Important aspects regarding Rolling of Wire Rods
Important aspects regarding Rolling of Wire Rods
Wire rods are the steel product normally of round cross section which is produced from hot rolling of steel billets. Wire rods are known for their long subsequent processing which they undergo in the secondary and tertiary processing units until the final end products are produced. Furthermore, the end products are used in many cases as vital parts in various industrial fields.
Over the last few years, the technological developments activities for the production of the wire rods have been driven by the increasing demand on the quality of the wire rods by the users for its many applications, such as the production of steel cords for reinforcing automobile tires (brass/bronze coated bead wire and cord wire), high strength cables, galvanized wire for suspension bridges and roadways, railroad switches, spring wire, reinforcement strands, fasteners, welding rods, rods for the reinforcement of pre-stressed concrete structures (PC-wire), saw wire to cut silicon wafers for photovoltaic industry, and music wire, etc.
In a wire rod rolling mill (Fig 1), for meeting the requirements of the secondary and tertiary processing units, various aspects have been tackled till now by the introduction of in-line heat treatment facilities, development of small-diameter wire rods, controlled rolling with the goal of omitting certain processing steps of secondary processing, improving productivity, and energy saving. In recent years, because of the ever-growing intense competition and with the intention of establishing stronger competitiveness in product quality, some aspects such as improvement in the dimensional accuracy and product metallurgical quality, improvement in equipment and operational efficiency have also become very important.
Fig 1 Wire rod mill
The increasing demand of wire rods of different grades both for small and large sizes are challenging the wire rod producers to enhance new technologies by combining high plant productivity and efficiency of superior quality finished products with extreme process flexibility. The possibility to change process route according to the final application of the rolled steel grade and to simplify production planning is one of the most important requirements presently in the wire rod mill for the success.
The technological properties of the wire rods depends on the production process, the billet chemical composition as well as its inspection/preparation, reheating of the billets, and the rolling temperature optimization besides the mill configuration.
Billet and billet preparation
Non-metallic inclusion – In most of the grades of the wire rod, the presence of non-metallic inclusions is extremely detrimental because of the possible promotion of microscopic cavities or metal matrix discontinuities which can cause the breakage of the wire rod during its rolling in the rolling mill or during its further processing. Such inclusions (mainly oxides, silicates, sulphides and nitrides) can originate during different stages of the steel production itself (alloying, desulphurization, and dephosphorization) or can have an exogenous origin, for example, the wear effect caused by the direct contact of liquid steel with the material of the refractory lining. Apart from the aspects related to overall quantity and distribution, the most important indexes to determine the danger of non-metallic inclusions is their shape and deformability. The aluminates are the most harmful ones because of their high melting point and in-deformability, while globular manganese sulphides (mostly produced during casting stage) are easily fragmented in fine ductile strings during rolling and can be further deformed during the subsequent processing stage.
Chemical composition – The chemical composition is an important aspect since the chemical composition determines the mechanical properties of the wire rods and its suitability for a certain application. Further, besides the chemical composition, ensuring narrow variation of chemical composition within a single heat and within different heats for a specific steel grade is fundamental in order to ensure the reliability during the rolling and further processing of the wire rods. A low level of harmful impurities such as phosphorus and sulphur is desired since these impurities can deeply affect the ductility of the wire rod during the subsequent drawing operations.
Segregation level control – The control of segregation levels in the starting billet represents one of the most important aspects to ensure reliable performances in the final drawn wire from the wire rod. In order to make certain an adequate segregation level of the wire rod, the continuous casting stage of the billet becomes important to produce billets characterized by the minimum occurrence of columnar crystal and the maximum incidence of regions with equiaxial crystals. Any dendritic segregation can lead to the formation of segregation bands which can still be present even after rolling and drawing, limiting the reliability of the wire performances. Moreover, even micro-segregation in case of the wire rods with higher carbon can lead to the formation of martensitic structures in the central area of the wire rod, increasing the risk of the wire breakage.
Ductility – The ductility of wire rod and its behaviour during following drawing operations are strictly dependent on hydrogen and nitrogen content. For this reason, it is very important to control the hydrogen and nitrogen content of the steel within an acceptable limit.
Billet size and billet conditioning – The definition of the most appropriate billet size and required surface quality is a hotly debated subject. Billet conditioning is a further crucial aspect for the wire rod production because it represents the link between the casting and the rolling technologies. Billet quality inspection norms are to be specified to define if the as-cast billet can be directly transferred to rolling mill or a conditioning is required for the billet for controlling the risk of product rejection. The target of billet conditioning lines is to ensure high surface finish with the absence of laps and bleeds, control of decarburization depth on the one hand, and to minimize the final product rejection on the other hand.
