the run-out tables.
Strip rolling is a complex combination of batch and continuous processing. As each piece approaches the mill, the machinery is to be individually set up for it. Even when rolling batches of the same product, considerable changes to the set-up of the machines are necessary to maintain quality because of changes in the mill machinery (for example, because of the thermal expansion effects and wear) and variability in seemingly similar slabs (differences in dimension, chemical composition, and thermal state). The control procedure for setting up the mill for the piece is termed the mill set-up.
When a particular piece is in a mill, it is continuously processed from nose to tail and is still subject to disturbances, again from changes in the machinery and from variability along the piece as it is rolled. Of particular significance is variation in the piece temperature. To maintain product quality along the length of the piece in the face of this variability, the mill actuators are modulated to eliminate disturbances by feed forward and feedback control. This is the dynamic control.
A further complicating characteristic of the hot strip mill is the large number of degrees of freedom in the process. For example, in principle, there are several possible strip reduction paths through the process from slab to finished coil. In practice, much of this redundancy is lost because of processing and machine constraints. In the practical control system, much of the functionality is devoted to guiding the piece safely through a multiplicity of constraints from slab to rolled coil.
The general parameters of a modern high-production hot strip rolling mill are impressive. Weekly throughputs exceed 60 000 t, and the hourly rate can peak considerably higher at more than 500 tons per hour (t/h). For narrow product, up to 60 pieces per hour can be rolled. Variability of throughput is consequent on the different processing times when rolling different products. Wide products are furnace limited while the narrow, heavy-gauge products are rolling mill limited and the light-gauge products are limited by the finishing mill. At gauges below around 2.5 mm, the finishing mill threads at its maximum speed, so for light gauges, throughput is around inversely proportional to the exit gauge. The rolling of the strip also involves high forces and powers on the mill stands with forces can reach 3,000 t and the power to drive each stand 10 MW or more. The main equipment and facilities of a typical modern hot strip mill are described below.
Following reheating and descaling, the slabs are initially rolled in the roughing mill which is equipped with an edger and state of the art twin drives each with a motor power of 9.5 MW each. The roughing mill stand is capable of exerting a maximum load of 60 MN (mega newton). This immense power gives the hot rolling and processing facility the capability and versatility to process and roll the broadest ranges of specialty metals in the industry.
The finishing mill is normally designed with six to seven 4-high mill stands, each equipped with 10 MW drives and a mill-stand load of up to 55 MN. All stands feature dynamic work-roll cooling. Additional systems include inter-stand cooling, work roll lubrication, a fume-suppression system, strip cross sprays, entry-guide cooling as well as looper cooling as the basis for optimum process parameters and equipment conditions. Continuous varying crown rolls are installed in the finishing stands, which operate in conjunction with L-type bending blocks and the work-roll shifting system as the decisive factor for assuring excellent strip profile and flatness.
The laminar cooling section comprises 54 top headers and 162 bottom headers which allow a maximum water-flow rate of 20,000 cubic metre per hour (cum/h). The cooling headers are flow-controlled on the basis of calculations from a microstructure target cooling model. Laminar cooling is split into a fast cooling and a normal cooling zone. Each top header with the associated group of bottom headers is separately regulated by a flow-control valve.
Two power coilers are installed in the coiling section, which are dimensioned that they are capable of coiling API X100 pipe-grade material at a thickness of 21.2 mm and a width of 1,956 mm, or API X80 pipe-grade material with a thickness of 25.4 mm and a width of 2,083 mm. The power coilers are equipped with servo-hydraulically controlled side guides, pinch-roll units, and four wrapper arms. They are also outfitted with pinch-roll polishers and quick-exchange pinch-roll units.
A complete suite of Level 1 automation systems and sophisticated Level 2 process-optimization systems with integrated tailored process models ensure that nothing is left to chance. Level 2 rolling systems include models for the precise control of rolling parameters such as profile and flatness control, roll bending, roll thermal crown and roll wear, material flow, roll flattening and roll shifting. The cooling section control includes models for parameters. Cooling section model includes temperature monitoring and control, heat-transfer and phase transformations. The coil transport is carried out with the newly developed ‘Modular Coil Shuttle’ system (MCS).
Developments of the hot strip mill technology
The continuous hot strip mill was a shift from a simple to a ‘complex’ technology. Steam driven hand mills were all of a piece with both the steam engine drive and the mill stands were made up of castings and simple machined moving parts. A simple brass foundry was all which was needed to supply the replacement journal bearings and the complete works could be readily maintained by skilled mill mechanics equipped with tacit knowledge and experience.
The hand mill was the product of a craft era. In contrast, continuous hot strip mills were complex in the sense that they drew of a range of inter-acting technologies and it was no longer possible for one person to understand all the features of the complete system of rolling. Controls used to synchronize mill stands were at the frontiers of electrical engineering. United Engineering was forced to rely on roller-bearing organizations for their expertise to solve the problem of friction from heavy rolling loads.
The size of the new mills posed new civil engineering challenges. In effect, the continuous hot strip mill was a package of technologies from different sources re-combined in a new way. This combination of breakthroughs from a range of engineering disciplines is what had made the hot strip mill fundamentally different from earlier, narrow strip mills which shared a common layout and approach to the new technology.
The invention of the hot strip mill came as the result of two converging lines of development namely from sheet mills and from strip mills. Sheet mills rolled sheet from sheet bars. Sheets were wide and short. Frequently they were rolled in packs. Strip mills, on the other hand, rolled billets and slabs into narrow and long strip shapes. The hot strip mill was developed as a continuous mill, which gave a product having the dimensional characteristics of both strip and sheet, being at the same time wide and long. An important innovation of this mill was the 4-high finishing stand. The 4-high stand allowed the work rolls to be small, supported by large diameter backup rolls, so that much greater reductions could be achieved on a single pass.
The development of the hot strip mill was dependent on a number of inter-related technical improvements such as (i) the adoption of 4-high stands, (ii) the manufacture of bearings suitable for the demands of heavy rolling, (iii) the availability of equipment for synchronizing the motors to prevent buckling of the sheet, and (iv) the existence of automatic control devices. All of these became available in the 1920s. Electrical motors and controls were essential to adjust the speed of the hot strip mill’s successive stands. An important part of the credit for the technological breakthroughs belonged to the plant supplier United Engineering Company of Pittsburgh.
