ATEC+ID performs steel sections hot rolling mill basic engineering projects, where key parameters and capacities are defined, among several options, allowing further detailed and construction engineering.
ATEC+ID performs steel sections hot rolling mill basic engineering projects, where key parameters and capacities are defined, among several options, allowing further detailed and construction engineering.
STEEL HOT ROLLING & ATEC+ID
ATEC + ID offers a long experience in the steel sector, especially in hot rolling of structural sections. The activity of the hot rolling steel mills integrates different aspects of the strategic and commercial, productive and technological enterprise level, and has evolved a lot in recent decades.
Each industrial group adopts strategies that suit them at all times or simply decide to go, that due to the very nature of the hot rolling, often bring organizational changes, need for technical and human evolution, and sometimes productive changes and/or technical and/or technological. These changes are difficult to perform in short periods when there is no external help because the regular business absorbs the available resources. However, both to address how to transfer the changes to those at intermediate and plant level, and to make process changes through trace, calibrates or rolls design change, or to develop new paths (including new initial sections) it is best to have the help of a consultancy like ours.
At the strategic level, companies are moving from address
In a company, even for any multinational, these changes have great media, economic and human impact, so given the difficulties to develop everything in-house, a collaboration with an specialized external engineering is very interesting, from consulting to the provision of planes machining, through design, which is what provides ATEC+ID. We have worked at all levels with different parts of the steel market, and on several continents so we have valuable experience, technological knowledge and technical capacity.
The high-level strategic approaches have historically had a strong influence of the authorities of each country, but today there are groups with their own corporate governance, able to decide on different directions to take, with respect to the environment and the market. Also in this area, we offer a wide range of possibilities for cooperation, and the organization and monitoring of benchmarkings (not only technical) in developing organizational policies
Process reengineering is fundamental in the life of a rolling mill, as the evolutions and changes are reflected, for example, in machinery, products and product ranges. The objective of rolling is always dual,
Changes in processes as well as the dimensions of the profiles also affect times, speeds and temperatures, which are a basic part and primary key variables affecting both rolling objectives.
In this section we want to show what is the hot rolling of steel sections, without addressing other steelmaking processes, such as the level at the Steelworks nor continuous casting except in technological and quality aspects that we may consider further relevant for hot rolling.
What's Hot Rolling of Steel?
Hot rolling normally refers to steel, although aluminum, but here only deal with steel, and especially focused on overall steel used in construction, both common qualities as microalloyed steel. Rolling seeks to get sections with different shapes from a thicker initial section with a typical rough external finish from the continuous casting and rough internal structure, by passing these sections between rods and/or rolls, commonly known as rolling mill cylinders housed in what are called rolling mill stands.
Perfil acabado en caliente
What is the purpose of rolling in a rolled section?
Rolling has two key objectives:
1. Get a metallurgical structure of steel, suitable to seek certain mechanical and chemical properties, fulfiling the requiered conditions by product standards.
2. Get a geometric section according to the applicable quality standards to dimensions, tolerances and surface finishing.
Steel quality table under EN 10025
Structural steel ready for sale
How the metallurgical structure is achieved?
The metallurgical structure of steel refers to the way in which the arrangement of the chemical constituents of the steel elements as defined material. Steel is an amorphous material, hence microstructure spoken to refer to the arrangement in groups of polyhedral grains or crystals having different orientations resulting in a polycrystalline metallic material. However, these structures have a number of preferences based on combination of elements, in this case the primary is carbon C, and the temperature at which the material is, these structures are also called phases reflecting the material state. Different microstructures steel can adopt are produced by cooling from its molten state, or from an intermediate state. At room temperature only the austenite does not correspond to a carbon steel, and this requires nickel, which makes it stable resulting stainless steels, which won't be covered here.
Steel Fusion Electric Arc Furnace (EAF), rotating dome OBT type
Liquid Steel Transfer From a Tundish to the Continuous Casting ingot for solidification
In this Hot Rolling of Steel Sections explanation we won't explain steelmaking, no vamos a explicar la fabricación del acero, except that details we consider important and have decisive consecuences for the rolled product, about how it is obtained or about the way it influences quality.