Reheating of billets – The strict control of heating of the billet is needed for the control of the quality of the wire rods. The reheating furnace is to ensure the billet surface quality, temperature uniformity, avoiding excessive soaking, and avoiding increase in the austenitic grain size. The surface quality of the billet has dependence on the formation of scale in the furnace. Further, the higher carbon grades have susceptibility to decarburization. The decarburization in the billets is to be controlled through the burner control while heating of the billet.
One of the most critical aspects to be considered for the reheating of some of the grades of the wire rod such as tire cord grades is to limit the head-to-tail temperature drop due to the rolling time in the mill. While rolling of the smaller diameter wire rods, the time needed is longer, even if rolled at the highest speeds. This means that the head-to-tail temperature drop at the continuous mill entry is the highest for wire rods of low diameter (say 5 mm). This problem can be limited by a proper control in the reheating furnace, with setting the heating pattern in the various sections so that it is possible to partially compensate the natural temperature loss in the billet tail.
Rolling mill equipment
Rolling mill equipments play a very important role in production of the wire rods. The rolling mill optimization, operational flexibility, and the process reliability play a fundamental role. In order to improve the finished product quality (in terms of size tolerances, mechanical properties and surface finish), a reliable and dedicated automation system is also needed. Different automation systems are implemented in modern rolling mills to control thickness/section, the angular speed of the rolls, and the tension between the stands and the related temperatures. Specific thermo-mechanical processes and automated control systems have been developed to enhance wire rod technological properties, tolerances and surface quality.
Descaler – In order to feed the mill with a billet characterized by a proper surface quality, apart from the billet conditioning and reheating practices, descaling is needed. This is a very important requirement. A perfectly clean surface is to be ensured in order to avoid irreversible surface defects in the following rolling stages (rolled-in scale). For this reason, primary scale removal is performed at the furnace exit by a high water pressure descaler in very short times (elevated billets speeds), to avoid detrimental surface overcooling.
Rolling mill stands and shears – The rolling mill stands represent the core of the rolling process and their configuration has to be proper to suit the dimensioning technological parameters such as steel grades product mix, rolled sizes, minimum and maximum productivity, minimum and maximum rolling time, required biting speed, required shears configuration, available upstream and downstream facilities, and the media availability etc.
It is easy to understand to which extent an inappropriate mill configuration can affect the entire rolling process. In the present day environment, for the wire rod mills in general and in a broader sense for the wire rod mills having capabilities to roll special steel products, there is the need of more and more process flexibility in terms of steel grade to be processed, rolling strategy to apply, and size changing quickness. This is becoming more and more stringent with the passing time. In modern wire rod mills, in fact, it is not unusual to have more than 250 size changes in a multi-strand rolling mill. For this reason, the possibility to reduce the size changing time and to simplify roll pass design is a priority.
The shear after the descaler is required to have the cutting force, especially when low temperature rolling is adopted in the wire rod mill. The rolling stands are to ensure the appropriate stiffness, with high axial and radial rigidity, to support the high rolling loads. They are to ensure quick change and reduced risk of damaging the hoses in case of a cobble.
Finishing mill blocks and reducing sizing mill – The blocks need to have a sturdy design and heavy-duty construction to withstand high rolling loads and transmit high rolling torques. Certain grades of wire rods are very sensitive to improper rolling temperature which can be forced by rolling block limitation. Further, as the possibility of obtaining tight dimensional tolerances is another fundamental aspect for both big and small sizes of wire rods, a reducing and sizing mill after the rolling block is needed.
Apart from the benefits achieved in terms of operational cost, plant efficiency and material yield, and the application of the ‘single pass family’ concept (all the products are finished on the reducing and sizing mill), the improvements resulting from the use of the reducing and sizing mill for various grades of wire rods are mainly reflected on the quality of the wire rods itself. The reasons for this are many and include (i) because of the optimized rolling sequence and tension optimization, it is possible to achieve tight size tolerances in terms of both absolute values and reliability/repeatability along the length of the coil and between different coils, (ii) small sizes can be produced with superior rolling speeds, thus reducing pure rolling times and head-to-tail temperature drop, (iii) big sizes have a better coil tail shape, because of the small distance between sizing block and loop laying head and the shorter water cooling line after the sizing block, (iv) the lesser length of the untreated (not water cooled) bar, (v) internal quality of the wire rod is ensured because of the proper reduction of area achieved in the reducing and sizing mill, (vi) thermo-mechanical treatments can be applied for ensuring a proper grain size control,(vii) the technological reduction in cross-section at breaking point is increased, (viii) the thermal profile is more easily controlled because of the multi-stage rolling and cooling sequence, thus avoiding excessive temperature increase in the finishing blocks (Fig 2), (ix) a lower spread of final technological properties can be achieved, (x) the scale appearance is drastically improved because of the reduced cooling necessary after the last rolling sequence, and (xi) the decarburization layer is thinner and more uniformly distributed along the perimeter of the wire rod in case of higher carbon grades.