Logically, ‘Generation 1’ were the first commercially successful hot strip mills ever built. It is now familiar that the first commercially successful continuous wide sheet mill started up at Ashland, Kentucky in 1923 and the first mill to hot roll wide coils was Butler in 1926. With Butler, Generation I had arrived. All over the world, 70 number ‘Generation 1’ mills were built between 1926 and 1960.
Since then, development of the wide hot strip mill has been driven by a combination of economic incentive and technical opportunity. Rapidly growing demand for steel and the imperative of scale economies drove strip mills to higher and higher outputs across ‘Generations 1 to Generation 3’ between 1920 and 1980. Each of these generations overcame a set of constraints on the path to heavier coil weights and faster mill speeds. ‘Generations 2 and 3’ were stages on a path to higher output and lower costs per ton, realized by securing economies of scale at a time when energy was cheap.
The first ‘Generation 2’ hot strip mill was developed in the USA. These hot strip mills represented a step change on what had gone before. They were designed to realize even higher outputs than the preceding fully continuous mills. Heavier slabs were rolled at higher speeds on more powerful mills. Slab weights jumped from 10 tons to 20 tons or more. Mill powers doubled from 30,000 kW (kilowatts) to nearer 60,000 kW. Maximum speed at the last stand of the finishing train rose from a stately 12 metres per second (m/s) to near 20 m/s. The ‘Generation 2’ mill was developed in the USA, initially by United Engineering of Pittsburgh. National Steel’s fully continuous 80 inch (2,000 mm) mill at Ecorse, Michigan commissioned in 1961 was the first true ‘Generation 2’ mill.
‘Generation 3’ are huge. In 1969, Nippon Steel commissioned at Kimitsu, the first mill in the world capable of rolling coils up to 45 tons and specific weights up to 36 kg per mm of width. The mill rolling train was 0.74 km (kilometer) long, contained in a building 1.35 km long. The maximum rolling speed was 1,400 m/min (23 m/sec). A total of five mills were built along these lines. These super mills represented the ultimate in speeding-up and scaling-up. The idea was to stretch the fully continuous mill to its utmost in order to achieve all the economies of scale available in hot strip rolling. As a result of this development, total mill power, mill exit speed and unit coil weight had increased six-fold within fifty years. These ‘Generation 3’ mills were a logical, if extreme, development of later ‘Generation 2’ mills. Their inordinate size pushed up capital costs in three directions, namely (i) they had to be stronger to take heavier slabs, (ii) they had to be longer to make the output, and (iii) they were chosen to be wider. At this stage, these new mills became too expensive for the majority of the organizations to finance.
Since ‘Generation 3’, mill size had gone into reverse. Specific coil weights drifted back towards the 18 kg/mm to 25 kg/mm range, around half that of ‘Generation 3’. JFE steel commissioned their Ohgishima hot strip mill in May 1979. Although a very wide strip mill (2,400 mm roll barrel), it reverted to the specific coil weights associated with the ‘Generation 2’ mills of 18 kg/mm. It is a three-quarter continuous mill, a layout with a long history in Europe prior to its adoption in Japan. The three-quarter continuous mill was a low cost solution which added just one extra roughing stand behind the reversing roughing stand of a conventional semi-continuous mill. This extra roughing stand takes a single additional pass in one direction as the bar proceeds to the delay table.
A fully-continuous mill is inherently wasteful since slabs pass through the continuous roughing train more quickly than through the subsequent finishing train. The roughing train is idle much of the time. Yet a continuous roughing train has enormous capital cost owing to its extreme length and high engineering requirements. A three quarter continuous layout offers the shortest layout for hot strip mills rolling specific weights of 18 kg/mm or above and the lowest capital cost per ton. But, the logic of completely eliminating the roughing train altogether is self-evident. In retrospect, the huge capital cost and poor energy efficiency of the ‘Generation 3’ look like a mill too far. Yet, without this step, there may never have been an opposite reaction towards ‘Generation 5’ mills.
The energy crises of 1974 and 1979 forced a rethink in design of the hot strip mills. Few hot strip mills were ordered worldwide after 1974. The existing stock of mills was more than adequate to meet slow growth in strip demand. The shock of higher energy prices, coupled with slower growth and constraints on finance led to pressures for cheaper, more energy efficient hot strip mills.
By 1982, a couple of developments had emerged. The first true ‘Generation 4’ mill was commissioned in 1982 at Nippon Steel, Yawata 3. The other innovation was the coil box incorporated into the design of Lake Erie Works mill. At first sight, these separate developments have little in common as they offered markedly different technical solutions to mill design. The Japanese mill was technically sophisticated and high in output. Lake Erie was simple and small. Yet they were both a response to four new priorities in the design of the hot strip mill. The need to cut initial investment and reduce energy consumption, while at the same time producing higher quality and rolling a wider range of products in rapid response to changing orders.
The key ‘Generation 4’ mill at Yawata 3, like Ohgishima and Nisshin Kure 2 mills, had a three quarter continuous layout. This mill incorporated more innovative features than had been seen in one mill building since Armco at Butler and Great Lakes at Ecorse ushered in ‘Generations 1, and 2’. It was a compact mill designed from the outset for hot direct rolling of slabs and hot charging to the reheat furnaces. The roughing stands were built with very powerful edgers. Key innovations in rolling practice included edge heaters and edge lubrication to reduce localized roll wear, four numbers six high stands on the six stand finishing train and schedule free rolling through side-shifting rolls. The product range was wide, the scheduling superb, but the roll technology outdated by European standards.
‘Generation 4’ hot strip mills accumulated earlier separate innovations. Walking beam reheating furnaces became standard. Hydraulic control was applied to edgers on the roughing mills and to gauge control on finishing mills. Finishing mills adopted new shape control technologies including six-high stands (developed by Hitachi), pair cross rolls (developed by Mitsubishi), and continuously variable crown mills (‘CVC’ developed by SMS Schloemann-Siemag). Although six-high stands proved a false start, other fourth generation technologies were to find their way onto ‘Generation 5’ mills. CVC technology has been fitted to all CSP plants and pair cross rolling has been adopted by Trico and Ijmuiden mills. It is also a short logical step from hot charging slabs onto a conventional roughing mill, to hot charging thinner slabs directly to a finishing train.
