Why steel coils are called Wire rod?

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It's an arbitrary designation. Wire is smaller in diameter than rod, rod is smaller than bar and bar is smaller than billet.

In general, steel stock thinner than rod can be shipped from the mill in coils and then easily straightened at the point of use. You cannot do that with anything thicker than wire.

Wire Rod

Wire Rod

Wire rod is a finished product in a steel plant rolled from the billet in a wire rod mill. It is mainly used for the production of steel wire which is then subjected to further processing. The steel for wire rod is produced by both the steelmaking processes namely the basic oxygen steelmaking, and the electric furnace steelmaking. Steel wire rod is normally cold drawn into wire suitable for further processing such as drawing, for cold rolling, cold heading, cold upsetting, cold extrusion, or cold or for hot forging.

Wire Rod is used for many products. It is the raw material for the wire drawing units. Different uses for the wire rod include wire ropes, springs, electrodes, barbed wires, steel reinforcement for aluminum conductor and pre-stressed concrete, wire mesh, fasteners, automobile components and hardware manufacturers etc. Some of the products of the wire rods are shown in Fig 1.

Fig 1 Some of the products of the wire rods

Although wire rod can be produced in several regular shapes (such as square, hexagonal etc.), the majority production of wire rod is in round cross-section. Round wire rod is normally produced in nominal diameters of 5 mm to 19 mm, advancing in increments of 0.5 mm. As the wire rod comes off the rolling mill, it is formed into coils. These coils are secured either tied with a wire rod or strapped with a strapping band. In each coil, wire rod is continuous without any break. Internal diameter of a wire rod coil normally varies in the range of 810 mm to 910 mm depending on the mill equipment. The external diameter of the wire rod coil depends on its weight and is normally in the range of 1,100 mm to 1,300 mm. The coil weight can vary from mill to mill in the range of 600 kilograms to 2.5 tons. A longer and larger cross section billet produces heavier coils. Wire rod coils, the method of coil securing, and the dimensions defining the coil is shown in Fig 2.

Fig 2 Wire rod coils

Depending on the capabilities of the wire drawer’s equipment and machinery, the requirement of coil weight can be restricted. Wire rod coil is held in a unit with at least four steel straps or four steel tying rods in the transverse direction. Before the steel strapping or tying rod is applied, the wire rod is to be sufficiently compressed. The strapping/tying rod is fixed in the transverse direction with a single circumferential strap so that the strapping does not slip and cause the coil to come apart. A number of lower weight wire rod coils are normally unitized and strapped to facilitate their handling at the mill end and in transport. In this case each unit is referred as a unitized coil of wire rods.

Wire rods are rolled in a wire rod mill. A schematic view of a wire rod mill is shown in Fig 3.

Fig 3 Rolling of wire rods in a wire rod mill

Plain carbon steel wire rod

Plain carbon steel wire rods are produced in various grades, or compositions namely (i) low-carbon steel wire rods (maximum carbon content of 0.15 %), mild steel wire rods (carbon content in the range of 0.15 % to 0.25 %), medium carbon steel wire rods (carbon content in the range of 0.31 % to 0. 60 %), and high carbon steel wire rods (carbon content higher than 0.65 %). Normally, sulphur and phosphorus contents are kept within the usual limits for each grade of steel, while carbon, manganese, and silicon contents are varied according to the mechanical properties needed.

Wire rods for the production of carbon steel wire is produced with implementing the controls and inspection procedures intended to ensure the degree of soundness and freedom from injurious surface imperfections necessary for specific applications.  Plain carbon wire rods are produced in various qualities as described below.

Industrial quality wire rod – It is produced from low-carbon or mild steel (carbon content less than 0.22 %) and is intended mainly for drawing into industrial quality wire. It is used for the production of various kinds of steel wires, nails, and wire entanglement.

Chain quality wire rod – Wire rod for the production of wire to be used for resistance welded chain is made from low carbon and mild steel produced by practices which ensure their suitability for drawing into wire for this end-use. Good butt welding uniformity characteristics and internal soundness are essential for this application. Wire rods for the production of wire to be used for fusion welded chain can be produced from specially selected low-carbon steel.

Fine wire quality wire rod – It is suitable for drawing into small-diameter wire either without intermediate annealing treatments or with only one such treatment. Wire rods of 5 mm in diameter can be direct drawn into wire as fine as 0.9 mm without intermediate annealing. Wire finer than 0.9 mm, for such products as insect-screen wire, weaving wire, and florist wire, is usually drawn in two steps first reducing to an intermediate size no smaller than 0.9 mm, followed by annealing and redrawing to final size. Fine wire quality rod is generally rolled from steel of very low carbon steel grades produced using techniques to provide good surface finish and internal cleanliness. In addition to these precautions, the wire rods are subjected to tests such as fracture or macro-etch tests.