Fig 2 Typical temperature profiles along the wire rod mill length without and with reducing sizing mill
The total reduction of the reducing sizing mill is well above the critical reduction for the whole range of product mix in the wire rod mill. Hence, all the modern wire rod mills are equipped with this technology.
High speed shears – Wire rod mills are normally equipped with high speed shears at various mill locations. These shears are designed for head and tail trimming of the wire rods at the maximum speeds of the rolling mill which can be in the modern mills upto130 meters per second for both plain and reinforcement water-quenched/self-tempered steel wire rod. The advanced design characteristics of the high speed shears include (i) compactness of the unit, (ii) single-pair blade holder/single-drive design which enables cropping and chopping operations to be carried out by the same pair of blade holders, (iii) advanced blade locking/centering system with faster blade changing, (iv) short-stroke electrically actuated diverter, (v) reduction of deviation angle amplitude (reducing friction and minimizing wear on diverter and conveyors), (vi) shorter deviation cycle, enhancing operation synchronism and efficiency well beyond the design speed, (vii) significant reduction in blade width, (viii) narrower blade-holder resulting in better operating efficiency, and (viii) less friction on guiding elements with lower wear rate and less noise at the highest speeds.
In-line inspection – The standard practice is to inspect the wire rods in the cold state after the rolling process has been completed. This practice is no more meeting the requirements because of the increase in the rolling speeds and customer demands for higher quality products. Hence, necessity has arisen for the new process and quality control techniques during the production of the wire rod in the wire rod mill. The general trend today is for testing to also take place on the hot product in the rolling mill itself. The on-line testing has two advantages namely (i) it identifies production problems early on so mill operators can intervene before more damage results, and (ii) it avoids the production of waste materials and prevents time and energy from being spent on defective products in later stages of production. The in-line inspection is normally based on eddy-current.
Loop laying head – A normal task of a wire rod mill loop laying head is to ensure a good coil pattern and a long pipe lifetime. With the increase of finishing rolling speeds in wire rod mills (as during rolling of smaller size), such need becomes even more stringent, and hence, it requires new technological solutions and alternative materials.
The very high rolling speeds (over 100 m/s to 130 m/s) imply head ends and especially tail ends formation problems, if not properly controlled by mechanical and automation systems. Further, with these elevated rolling speeds, loops centering in the cooling conveyor becomes more difficult. Such aspect is fundamental for some grades because of the completion of the thermo-mechanical treatments.
The new generation of the loop laying head rotor, besides having the well-known ‘oil-film bearing’ for rotor support in order to have vibration-free operation, has advanced design, specifically studied using 3D kinetic-dynamic simulation for providing operation stability and optimization of wear-rate at high finishing speeds. The new design and material choice for the progressively-curved shape of loop laying head pipe give wire rod an ideal path, ensuring its constant contact with the inner wall along the whole pipe length and thus resulting into even wear distribution.
Controlled cooling conveyor system – At present, one of the most common in metallurgical practice in a wire rod mill is the controlled cooling conveyor system. The wire rod cooling technological section is termed a two-stage water-air cooling line (Fig 3). On such a line, after leaving the finishing stand of the wire unit, the rolled wire rod is cooled first with water by special nozzle devices, and after it is laid in coil rings by the loop laying head on the cooling conveyor, by air flows blown by the air blowers from the bottom up on to the conveyor.
Fig 3 Controlled cooling conveyor system
In order to create effective metal cooling conditions, design features of the controlled cooling conveyor system equipment are being constantly improved. For example, in the production of rolled wire rods from medium- and high-carbon steel grades, a standard ‘short’ controlled cooling conveyor line is used, and for low-carbon alloyed steels, including complex-alloyed steels intended for welding purposes, either a slow cooling or a retarded cooling mode is needed, for which a pre-determined ‘long’ controlled cooling conveyor line is more effective and universal.
Design features of a controlled cooling conveyor system enable implementation of various modes for cooling the rolled wire rods. Accelerated cooling of the wire rods takes place due to air supply to the conveyor by air blowers. If cooling rates are at least 15 deg C per second, rolled wire rod micro-structure consists mostly of pearlite. Uniform distribution of the structural components over cross-section of steels of the pearlite is particularly important in the case when the wire rod is subjected to the high degree of cold plastic deformation during its further processing. Pearlitic structure is the most favourable structure for the production of the high-strength cold worked products with a high degree of deformation.