The coil box was invented and developed in the 1970s to reduce the cost of a new hot strip mill. The coil box was prompted by conceptual planning of a new green-field steel plant on Lake Erie, at Ontario. Stelco wanted to build their new hot strip mill at the site as cheaply as possible, but there were two radical features. The mill was laid out for inline rolling directly from the continuous casting machine to save energy. And overall mill length was reduced by use of a new coil box in place of a lengthy delay table between the roughing stand and finishing train. This improved energy efficiency and reduced the capital costs of the hot mill. The coil box was conceived for a hot strip mill with an unorthodox ‘U’ shaped layout with the roughing mill and finishing mill running side-by-side in opposite directions. The coil box emerged as a device for temporary bar storage between the roughing mill and the finishing mill. In the event, the new Lake Erie mill was built with a straight line layout, but the coil box proved a spectacular success in a conventional setting.
A typical roughing mill operation reduces a 250 mm × 10 m slab to a transfer bar 25 mm to 35 mm thick with a length of 70 m to 100 m. The transfer bar is then rolled through five to seven finishing stands to produce the final product thickness, ranging from 1.2 mm to 19 mm. It takes around 100 seconds to roll the product through the finishing stands and, in that time, the tail end of a 25 mm transfer bar loses around 160 deg C (temperature rundown). In hot strip mills without a coil box, it is essential to compensate for this entry temperature rundown by gradually accelerating the mill throughout the rolling of the product to reduce the temperature loss through the finishing stands. By increasing the mill speed, up to 50 % on lighter gauges, it is possible to maintain constant finishing temperature in spite of the falling entry temperature.
The power required to roll the cooler tail end at the increased speed is as much as 80 % higher than the head-end which needs additional capital expenditure. The full motor power, however, is only utilized for part of the product mix the widest limits of the lighter gauges and only for the tail end of these products. This extra power and cost, hence, are only used for 1 % of the total mill rolling time.
The technology was developed to avoid this extra costs. It removes the basic problem of temperature loss and temperature run down by coiling the transfer bar and reducing the surface area exposed to radiation by around 30:1. Hence the heat loss is reduced by a factor of 30 to 1. By reversing the transfer bar in the coil box so that the tail end of the roughing mill becomes the new head end, temperature run down is eliminated.
A coil box simply receives the hot breakdown strip in the final pass from the roughing train as a large, open coil. Coiling the strip obviates a lengthy delay table. Coiling also promotes heat retention and temperature homogenization so that the bar is at a more uniform temperature. The coil box was put on trial as a sequence of prototypes on Stelco’s existing Hilton Works hot strip mill. Commissioning of Stelco’s new Lake Erie works was delayed, Westernport hot strip mill in Australia became the first new hot strip mill to commission a coil box in 1979.
The coil box was widely adopted both as a technique for saving energy and improving the strip quality through more uniform heating. It proved an ideal way of rebuilding roughing mills of ageing ‘Generation 1’ mills. It has enabled Japanese mills to develop true endless rolling. Coil boxes became central to development of some ‘Generation 5’ hot strip mills. Several hot strip mills installed medium thickness slab continuous casting machines coupled to coil boxes. Casting a thicker slab brings metallurgical benefits arising from improved central segregation, plus less scale formation and better temperature control. In this way, direct rolling has hijacked ‘Generation 4’ coil box technology to solve ‘Generation 5’ problems. By 2000 some 43 coil-boxes had been installed – half of these since 1990. Fig 2 shows coil box and its operating mechanisms.
Fig 2 Coil box and its operating mechanisms
‘Generation 5’ hot strip mills have proved a route to new strip products. In Europe, SMS Schloemann – Siemag addressed the imperatives of lower capital outlay and better energy use in a different way to the coil box. The technical breakthrough pioneered by SMS was the commercial development of thin slab casting which eliminated the roughing mill of a conventional hot strip mill. Adoption of a long continuous tunnel furnace in place of conventional reheating furnaces enabled uninterrupted casting and rolling in a continuous sequence.
Thin slab casting and direct rolling was announced to interested parties around the same time as the ‘Generation 4’ hot strip mill. The Tokyo Rolling Conference in 1980 was a key event. Neither ‘Generation 4’ hot strip mills nor thin slab casting appear in the papers, yet gossip relating to both technologies circulated at the conference.
The limited product range of thin slab direct rolling units was of less concern to established producers in Europe. Thyssen Krupp Stahl has four conventional strip mills in addition to their thin slab unit. Hoogovens have a large ‘Generation 2’ mill at IJmuiden close to their new direct rolling mill. For both the steel organizations, direct rolling of thin slabs was an attractive route to making thin gauge products. The small capacity of these plants suited their plans for expansion by developing new products in a slow growing European market.
The market for hot rolled thin strip is growing fast, but there are considerable operational difficulties, notably roll wear, mill stress, and the slow pace of production which influence against production of thin strip on conventional mills. The attractions of ultra-thin gauge hot rolled material are the scope to replace cold rolled material, the opportunity to replace thicker gauge hot rolled material in some applications and its potential as substrate for coating.
Both Krupp Stahl and IJmuiden have the advantage of a basic oxygen steelmaking feed which makes very high quality ultra-low carbon steels. Both the plants have ladle steelmaking facilities, not only for quality and scheduling, but also to provide for calcium injection and the high temperatures needed for thin slab casting. Both German and Dutch mills incorporate a variety of innovations, including very high power descalers to improve surface quality. In addition, IJmuiden has ultra-fast cooling on the short run-out table and has eliminated down-coilers in favour of carousel coilers suitable for thin gauge semi-endless rolling. The interesting part is the software which simultaneously controls the mill power and cooling.
Arguably IJmuiden is the first true ‘Generation 5’ mill as it was designed from the outset specifically for semi-endless rolling of ultra-thin strip and production of ferritic strip. It is argued that all metal industry processes migrate towards continuous flow processes. A conventional hot strip mill is a batch process. IJmuiden is a stage in the transition towards non-stop strip production. A semi-endless flow of strip and ferritic rolling both have marked effects on mill layout and design.