The steel produced in a continuous casting does not have a uniform grain structure, being typical to show an structure based on big and vertical grians, in the solification direction. This structure is brittle, with weak grain boundaries. Only in the central area will exist a non oriented grain area.
Rolling is performed in the presence of different phases, austenite, cementite and ferrite basically, we will try to avoid martensite, bainite and cementite, and will seek get a finished product with a distributed structure among ferrite, perlite (mixture of ferrite and cementite) and cementite. Common construction steel, is identified in all the world as low carbon steels, clasification used up to a 0,3% C content, though most of these steels present a range of 0,08% to 0,1% C content, up to 1,65% Mn and a maximum of 0,6% Si. The more C, more perlite and less ferrite, being soft and ductile. Rolling is performed above recrystallization temperature, allowing to break the previous structure in order to generate a new one, much more uniform, with much more resistant grain boundaries, even being able to break inclusions, spreading its parts about the structure.
To get an adequate structure it is important that chemical composition adjustments observe certain rules, especially with respect to trace or residual elements than can brittle the material or generate microstructures shapes that may favor. Furthermore, it is important that the blocking of the structure (if necesary for improved qualities) or the grain size is performed at the right time at the end of the rolling, and to do so, chemical composition is used (microalloying elements) and thermomechanical treatments (TMT), and we won't forget in certain parts of the process undesired thermal treatment can happen, as in the reheating furnace, continuous casting cooling, cooling after rolling and others.
How is achieved geometrical shape?
This is one of the most important areas where we will stop, sine it is one of ATEC+ID's special field, where we can perform complete datailed engineering to fulfill basic rolling mill engineering, and one of the areas all Iron & Steel companies work in because it has direct influence in the offer they can show the market or product mix, surface finishing quality and can even determine their acceptance for certain contract tenders.
Getting the initial sections (Beam Blank here) in the Continuous Casting and Stock Yard
Profiles, sections, beams, etc (they are all the same), are obtained by reducing initial section thicknesses and distributing the initial section and material conveniently. We will take into account thermomechanical properties of steel under hot conditions, in all the temperatures range from 1280ºC and room temperature. We talk about room temperature because the section won't establish its dimensions until it is straightened, although the last big conforming is generally applied between 900ºC and 1050ºC, and being posible to apply it down at about 775ºC. Initial sections are Slab, Bloom, Billet, Beam Blank (or dog bone), and Near Net Shape (Beam Blank with thicness lower than 70 mm). Slab is only used in the Kawashaki method (one in Japan) and in Teeside (UK, from Tata Steel, former British Steel) as far as we know, but we won't speak about it here since it didn't have many adoptions, however bloom has been used extensively all along the world to get big sections and billet for small sections and especially angles and U channels, the dog bone and Beam Blank has been the great success for western rolling mills, since with less resources a wider product range can be reached, with higher productivity and lower costs, and finally the Near Net Shape that didn't reach a great success due to the detailed process control needed at many points of the process, especially at the Continuous Casting making it very susceptible to propagate and develop defects.
The material from the continuous casting, baing steel due to chemical composition and structure, a rough structure with big size grains, etc, is usually at a medium temperature (between 700ºC and 300ºC), being posible to send it to stock and cool down, either conduct it directly into the reheating furnace, where it can be reheated with an objective homogeneous temperature across the section of 1240-1250ºC. There exists also a variation of rolling called warm rolling in which initial temperature is about 1210ºC even lower down to 1140ºC.
The first step is to convey the heated initial section to the first stand where first thicness reduction is applied and/or material distribution. The most common being going directly to the breakdown stand or mill, into a groove with a certain shape, although in the last years (one decade and a half) a previous stand and/or simultaneous has been introduced, any both cases to adjust initial section width, such stand type being called vertical stand or edging stand, being similar in function to square passes stands used in round rolling. In these breakdown roughing stands a huge variety has been seen, since there were used three high stands, two high stands, and all of them with closed grooves before, and open grooves later. It should be noted here that also combining open and closed grooves practice has been used in the west and in China, but the higher control complexity and higher costs have banished closed grooves, exception made of certain rollings like rail and sheet piles.