In case of wire rods with higher carbon, the main aim for the wire rod from the metallurgical point of view is to get the highest possible unresolvable pearlitic structure, thus minimizing presence of resolvable pearlite and structure with free cementite or ferrite. The presence of pro-eutectoid ferrite determines a ductility reduction in comparison with a fully pearlitic micro-structure, because of the higher possibility of crack initiation sites at ferritic-pearlitic interface. For this reason, the amount of pro-eutectoid ferrite has to be as limited as possible (1 % to 2 %), so that the mechanical properties can generally be described by a ‘Hall-Petch relationship’ defining tensile dependence from inter-lamellar spacing. The Hall–Petch relationship tells that the strength in materials can be increased to as high as their own theoretical strength by reducing grain size. Indeed, the strength of the materials continues to increase with decreasing grain size to around 20 nanometers to 30 nanometers where the strength peaks.
In the rolling mill area the core of the wire rod production is represented by the temperature control of wire rod in the cooling conveyor. In this area, indeed, the steel phase transformations occur. Depending on size and grade, the first 30 seconds to 50 seconds of treatment in the control cooling conveyor become of strategical importance to get the best technological properties.
The control cooling conveyor by a proper selection of cooling regime accommodates production of all conventional grades of steel as well as special product grades with alternative cooling modes. It also optimizes processing of carbon and alloy steel grades when used in conjunction with the reducing and sizing mill for low-temperature rolling and controlled cooling, producing a very good combination of properties and dimensional control.
The control cooling conveyor facilitates processing in a wide range of conditions, including both fast-cooling and slow-cooling modes within a single system. This capability enables the wire rod mill to produce a broad spectrum of plain carbon and alloy steels, as well as stainless steels and other specialty grades. The control cooling conveyor results in improved as-rolled properties of the wire rods, which enable the production of more grades in a directly usable condition, and reduce or eliminate downstream processes, such as, spheroidize annealing.
In-line thermo-mechanical treatments – In the present day environments, the wire rod users have become more and more demanding in terms of overall quality of the product and, for specific application (e.g. tire cord), dedicated process routes are needed to be applied to ensure the required performances. The application of in-line heat treatments in the wire rod mills has deeply modified the process dimensioning approach for the rolling of the special steel wire rods. The in-line heat treatments combine simultaneously process flexibility, to increase the added value to the final product, and high productivity and material yield, to minimize production cost and environmental impact.
Thermo mechanical treatment in the wire rod mill refines the final grain size as a result of dynamic recrystallization. Combined with final in-line water cooling and the superior controlled cooling on the controlled cooling conveyor system, thermo mechanical treatment plays a significant role in determining final product properties. This is particularly beneficial for low- alloyed and medium-alloyed steel products which are subsequently spheroidize-annealed during downstream processing. The ability to strongly control grain size also influences subsequent transformation to hard products such as bainite and martensite by shifting the transformation start time and temperature. Thus, thermo mechanical treatment can minimize direct downstream cold working and reduce annealing times.
The combination of processing on the controlled cooling conveyor system and low rolling temperatures provides the capability to reduce hardenability in some critical grades of wire rods. Ultimately this promotes ferrite formation and retards the evolution to bainite and martensite. The refined grain size achieved through thermo mechanical treatment improves diffusion during heat-treating and can result in reduced heat treatment times and temperatures. For those rods which are not heat-treated, the refined and complex structures increase tensile pickup during cold deformation, producing several advantages such as (i) reduced as-rolled tensile strength, (ii) improved downstream response, and (iii) increased work hardenability. The improvements stem from grain refinement and micro-structural control. The good control of the cooling process at the controlled cooling conveyor system combined with the reduced hardenability of the wire rods makes the process very stable and reduces the chance of forming unwanted hard phases.
Control system for the control of technological properties and scale optimization – The control of rolling temperature is a key aspect to achieve constant strain loads, optimum dimensional tolerances, elevate technological and metallurgical characteristics, improved product homogeneity, and adequate scale properties, both in terms of quality and quantity.
In wire rod mills normally, water cooling is carried out in strategic positions to manage rolling and coiling temperatures, allowing adequate time for the self-tempering of the bar, thus ensuring minimum temperature difference between surface and core at wire rod blocks inlet. For some steel grades, the optimal conditions are normally represented by what is generally called ‘normalizing rolling’ temperature range, finely controlled by the temperature close-loop system which is to manage the high pressure water cooling nozzles.