Semi-endless rolling needs an accumulation of evenly heated slab in the tunnel furnace since the discharge speed of the finishing train is higher than the casting speed. Ultimately, the faster pace of rolling catches up with slab supply. This implies either an ultra-long heating tunnel, or a fast reheating speed with consequences for furnace maintenance and fuel consumption. The furnace tunnel at IJmuiden is 1/3 km (kilometre) long. Semi-endless rolling needs sustained operation of the finishing mill. Drive motors cannot be run in the conventional way with intermittent bursts of power followed by recovery. Instead, drives are installed for prolonged operation at rated capacity. Both Krupp Stahl and IJmuiden have motor powers on their finishing mills comparable to all the drives throughout an early ‘Generation 2’ continuous mill.
The concept of endless hot rolling was introduced to meet the increasing demands for hot-rolled steel sheet using a cost-effective process compared to the conventional batch-type process. Endless rolling involves the joining of the tail end of a preceding transfer bar with the head end of the following transfer bar after the roughing stands. The repetitively joined hot bars are subsequently finish-rolled, which results in the endless rolling. For enabling endless rolling, Primetals technologies has developed the related equipment for induction-heating joining and super deformation joining. Also, the dynamic Pair Cross mill can also be installed to deal with flying strip-thickness changes and increasing thermal crown resulting from continuous rolling.
This process has several advantages over the conventional batch-type process which include (i) expansion of product line-up consisting of production of thinner, wider, and harder steel sheets as well as stable rolling of thin and hard-to-roll materials, (ii) increase in productivity because of great reduction in mill idle time, and constant rolling speed with reduction of mill acceleration and deceleration time, (iii) improvement of product quality consisting of improvement in accuracy and uniformity of strip thickness, crown, and width, (iv) energy savings because of reduction of the mill acceleration and deceleration frequencies, and (v) improvement of yield due to minimizing of miss-rolling by applying tension at head and tail of strip.
High rolling speeds up to 20 m/s are necessary to maintain austenitic end rolling temperatures. Rolling of ultra-thin gauges is achieved by semi-endless rolling. After initial threading of the first coil onto carrousel coilers, a step gauge reduction and mill acceleration takes place to coincide with a subsequent coil switch. In this way thin gauge material can be fed onto coilers under tension without the difficulty of initially threading a head end of thin material. The lead coil is to be of relatively thicker gauge, but thereafter thin material is rolled until the accumulation of hot slab is exhausted. Other pioneers of thin gauge strip rolling using conventional down-coilers make a sequence of ‘warm-up slabs’ before rolling thin material.
The need to roll ferritic grades needs a rapid cooling section between initial ‘roughing’ and final ‘finishing’ stands in the finishing mill. In effect, the strip is roughed down in the austenitic phase and finished in a fully ferritic phase. The precise temperature drop depends upon carbon content of the strip, but these products are all very low carbon steels. The technique was developed by CRM group in Belgium from 1990 onwards. IJmuiden has a ten-metre long rapid cooling section between No 2 and 3 stands of the finishing train. Ferritic rolling makes a soft and formable material. In particular, ferritic rolling preserves the desired crystallographic orientation of the steel (the ‘texture’) best suited for formability which otherwise has to be imparted by cold rolling and subsequent annealing.
Hot strip mills continue evolving in terms of product range and design. The story of hot strip mill development has been one of gradual evolution, with occasional step changes of direction and evolution as the engineers pursued the next incremental development. Changes of direction as economic imperatives has forced a reevaluation of the whole path of technical development. Each ‘Generation’ has accumulated the ideas of the past, but added more in response to changed circumstance. The first three generations pursued economies of scale in a growing market. The next two generations represented a shift in direction towards lower capital outlay per ton and lower specific energy use. ‘Generation 4’ has sought to cut investment and energy consumption while maintaining output. ‘Generation 5’ mills has cut capital outlays and energy use still further and has proved that hot strip rolling is viable on a small scale. It now remains to pursue the technical opportunities opened up by direct rolling of thin slab.
Main features of a modern hot strip mill
For achieving the demanding requirements, several important features are incorporated in the modern hot strip mills. Fig 3 shows some of the different technologies which are used in the modern hot strip mills. Some of these are described below.
Fig 3 technologies for hot strip mill
Reheating furnace – Critical to the hot strip mill is the reheating furnace. Modern hot strip mills are equipped with state of the art walking beam reheating furnaces which have replaced and out-performed older pusher type reheating furnaces. These reheating furnaces are normally computerized controlled and normally rated to produce heated slabs in the range of 250 tons per hour to 300 tons per hour with a capability of producing up to 25 % extra to their rated output with some sacrifice in slab temperature uniformity.
The reheating furnaces uniformly heat the slabs 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 slabs as the charge material in the furnace.
Depending on the furnace design, the interior of the furnace is divided into several (five to ten) zones for temperature control. The preheat and heating zones combust a mixture of fuel gas and preheated combustion air with the burners located on the roof and on the side walls of the furnace, both above and below the skids, to heat the slab nearly to its discharge temperature.
Most of the preheating of the steel slab is achieved by the hot exhaust gases rushing past the slabs on the way to the recuperators. Whatever heat is left in the exhaust gases preheats the incoming combustion air to around 500 deg C to 550 deg C in the recuperators. In the heating zone the steel is primarily heated by radiation by the hot furnace walls. In the soak zone, the burner sizes and locations are such so as to maintain a uniform temperature within the zones to equilibrate any cold spots in the slabs.
Descaler – Descaling of heated slab is a must in the hot strip mill for achieving good surface quality of the hot rolled strip. Descaler operation needs to be optimize in order to ensure maximum scale removal and hence enhanced cleaning at minimum cooling of the heated slab. Present day descalers employ state of the art nozzle technology with highly effective application of high pressure water.
After leaving the reheating furnace, the slab passes at a speed ranging from 0.15 metre per second (m/sec) to 2 m/sec through a descaling unit which is an enclosure employing two pairs of spray headers. These spray headers are of simple, maintenance friendly design and spray high pressurized water (pressure of water up to 400 kilograms per square centimeter (kg/sq cm) on the slab to remove the oxidized iron layer which forms at the surface of the slab in the oxygen rich atmosphere of the reheating furnace. These headers are normally equipped with advanced nozzles to spray water effectively. The descaler is normally of closed design to prevent water from escaping and there is optimized water flow inside the descaler. The water consumption for descaling ranges from 200 cubic metre (cum/h) to 700 cum/h.