Rolling Pass in a Vertical Stand
The resultant section of this phase is called the Leader Pass, and is characteristic for a key reason, because it is the section that with a certain geometrical proportion between the parts of the section (web and flanges), allows to reach sections with certain final geometrical proportions (web and flanges), after a certain number of passes in the universal group. With this, we mean that the initial rough and imperfect structure from the reheating furnace (and from the continuous casting), is strongly conformed in the breakdown stage and Leader Pass attainment, conforming understood as the distribution of material areas to the web and flanges here (for rail and sheet pile, corresponding parts would be considered). Afterwards, the leader pass undergoes a thickness reduction using the universal method (Universal + Edging) reducing web and flange thicknesses simultaneously until the last pass made at the finishing stand (universal type). There exist different disposal and configurations also at this part of the process, especially for U (channels) rolling in universal stands, and in case we compare multisection mills (western) and small mix mills (China).
Rolled bar in the breakdown, before entering the Universal Group
It is very important to take care of certain aspects in different parts of the process, i.e. in the breakdown it is important to take care of the geometrical distribution of the material to achieve an stable process with respect to geometrical magnitudes to be obtained and with respect to corrections to be made if needed. In the universal group, it is key to look after the proportion of thisness reductions among parts of the section, so as the stability and speed of the process, in order to avoid speed-temperature changes play a trick about final product dimensions. Shall we add, that in this final part, and especially in the finala pass, where we will determine the final resultant properties for microalloyed steels. Anyway, the metallurgical final steel structure will be changing its characterization and its grain size, that will reduce, increasing resistance both due to grain refinement and loss of temperature. ATEC+ID does offer detail engineering services throughout this area of rolling claiente or formed steel, through design paths, calibrated cylinders, with their guides, etc, contact us!
Description of a Hot Rolling Mill Machinery
The machinery of a hot steel sections rolling mill refers to the resources needed to achieve both steel conforming and steel section conditioning for optimal supplying conditions. The description of the machinery and each type of stand is made in the "Rolling Mill Stands" section, that briefly is described here.
For the reheating of the bars obtained in the continuous casting, a reheating furnace is used where several technologies are available among which walking earth and walking beam have been imposed, and also adding "digital" characteristic to them, as the most economic in the life cycle and also providing the best quality. It's afurnace whose heating zones are separated and work at full burning load when on, or are off instead, and where bars are separated ones from others, facilitating and maximizing the heat utilization and a faster heating and more homogeneous temperature distribution accross the section.
Billet ready for discharge in a rolling mill furnace
Rolling mill stands are different in the breakdown zone and the universal group/s, in the first case being used reversing duos, with authomatic lateral guides at the entrance and exit, to apply a sequence of several passes making the bar pass through different grooves or channels machines in the cilinders. These cilinders are driven by a motor through a reducer, currently without it using just a distributor for each roll (in case of two high), motors being heavy duty motors whose specific characteristics and control electronics depend on application detail, i.e. on the conforming magnitude and conditions we want to apply, defining a needed power and torque, speed, etc. In this stand the motor must give a high torque for low speeds since rolling speed is variable in the same pass, being very low at the entrance of first pass of first channels, accelerating after bite at the entrance usually between 1,5 and 6,5 m/s for medium-heavy sections. Speed usually increases with passes, as well as the bar elongation, since thicknesses are reduced increasing the bar length. When a vertical stand is placed previous to the breakdown, its function is to adjust the initial section width in one or several passes, and to do so this stand motor is smaller.
Key mechanics of a walking furnace
In the universal zone we could find several universal and edging stands, either one and one, or arranged in a tandem Univ-Edger-Univ, being needed in the first case a susequent universal finishing stand and in the second a finishing group edger-Univ, or selecting a universal finisher stand as second universal in the tandem, which is the so called X-H rolling technology, please look at the "Roll Pass Design" section for more detail. In this part universal stands motors are much more powerful than edger ones for obvius reasons, please never forget that the required power depends on the reduction characteristics to apply (material, temperature, reduction magnitude, and speed), and that such characteristics have secondary variables dependant on previous four cited. A universal stand motor drives the horizontal rolls being those that bite and track the section inside the four rolls (two horizontal and two vertical), since vertical rolls are idle rolls. The edger stand can show different roll shapes, as a result of multiple developments to try to get a better flange tip finishing and a perfect simetry. Usual speeds in a universal group can be found in the range from 4 m/s in the first passes of an starting campaign, up to 12 m/s and even 17 m/s in a medium size rolling mill.