Steel grades which are very sensitive to any improper control of rolling and coiling conditions can lead to problems either substantial, affecting the final technological properties and the following drawing operations (e.g. density of pearlitic colonies), or merely aesthetical, as the ‘red rust’ aspect.
There are two kind of iron oxide formed during the wire rod production. One of them is the primary scale, while the second one is the secondary scale. The primary scale is formed in the re-heating furnace before wire rod rolling on the surface of steel billets and is removed in the descaler. The secondary scale is formed during the wire rod rolling and after laying on the control cooling conveyor. The structure of secondary oxide scale of the wire rod is composed of three layers namely (i) wustite (FeO), magnetite (Fe3O4) and hematite (Fe2O3) from the inner to the outer layer. However, for some steel grades only two layers are substantially present, because of the low amount of hematite. The thickness of such scale is not-linearly proportional to temperature and time of oxidation i.e. over 900 deg C and especially in the first 20 seconds of oxidation the growth of FeO is fast, then it is more linear, while the thickness of Fe3O4 remains approximately constant. Further, the oxide thickening has a high rate at all temperatures except when the temperature is reached 650 deg C. After this point the oxidation rate slows and the thickness of the scale remains almost constant or grows very slowly. Hence, the thickness of the secondary scale is very much dependent on the mode of cooling in the control cooling conveyor system.
According to the needs of fastener industry, the scale quality and quantity is an important aspect to be controlled by a proper thermal treatment. During further processing of the wire rods, either mechanical or chemical descaling is used. In order to ensure optimal wustite scale and to facilitate scale removal by mechanical descaling before drawing, high coiling temperatures ( higher than 900 deg C) are suitable, while lower temperatures (around 850 deg C) are used for facilitating the chemical descaling, since in that case thin and dense scale is formed to reduce the metal loss and the pickling time. Anyhow, the best overall technological properties of the two cooling stages (forced water cooling during/after rolling and accelerated cooling in the cooling conveyor) are required to be controlled.
In recent years, for productivity, economic and environmental reasons, the requirements of wire rods suitable for mechanical descaling have been increased, because of the improved technologies available for scale mechanical removal. For some applications a perfect scale-free surface is required, so chemical descaling is used.
Handling of wire rod coils – Since wire rods coils are in most instances transported in an unwrapped condition, they are therefore affected by rust. They are sometimes stored in the open prior to their transport, so it is not uncommon to observe water dripping out of the bundles when they are transported. This is a hot roller product which is subjected to further processing in order that it might be directed to a large range of end uses, such as the manufacture of nails, galvanized wire for fencing (including barbed wire), road mesh, and wire for pre-stressed concrete to mention a few applications.
Many dispatches of the wire rods are eventually destined to be cold drawn. During this process the wire is forced through dies which reduce its gauge, and cause it to increase in length. Because of this, kinks and nicks in the wire are inadmissible, as when being drawn through the dies the wire can break. Even if the wire is not for redrawing such defects are undesirable, e.g. in the manufacture of road mesh, as these imperfections show up in the finished product. Disintegration of bundles during the transport, caused by bad stowage, crushing and breakage of the strapping bands, is to be avoided as this leads to the loose turns of the wire rods which develop into tangling, intertwining and twisting of the wire rods. As a result of this, parts of the coils may have to be cut off and scrapped. If this is not the ultimate solution, depending upon the uses to which the wire rods is intended, tangling and twisting of the turns in the bundles results in loss of time on the production line.
When the wire rod coil is wrapped, this is an indication that the wire rods are destined for a fabrication of a more delicate nature, e.g. wire for musical instruments. Special steels wire rod coils are usually protected from corrosion and mechanical stresses (e.g. scratching and buckling) and are generally provided with multilayer packaging using corrosion protection (e.g. oiled or ‘vapour corrosion inhibitor’ paper) or film-coated packaging paper and plastic films.
Wire rod coils are to be handled carefully owing to its sensitivity to mechanical damage. Damage can be prevented by correct handling and the use of suitable handling and slinging equipment (e.g. crossbars, C hooks, coil mandrels, webbing slings, and chain slings etc.). Lifting or setting down the wire coils with excessive force results in distortion, which is detrimental to further processing, since the wire coils can no longer be properly unwound and further processed.
Wire rod coils are to be transported in vehicles or railway wagons having a headboard and side walls (stanchions) with sufficient strength and loading capacity. Nonslip materials are also to be placed under the load and between layers. Gaps in the load are often unavoidable due to the handling methods used and vehicle characteristics (load distribution), so the load is to be secured in accordance with anticipated accelerations by direct securing (e.g. tight fit, loop lashing) and/or by frictional securing (e.g. tie-down lashing).