Slab sizing press – Modern hot strip mills are equipped with a sizing press in place of an edger. A slab sizing press in the roughing mill area has the technological advantage over a conventional edger. Besides large width reductions (up to 350 mm), it results into a distinctly better through forming of the slab right to its centre. Slab sizing press produces flatter dog bones leading to reduced respreading and greater sizing efficiency. It offers more flexibility in production. Slab sizing press improves width tolerance along the entire strip.
The sizing press offers distinct advantage of far more flexibility in the hot strip production. The width reduction in the sizing press pass enables the number of sizes in the continuous casting to be standardized to a few widths which in turn helps in the enhancement of the productivity in continuous casting machine. A special short stroke mode at the slab head and tail ends results into less cropping losses and higher yield.
Hydraulic edger adjusting system – The width of the strip from its head to its tail is controlled by this system. The quick dynamic response of this system enables the fast corrective movements at the material head and tail for reducing cropping losses and in controlling the width over the length of the strip. Latest generation edgers are fully hydraulic facilities without any additional electro mechanical adjusting systems.
Secondary descaling – Between the crop shear and the first rolling stand of the finishing mill, there is normally second scale breaker, whose task is the final scale removal. Water sprays above and below the transfer bar at around 200 kg/sq cm pressure break up the scale which has re-formed (secondary scale), as well as any scale which has persisted through earlier descaling operations. Level adjustment of the top spraying headers and water collecting troughs enables optimum adaption of the transfer bar being handled. Due to the special nozzle arrangement, different degrees of cooling on the transfer bar upper-side underside are minimized.
After secondary descaling, the bar is pinched by a pair of pneumatically-actuated rolls to mechanically loosen any remaining scale, which, as the processing temperatures cool off, becomes increasingly sticky even as it returns ever more slowly to the surfaces of the still red-hot steel.
Motors and drives – The motors and drive systems are the heart of any hot strip mill. Drive systems in the hot strip mills have to face hard work and high demands in daily operation, constantly high loads, highly dynamic acceleration and braking operation, and mechanical and electrical shocks from heavy passes. Hence, they contribute to the mill’s productivity and availability figures, but they also have an important impact on maintenance costs, product quality, energy consumption and grid quality.
For the high dynamic requirements especially in hot strip mill, non-salient type rotor for the main drive motors have been developed, reducing the moment of inertia and the torque ripples while providing a very robust motor. For the widest range in drive solutions, ‘Cyclo Converters’ are proven and simple. For superior requirements, the newest ‘Voltage Source Converters’ provide high power scalability with the newest technology. One of their advantages is that no compensation devices are needed and together with the AFE (active front end) even compensation for other reactive loads can be provided.
As a result, the solution best meets the requirements of the mill stand and prevents oscillation and chatter effects right from the beginning. The selected drive solution also contributes to the construction environment as well as to the electrical boundary conditions. This applies to the design of new mills and also to mills which need a modernization of their drive systems. The conversion from DC to AC drive systems is a typical example of this.
Coil box – It is installed between the roughing mill and the finishing mill to form coils of transfer bars, hence serving as both material and heat accumulator. During uncoiling, tail end of the transfer bar becomes head end of the transfer bar as it enters the finishing mill. Coil box enables a shorter distance between the roughing mill and finishing mill. It also minimizes the temperature drop of the transfer bar entering the finishing mill. Coil box can be with mandrel or it can be mandrel less. The mandrel less coil box contributes further to reducing the temperature drop at the coil inner wraps in comparison to mandrel type coil box. Mandrel less coil box also makes space to install the side heat shields which contribute to reducing the temperature drop at the strip edge. Fig 4 shows mandrel type and mandrel less coil box.
Fig 4 Mandrel type and mandrel less coil box
Automatic width control – In general, the width performance of the hot strip is evaluated using two metrics namely (i) average body width deviation, and (ii) width variability at the coil ends. As the control of in-body width improves, the over-width amount can be reduced without the risk of being under-width. Good width control ensures that both the end and in-body widths be greater than or at least a little above the minimum aim width. For the width in bar, the width profile along the length is the most important performance indicator.
For squeezed slabs, the profile shows high width-drop and for spread slabs high width flare at the two ends of the coil’s length. The majority of slabs are squeezed, that is, the slab width is greater than the target width. Narrower width at the two ends (head end and tail end) is very typical for the squeezed slabs. This characteristic is called width-drop or width necking, and can originate in either the roughing mill or the finishing mill. Although a large width drop do not result in a whole coil rejection, it can require a lengthy end crop which impacts the yield and increase the processing costs.
A small percentage of the slabs whose width is narrower than the target width spread to the target width. The width at the two ends of these coils is typically wider than the body width and is called width-flaring. There are several efforts devoted to this issue of width drop. Basically, optimization of the short stroke movement using a hydraulic edger is the best approach to smooth the width profiles at the two length ends generated in the roughing mill.
Several single stand and reversing roughing mills are equipped with hydraulic edger-adjusts with a sophisticated automatic width control function. Automatic width control with its hydraulic movement is an effective tool to reduce the in-bar width variation, especially in the two ends. In a multi-stand roughing mill, edgers with electro-mechanical screw-adjusts are more typical. Experience shows that the hydraulic system not only needs much higher capital investment but also increases operational maintenance costs. Moreover, hydraulic edgers do not have any impact on the in-bar width variation generated in the finishing mill, especially on the ends. In addition, the width variation differences with or without automatic width control are hardly significant for the width profile generated in the finishing mill. In other words, the importance of automatic width control to the width performance cannot be compared to the automatic gauge control to the gauge performance.
Automatic gauge control – The automatic gauge control system is provided in the hot strip mill for the purpose of achieving the desired thickness of the hot strip. It is the system for the regulation of the thickness. The control of the gap between the work rolls, and in other words the strip thickness, is made in two steps with two control modules (Fig 5). The first module is called automatic gauge control. It receives thickness set points, ‘s0’, from a higher-level system. The automatic gauge control compensates for such things as roll wear, heat expansion in the rolls, stretch of the stand, and other unmeasurable quantities which affects the gap. These quantities are all calculated using models and summed into a total gap deviation.