Compact Universal Stand Rolls and Guides (guards)
There are also continuous rolling mills, although Existen también trenes de laminación de perfil continuos, though in practice reduced to mills for small sections (up to 200 mm), or angles and y U channels. Out of such products it's not usual to find them apart from China, or in some cases in Japan. Continuous rolling mills offer a more compact process, but much less flexibility in terms of obtainable product mix, and in general, have been disapearing from more advanced technology countries or companies, after U staretd being rolled in universal mills and angles passed to comercial sections mills.
Rolls positioning accuracy is very important in any stand, which is autimatic, pass to pass. In the breakdown this positioning can allow certain higher tolerance, but must be repeatable at least for the Leader Pass, and in any case has to be very low at the universal stands, and repeatable in any case for the finishing universal stand, ie not switch among passes and/or bars cycles. It must be minimum at the finisher which complicates and makes it more expensive when used in a tandem applying several passes, and it is one of the key reasons why the UF is placed apart from the universal roughing group. This also facilitates controlling the variation in the resulting measurements, being due just to roll wear, which is minimum for just one final pass, vary different from the case of the so called X-H rolling where wear is much higher for the same roll quality.
All stands need bar guiding systems, ensuring a correct entry of the bar into the gap defined by the cylinders positioning. These systems comprise external guides for centering the bar to the rolling line, and that move and adjust each pass when mobile, and other type of guides (guards, internal) comprising vertical and horizontal alignment closed to rolls ensuring good quality guiding and influencing final geometry. Guides drives can be electro-mechanical or hydraulic. Guiding makes a difference in the final section geometrical quality, and especially in the dimensional stability that can be obtained in the process (process capability). Continued progress is made in this field, most common in current days, to get a quick and accurate mounting/adjunting process.
Mounting of a lower horizontal cylinder at the universal finishing stand (UF)
Cooling is a key factor for rolling stands, being determinant for rolls wear and to ensure failure, cracks, rolls breakage incidents, etc. There are three main aspects to focus atention about rolls cooling: rolls surface, chocks (bearings), and guides. In each case cooling has an specific task and objective.
After geting the final hot section, the bar is left for cooling, is cooled rapidly and/or undergoes hot cutting to customer order length. If the long bar is left for cooling, cold cutting is made after straightening, and if it is hot cut, it is straightened afterwards.
Rapid cooling after final pass is part of what it is called Thermomechanical Treatments, looking to apply the final pass at a lower temperature than (usually bellow 940ºC) and to block grain size by cooling rapidly. This practice is commonly used to get S 355, S 420 y S 460 steels, being in place an adequate chemical composition. Applying last pass under certain temperatures depnding on microalloying element (V, Nb, Ti) and steel quality to be obtained, is also used as thermomecanical treatment. When improved properties for impact (Charpy test) is the objective, especified by J2 (at -20ºC), J3..., anyone of previous muroalloying elements may be used, but for Vanadium and Niobium and not due to price it wouldn't help if it wasn't applied under 920-940ºC for the first and generally 950 - 975ºC for the second, that due to price, it's not used alone as microalloying element but in combination with V and even with Ti. To increase resistance and yield from S 275 towards S 355 steels, the european norm EN 10025 and/or A 572 steels from ASTM american norm, previous practice is used.
Rolled steel quality doesn't refer just to steel quality, nor achievable properties for the same chemical composition, but also to geometrical, shape and finishing quality. Here we talk about dimensional and shape in roll pass design section including roll pass schedule, which are key for them, and surface finishing we treat it globally and in the rolling problems section, since roll pass design is key but there are other influencies like continuous casting, cooling, wear, roll wear, roll guiding, chocks fixing/positioning, etc that can determine final state.