Fig 5 Simplified control system for AGC and HGC
There is no thickness feedback control in the hydraulic gap control. A thickness measurement is made during the passes, but is not used in the hydraulic gap control. Instead it is used in the automatic gauge control. The thickness feedback is passed through a ‘proportional integral’ (PI) controller and added to the total gap deviation. It can be seen as a model error compensation. This compensation changes the cylinder position reference in the hydraulic gap control. The total compensation, delta ‘s’, calculated in the automatic gauge control is sent together with the set points received from the higher-level system to the hydraulic gap control.
Hydraulic gap control – The hydraulic gap control is the second module in the gap control, controlling the gap between the two work rolls. The hydraulic gap control uses the higher-level set points as roll gap reference values. Measurements from position transducers on the cylinders and the current compensation are used to calculate the true roll gap. The hydraulic gap control is position controlled and uses the reference and the true gap to control the strip thickness. A simplified description of the hydraulic gap control is shown at Fig 5
To close or widen the gap, the hydraulic gap control steers the servo valves for the hydraulic cylinders. The opening or closing of the valves increase or decrease the rolling force. The signal from the load cells gives a good approximation of the size of the rolling force and is used in the hydraulic gap control to calculate the gap compensation. There are two position transducers on each of the hydraulic cylinders which are measuring the cylinder stroke. The hydraulic gap control is implemented as a PI controller.
Both the reference and the control signal are ramped and have an upper limit. This means that the signal has a maximum value and that the rate of increase is limited. This is done to avoid heavy strains on the mechanical parts.
Work roll bending – In a hot strip mill, the backup rolls have a large diameter for supporting the work roll and prevent it from bending. But in practice even backup rolls get bent when the force on the bearing housing becomes larger. This makes the force applied on the work roll higher closer to the bearing housing and smaller at the middle of the roll. The strip on the other hand operates with a reactive force. This acts onto the middle of the roll. This in turn bends the work roll and affects the strip profile (Fig 6).
The available profile control range without visible flatness defects (centre buckle and edge waves) is correlated with strip exit thickness. The higher the strip exit thickness, the higher is the profile’s capability to change without generating a flatness defect. The important factor in the design of the bending system is the impact range of the bending actuating device on crown and flatness control. When the rolled material is thick, the impact of bending on the strip crown ratio is limited, whereas when it is thin, the impact is very high.
The bending control range depends on two main factors namely (i) the power of the crown control actuating device used on the stands, and (ii) the position along the mill stand where these actuating devices are installed. For increasing the crown and flatness control capability of the hot strip mill, it is necessary to install the actuating devices in the right position to optimize their efficiency.
The work roll bending device because of its usability and easy-to-install structure, is the most commonly used shape control actuating mechanism for the hot strip rolling. Although the ideal work roll bending load capacity is a little less than one-tenth of the rolling load, it is in many cases difficult to achieve even if work roll bending is combined with negative bending to improve the total work roll bending load.
A double chock bending device, equipped with two chocks at a roll neck of work roll, is an effective work roll bending method which is able to easily achieve a balanced design among the three strength constraint conditions (roll strength, bearing strength, and chock strength) and achieve a large bending capacity in a limited space. However, these types of rolls have roll axis ends which slightly protrude and cannot be easily installed by converting an existing mill. Work roll bending has a relatively simple structure, and is frequently installed with other shape control actuating devices. Fig 6 shows work roll bending and shifting of geometric shaped rolls.
Fig 6 Work roll bending and shifting of geometric shaped rolls
Roll shifting technology – Different roll shift shape control technologies have been developed. These technologies basically consist of (i) a method by which the shape control effect can be improved by shifting rolls, reducing the contact portion between rolls outside the strip width and as a result to improve the deflection of the rolls, and (ii) a method by which the shape control effect can be improved by shifting specially shaped rolls, and producing the geometric roll gap distribution change effect in the width direction. This is shown in Fig 6. Out of various roll shift methods, the method based on the effects of geometrically shaped shift rolls has the advantages in that the shape control effect can be freely set by the geometric shape. A numerical control roll grinding machine makes it possible to more freely set the grinding roll curve, and more easily improve the effect of the optimized roll curve.
Basically, the same shift mechanism is used in the above mentioned two methods but the geometrically shaped shift rolls are more effective. The geometrically shaped shift rolls have a roll profile which has simple concave and convex combined curve (S-shaped curve). The shift rolls profile can be optimized and the optimally-shaped shift roll is also sometimes called as a ‘combined numerical profile’ (CNP) roll. Roll shift method is further aided by an automatic setting system which simultaneously calculates the setting values for the shift positions and roll bending pressures and provides commands, according to the roll shape.
Further, the shift roll shapes can be optimized for each rolling mill plant or rolling mill stand can be optimized, and the shape of shift rolls of a rolling mill stand can be optimized one by one so that the control effect for the strip width with a higher production ratio becomes higher. To thoroughly optimize the roll shape, the shape is frequently determined not by using specific functions, but by using numerical data. The work roll shift method can also be used for roll wear dispersion to reduce step wise uneven wear on rolls in hot rolling of the steel strip.
A shifting device can be installed at either the operation side or drive side, depending on the workability and serviceability needed in the rolling mill. Roll shift shape control actuating devices are used more frequently than before since they are able to produce flexible control effects depending on the selected roll shape, and they can be installed on a large sized hot strip mill.
Roll eccentricity compensation – Roll eccentricity is a periodic disturbance caused by a structure of back up rolls in rolling mills, and it affects product thickness accuracy. It cannot be measured directly by sensors, so it is to be identified by measured thickness or measured roll force. When there is a large difference of diameters between top and bottom back up roll, the performance of roll eccentricity control using feedback signals of roll force or thickness has not been so good. Also it has been difficult for the control to be applied from the most head end because it is necessary to identify the roll eccentricity during rolling. Roll eccentricity compensation control identifies top and bottom roll eccentricity respectively from one signal of roll force and it starts the control from head end.
Work roll lubrication – By lubricating the barrel of the work rolls, the surface defects are avoided and roll wear is reduced. The advantages of applying lubrication media onto the work rolls include avoiding scale build up and peeling of the work roll surface with a minimum of rolling oil, improved strip quality, reduced operation costs, extended mill utilization times, extended mill limits and reduced energy consumption.
Intensive cooling system and edge masking – The strip cooling system cools the strip rolled by the finishing mill and it is located on the run out table between the finishing mill and the down coiler. The cooling system is designed as a laminar flow system on both top and bottom, incorporating a line side head tank system. The cooling zone is divided into the required cooling banks which consists of intensive cooling banks for faster cooling, normal cooling banks for regular cooling, and the cooling banks for fine temperature control in order to achieve the desired cooling patterns and coiling temperatures for dual phase and TRIP (transformation induced plasticity) steels. The combination of laminar cooling and edge masking system prevents excessive cooling of the strip edges, thereby minimizing stress differences across the strip width.
Advanced down coilers – A hydraulically adjustable entry guide is provided at the terminal end of the run out roller table serves to centre the strip before it enters the downcoiler. The strip, running in at the finish rolling speed, is reliably siezed by the pinch roll unit and directed onto the coil mandrel. The features of the down coiler include hydraulically operated wrapper rolls, controlled hydraulic spreading of the mandrel, automatic calibration and exact hydraulic adjustment of the gap and the mandrel rolls, controlled limitation of the wrapper roll forces as a function of strip dimensions and material and strip tension at the strip head built up after two to three windings.
Other trend in the technological features of hot strip mill
Rolling speeds are becoming faster to increase productivity and reduce temperature loss. At the same time, rolling force capacity and reduction in strip thickness are increasing to produce high strength steel and thinner strip. It is known that higher rolling speed, larger rolling force, and higher reduction in thickness amplify mill vibration. Large mill vibration not only causes reduced life time of the mechanical parts, but also reduces the operational stability and manufacturing efficiency of the mill. As a result, a mill stabilizing function to reduce mill vibration is needed to produce high strength steel and thinner strip at higher rolling speeds.
Pair cross mill with mill stabilizer device – The pair cross mill with mill stabilizer device has been developed to achieve the requirements of higher strip crown control capability and stabilization of mill vibration. This system is mostly installed on the upstream stands of the finishing mill. The mill stabilizer device consists of hydraulic cylinders with orifices which act as a damping system and eliminate the clearances between the roll chocks and housing, thereby reducing the thrust forces along the axial direction of the mill rolls.
This function reduces the differential rolling forces between drive side and operator side. As a result, strip steering stability is improved. Mill stabilizer devices have been increasingly installed on downstream finishing mill stands to improve pinching trouble caused by strip steering and reduce maintenance requirements of clearances between roll chocks and housing. The mill stabilizer can be applied for any type of mill. This includes conventional mills as well as work roll shifting mills.
The pair cross mill was developed in the 1980s for superior strip crown and flatness control capability. Since then, Mitsubishi-Hitachi has enhanced the pair cross mill. The latest design of the pair cross mill is now in its third generation. In addition to the superior strip crown and flatness control capability of the first generation pair cross mill, the third generation pair cross mill has the advantages of (i) simpler mechanism for easy maintenance, and (ii) higher reduction in thickness with stabilization of mill vibration by mill stabilizer device.
The number of parts in the pair cross mechanism of the third generation has been reduced to less than 25 % of those of the first generation. The cross mechanisms are installed on the bottom delivery of the operator side and top delivery of the drive side.
The mill stabilizer devices are installed on the entry side of the housing posts. The mill stabilizer devices, which consist of hydraulic cylinders, push the work roll, and back up roll chocks against the delivery side housing posts and eliminate the clearance between roll chocks and mill housing.
The mill stabilizer devices function in place of the entry housing liners, and eliminate clearance during operation, but can be retracted to allow necessary clearance between roll chocks and housings during roll changes. Hence, the maintenance of entry side housing liner and the housing window gaps between entry and delivery side housing liners is not needed. This feature reduces the maintenance requirements for the housing liners to around one-third of those of a conventional mill.
As mention above, it is known that faster rolling speeds, larger rolling forces and higher reductions in thickness amplify mill vibration. The phenomenon of mill vibration in hot rolling consists of the upper and lower work rolls vibrating in opposite directions of each other, mainly in the horizontal direction. The mill stabilizer device is installed between the roll chock and housing, increasing the dynamic rigidity of the mill housing in the horizontal direction and hence reducing mill vibration. The hydraulic cylinders of the mill stabilizer device are provided with orifices. The orifices create a damping effect on the stabilization of mill vibration and further increase the dynamic rigidity.
New steel grades – The development of steel grades has by far not yet reached its end. The market, together with its product requirements and competing materials, like for example aluminum, drive this continuous development.
Developments in the dual and multi-phase steels are worth special mention. Mill technologies accompany these developments. Sometimes, the rolling mill builders also take part in the steel grade development, with the vision of building a ‘metallurgical perfect’ mill.
It is also worth to mention say that in the last 20 years the mechanical strength of hot rolled steel has doubled from around 300 MPa to nowadays 600 MPa to 700 MPa as an average (for automobile material and high strength steel applications). Fig 7 shows an overview of the most important steel grades in the area of hot-rolled carbon steels.
Fig 7 Steel grades in the area of hot rolled carbon steels
Cooling – The successes of new materials are accompanied by an increase in the requirements which are placed on the classical material, steel. It is not only automobile industry which expect to gain an advantage from thinner but still very strong types of steel sheeting which makes their vehicles more efficient and more environmentally compatible. Other branches of the industry can also profit from strength-relevant load-bearing structures and energy-absorbing components made of dual-phase, multiphase or high-strength steels such as TWIP (twinning induced plasticity) and TRIP steels. In addition to the alloying elements, the cooling section is decisive for the properties of these steels. Precise and highly flexible control of the cooling process in the cooling section is hence extremely important and provided by (i) laminar cooling / turbo laminar cooling, (ii) quick switch header, (iii) intensive cooling, (iv) microstructure target cooling.
While the laminar cooling with turbo cooling headers achieves a higher throughput and the quick switch cooling headers take care of higher accuracy in the coiling temperature, the intensive cooling is specially designed for the need of advanced steel grades.