Finishing mill processes
After geting the hot final section there start a series of very important processes combining cooling and metallurgical transformations. During cooling big changes take place in the steel structure, and that's why it is not recommended to interfere in them, for example through a highly forced cooling since the result could be negative. There are some temperature ranges where better not to act on the section, and others where no problem appear. Once the bar temperature is under 100ºC inside, we can start the straightening process, better to be applied on the whole bar before cutting, in order to avoid a higher number of heads and ends where posible action is very low.
Two roll straightener types
Bar marking is usually made after the last pass, commonly at the entrance of the cooling bed, and indelibly after straightening, although technologies exist to do it just after the last pass, being expensive.
The cutting process offer big differences if the practice is hot cutting or cold cutting. Hot cutting is quicker and lower cost, but can constrain production capacity, cut quality and starightening quality, while cold cutting is usually higher quality, more expensive and can also be a bottleneck. In any case, the technology is abrasive cutting with toothed saw for hot cutting and both toothed saw or band saw for cold cutting. it is important the tool (saw teeth) material, the motor power, and the teeth shape, in order to determine the cutting speed capacity.
Sections Mill Cooling Bed and Cold Cutting Time Margin Assessment for 200 t/h
Afterwards bars are organized in sheets of bars or mantles, for stacking in several rows to form packs, which are bound with wire or strap, and labeled as traceability and product data. Labeling is usually the visual story both in the product quality system and in the quality management system, commonly under ISO 9001, and also for the production control system and or business process management, and even for data for the general ERP of the company.
The information in a Rolling Mill
Information in a Rolling Mill is characterized by very different tentacles, that we'll try to expose with sense. The process offers appropiate characteristics for organization and management systems like Lean Manufacturing and Total Quality Management systems (TQM) and even six sigma or Lean Six Sigma, though these tools and information systems processes are relatively new and almost nonexistant in a wide part of the non western world. Information entry for production is generated from commercial and/or logistics with customer orders registered in the production planning and scheduling system, which starts and never gives up organizing and determining campaigns by section and length. A first schedule can follow cetain rules like the minimum number of rolls changes, the minimum size to provide time to prepare the next roll change, the optimum for roll wear, an optimum rotation about the product mix, etc, contents deriving from technology and available resources in place. the validated program is sent to the steelworks in terms of number of initial section bars of certain weight, planning the raw materials (scrap, lime, microalloying elements, carbon, etc) for the needed casts (batches of x tonnes, commonly between 100 and 250 t), with the lengths of each initial section bar, steel grade to be obtained in the continuous casting. Nowadays we tend to synchronize continuous casting and rolling mill production to take advantage of bars hot charging in the rolling mill reheating furnace. This synchronization is not easy, because both the continuous casting productivity per section, and optimal productivity per section in rolling mill are quite constrained, forcing adaptations, many times making to mix cold and hot charging. Rolls and guides mounting out the production line is performed based on the rolling schedule and available rolls, deriving different planning and scheduling of machining and mountings, subjected to different terms for the same section, both due to machining method/process and mounting needed time.
This described type of planning, adapts well to western markets and services (Europe, USA, Japan, etc), and even for other parts of the world but is quite different from others like most of China. In China there are lots of companies focused in rolling a very narrow product mix, but in huge amounts, reaching a high efficiency in their installations, but lacking individually of a minimum acceptable offer scope for western markets. This is solved because they are organized around state-controlled industrial asociations, and that's why they don't compete emong them developing one grouped offer to the markets. Certainly there exist a wide variety of production organization and planning systems, but all of them are customer and market oriented, that's why organization and management systems are imposing.
Bars arrive in the rolling mill from the continuous casting with three key data, cast number, quality and length, being rolled after leaving the reheating furnace at about 1240ºC commonly. The furnce keeps important information, like gas consumption, and if we developed apropiate software, we will know the heating story at any time. For heating, the furnace is provided of optimal heating curves, with also objective temperatures to be reached at certain points, following a heating dynamic type, together with several systems to control combustion usually based on PID with continuous correction or digital type (on or off), sometimes supplemented with fuzzy logic to collect practices or to smooth changes of behaviour under certain circumstances. The material put in can be registered in the furnace, but also can be taken from the continuous casting information system, which in turn registered its production, and that eventually registered all process conditions during manufacturing. There is no technical reason for not arranging a complete and consistent traceability.