Intensive cooling – With intensive cooling, an increase in cooling capacity of around 200 % can be achieved compared with conventional cooling. This water-pillow cooling offers a high cooling power density (up to 5 MW/sq m), high cooling rate (e.g. 750 deg C/s for a strip thickness of 2 mm) and a large control range (cooling power density control range 1:10). For high strength low alloy (HSLA) steels and ultra-thin strip, the intensive cooling header is installed between the finishing mill and the laminar cooling section. In case of advanced high strength steel (AHSS) grades e.g. DP (dual phase), TRIP and multi-phase steels, a two-step cooling is preferred and a second intensive cooling header is placed between the laminar cooling section and the down coiler.
High product quality – Hot strip mill managers face an increasing demand for high quality products which not only remain at a high level but also even grow even further. Among these are the geometric properties, such as uniformity and replicability of width, thickness, profile, and flatness. Surface properties are equally important, since damage to the hot rolled strip is permanent. Suitability of strip for automobile production is a step which is already determined in the hot strip mill.
Level 1, Level 2, (and higher) automation – The nowadays advanced processes demand more and more reliable automation systems, based on exact process-data-measurement (sensors) and actuators which are to be as fail-safe as possible, and / or redundant in order to avoid any discontinuity of the process. Coupled lines, such as continuous hot processing mills, just became possible after these reliable automation systems. The response time of such systems are in milliseconds (always have been) when thinking about fast hydraulic servo systems installed in automatic gauge control or automatic width control. Imagining that the most modern tandem mill in 1974 had a memory capacity of only 64 kB (kilo Bytes) of the control computer and 40 kB on the process computer, nowadays the memories are in the range of around with 60 GB (giga Bytes), or even terra Bytes, which is the magnitude of a million higher than what were in 1974.
To achieve the best qualities in the tight and tighter tolerances which the markets are demanding, process automation Level 2, with precise mathematical-physical set-up and control calculations with a fast and sophisticated learning is necessary.
The automation provides models for all aspects of the rolling process. These include set-up calculations for tight material property control solutions as well as precise mill set-up calculations for speed and gap. Profile and flatness systems complement with advanced ‘Crown Control’ systems. The adaptive model for set-up calculation uses a model for friction and for – yield, and is able to consider conditions with and without rolling oil and adapting physical parameters as depicted in.
The temperature set-up for new steel grades is very important, since the material properties depend very much on the precise and flexible control of the temperature and its course in the mill. Hence, a ‘Model Predictive Control’ (MPC) in the finishing mill as well as in the cooling section has to be implemented. This control takes into account the actual incoming temperature course and uses the best available actuator (inter-stand cooling or speed change) selected by an online optimization to achieve the minimum deviation over the whole strip at the exit of the mill or an intermediate target.
Crown and flatness control – Special work-roll contour together with the ‘roll shifting and bending’ for strip profile and flatness control are used, enabling tighter tolerance values and reduced off-gauge strip lengths. The roll contour itself can be described as a sum of a sinusoidal and a linear function which results in an unloaded roll-gap profile which is always cosine-shaped. With the use of a sophisticated profile and flatness model, work rolls are shifted to adjust the roll-gap contour in order to achieve the desired strip profile or to match the roll contour to the crown of the incoming strip. The principal features and benefits of this solution can be summarized as (i) significantly enhanced profile and flatness control range, (ii) lower number of work-roll sets in use, (iii) replacement of all conventional roll crowns with one single contour, (iv) more flexible rolling program and pass schedule design, and (v) avoidance of all kind of buckles including quarter buckles.
Microstructure monitoring – A decisive criterion for the quality of hot rolled strip is whether it lies within the tolerance range specified by the customer for tensile and yield strength. To document this quality, the rolling mill operator has traditionally been forced to subject samples taken from ongoing production to an extensive (and expensive) series of tests. This testing not only slows down production speed but can also provide unreliable results depending on the intervals between the samples.
The solution to overcome these shortcomings, a new ‘Microstructure Monitor’ system is used, now determines these quality parameters online during the production process, hence reducing the need for costly laboratory measurements and the time needed to conclude those tests. Additionally, the ‘Microstructure Monitor’ enables process parameters, such as coiling temperature, to be optimized with regard to target mechanical values.
Trends in process technologies
In order to be able to understand the trends in process technologies, it makes sense to take a look at the history of these technologies, in this case, the history of hot strip production. Hot strip mills are described in 5 generations of mill types. Casting and rolling are still de-coupled in all these 5 generations. A breakthrough, which has resulted in a new, sixth generation of hot-strip production, is the endless strip production. The casting and rolling are coupled in one production process.
The total length only amounts to 190 m and industrial strip thicknesses of less than 1 mm can be produced. An annual capacity of 2 million tons can be achieved with one strand. The endless strip production concept sets the new benchmark in the area of compound plants. In the area of successful continuous strip production are the SMS Schloemann Siemag CSP process, and Danieli QSP technology.
For new operators who enter in less developed markets or have constraints in qualified and specialized labour and services, the Steckel mill and the Plate mill in any combination built, is an excellent alternative. Mexican and Brazilian (beside Chinese and even African) operators have recently invested in such mills, which have a very wide range of products. The so called ‘Plate-Steckel-mill’ can do in one single stand plates and coils with a very wide range of dimensions.
A combination of both is to be seen in Brazil (Gerdau) where the maximum flexibility can be reached of a combined mill, regarding width and thickness, but also regarding grades of steel. Such a mill can achieve 3 million tons a year (from 2 mm to 150 mm thick and 900 mm to 3,800 mm wide). The three reheating furnaces are positioned in the centre of the plant and can thus feed alternatively the plate production (roughing mill and finishing mill) as also the coil production in the Steckel mill.
For high productivity mills the classical hot strip mill still its strong position. The USIMINAS hot strip mill, recently built and very successfully ramped up, is one of the most modern and productive mills on the globe.
The hot strip mill is still in a very good shape. It is living through its fifth and sixth Generation and new and important developments are on the way. The discontinuous mill is more and more being replaced by continuous mills. Automation systems release the stress on the operators and ensure the best quality and performance. In the area of roll-materials a continuous development can be registered. Environmental friendly mills are the ‘issue of today’ and any saved kilowatt-hour helps to get the production costs down.