Rolling Mill Information for Operations Control
Bars start to be shaped using a rolling schedule in each stand for an specific section, schedules being registered in the information systems and produced via an initial calculus and released by experience and industrial tests and practice. Each schedule can be turn fed by motors electrical consumption, and rolling conditions as like temperature in different points, steel quality, etc, being still nowadays essential the final section quality control (geometrical and surface quality), that must include process conditions according to quality norms where applicable, as it is temperature or thermomechanical treatment temperature. With that information, the rolling mill responsible takes appropiate measures to maintain the process under control.
Rolled bars are hot marked with their heat number, or other similar, allowing to follow traceability through the process, also at the hot curtting, each bar can be automatically marked.
The straightening process can be planned similarly to rolls planning (rolls and guides), since it is determined by the rolls geometry to be used and the needed mounting time. Currently this mounting is made in a few hours out the line, and roll change can take about 30 minutos, even less. Straightening is performed using a set of predicted "pressures", "distances" or "gaps" determining the curvatures the bar undergoes in order to obtain an straight bar at the exit, according to applicable norms. There are many variables affecting straightening, among them, the section dimensions, that's the reason for having these data, the same as the cooling conditions and the applied thermomechanical treatment, it it is the case. Afterwards the section is cold cut, to customers length, looking for the optimal cutting length combinations that were also previewed at the continuous casting production planning stage, and all bars are marked indelibly.
Bars are organized in mantles or rows and stacked to reach a certain standard weight, usually not to surpass 5 t, due to the most extended practices and warehouses cranes, then are tied and labelled, even some manufacturers label each bar. This final labelling is key because it is the feeding system for the company production control system and even for the company ERP (orders fulfillment, stock yard, etc) and also feeds the quality system.
The quality system needs to include, bring and collect several information, starting from orders and finishing with the final product quality certificate, certification that in case of neeeded tests can't be delivered at the labelling stage, so there may be some virtual production to manage.
Information and Parallel Processes in a Rolling Mill
All the rolls planning (rolls and guides) is derived from the roll shop management, management for which there are different systems. The type of practice or management also depends on the product mix that can be manufactured and on the established finacial practice. The traditional system has been the availability of all rolling mill rolls needed for each section and their guides, deriving to a continuous roll batch machining and a finantial assets management based in depreciation and provisions in its case. However there are also other type of roll shop management, especially with reference to roll management based in having a few set or roll pairs, being machined to lower dimensions, campaign after campaign. This practice allows to maintain very few rolls for universal stands, thereby reducing the rolls stock considerably, and its value and finantial cost, in like manner allows a predetermined and very simple production planning and scheduling but has two major problems. The first one derived from the need of production to stock and the imposibility of producing just to stock, nor flexibility to orders, and the second one is a huge risk in front of undesirable incidents like rolls breakage, being both reasons why Lean Manufacturing practices can't be implemented in this last case.
Cut to customer ordered length and stacking and binding operations
Rolls are machined according to a geometry which is determined by the company technology, materializing the way the material will be conformed or rolled to get the final product, and is expressed on various media, in constructive and machining drawings for rolls and guides, in rolls mounting tables, in computer design drawings, in roll stock design tables, etc. Nowadays it is common to access drawings fron the lathes, grinders, etc, but the est of the information is not common to be freely accessed and even in computer support, just where unique developments ahve been made or purchased for roll stock and machining management.
The rolling mill maintenance is key, because all rolling mills are unique projects, at least in western countries, and as such, are constituted by machinery and installationscommonly designed and built with unique solutions under order. When talking about heavy industry, it is usually refered to big magnitudes in its manufacturing processes, in many cases derived from assuming process conditions of difficult calculations or in the limit of control and manageability. This type of industry nearly always needs big investments and adjusted margins in multiannual averages, therefore requires a high utilization rate to be efficient and profitable appart from other market circumstances. In this scenario, several maintenance practices can be applied, as repairing what breaks, or evolve to predictive maintenance, which is needed if quality certification under ISO 9001 standard type is wanted. Nearly all installations in a rolling mill are likely to find a certain event that puts them in a weak position with respect to its condition of use, in an environment where everything is squeezing, which linked to customization, makes it necessary specific personnel and expertise.