Cold finishing quality wire rod – It is intended for drawing into cold finished products. The production of such product is controlled to ensure suitable surface conditions.

Cold heading, cold extrusion, or cold rolling quality wire rod – This is the rod used for the production of cold heading, forging, cold extrusion, or cold rolling quality wire. The wires are used for making bolts, nuts or screws used widely in automobile parts and industrial machines. The wire rod is produced by closely controlled manufacturing practices. It is subject to mill testing and inspection to ensure internal soundness and freedom from injurious surface imperfections. In the production of wire rod for heading, forging, or cold extrusion in killed carbon steels with nominal carbon contents of 0.16 % or more, both austenitic grain size and decarburization is to be controlled. Such steels can be produced with either fine or coarse austenitic grains depending on the end-use.

Wood screw quality wire rod – It includes low-carbon resulphurized and non-resulphurized wire rod for drawing into wire for the production of slotted-head screws only, not for recessed-head or other special-head screws.

Scrapless nut quality wire rod – Wire rod to be drawn into wire for scrapless nuts is produced by specially controlled production practices. It is subjected to mill tests and inspection designed to ensure internal soundness, freedom from injurious segregation, and injurious surface imperfections, and satisfactory performance during cold heading, cold expanding, cold punching, and thread tapping. Wire rod for scrapless nut wire is normally made from low-carbon, resulphurized steels. Non-resulphurized steels are also used, but these steels normally are made only in grades containing more carbon than the resulphurized grades and with the phosphorus content of 0.035 % maximum and sulphur content of 0.045 % maximum. In the resulphurized steels, the specified sulphur range is either 0.08 % to 0.13 % or 0.04 % to 0.09 %.

Severe cold heading, severe cold extrusion, or severe scrapless nut quality wire rod – It is used for severe single step or multiple-step cold forming where intermediate heat treatment and inspection are not possible. Wire rod of this quality is produced with carefully controlled production practices and rigid inspection procedures to ensure the required degree of internal soundness and freedom from surface imperfections. Fully killed fine-grain steel is normally required for the most difficult operations.

Welding-quality wire rods – These rods are used for general welding electrode wire, for CO2 gas shielded arc welding wire and for submerged arc welding wire used as filler metal. Welding-quality rod can be made from billets of low-carbon killed steel. It is produced to several restricted ranges and limits to chemical composition. An example of the restricted ranges and limits for low-carbon, arc welding wire rod is carbon – 0.10 % to 0.15 %, manganese – 0.40 % to 0.60 %, phosphorus – 0.025 % maximum, sulphur- 0.035 % maximum, and silicon – 0.030 % maximum.  Precise component control is required to ensure weldability and deposit metal quality. The wire rods are produced by the slow cooling operation on the cooling conveyor and the technical specification deviation control to ensure the rod drawability.

Medium carbon and high carbon quality wire rods – These wire rods intended for drawing into such products as strand wire in wire ropes, lock washer wire, tire bead wire, precision spring wire, rope wire, screen wire (for heavy aggregate screens), aluminum cable steel reinforced core wire, and pre-stressed concrete wire. These wire qualities are normally drawn directly from control-cooled rod. When drawing to sizes finer than 2.0 mm (from 5 mm wire rod), it is customary to employ in-process heat treatment before drawing to finish size. Medium carbon and high-carbon quality wire rod is not intended for the production of higher-quality wires such as music wire or valve spring wire. High carbon steel wire rod needs the micro structure of fine pearlite to maintain its high strength and to ensure its wire drawability.

Spring steel wire rod – The main usage of spring steel quality wire rod is for coil springs and stabilizer bars. There are two types of spring steels namely (i) coil spring and torsion bar produced through hot forming and used for automobile suspension system, and (ii) wire spring made by cold forming and used for electronics, precision machinery and furniture. Wire rods for springs are to have high elasticity limits to prevent plastic deformation. It is also to possess defect free surface to ensure high fatigue limits and good dimensional precision for improved operation.

Bearing steel quality wire rod – Bearing steel quality wire rod requires a high degree of durability to withstand repeated loads and high speed rotations during operation. This type of wire rods are used in a variety of industrial machine parts, including ball bearings and ring bearings.

Wire rod for tire cord – This quality of wire rod is drawn in the range of 0.15 mm to 0.4 mm. Tire cords are usually made from high carbon steel wire rods. Steel cord is used to reinforce the durability of automobile tires. This type of wire rod requires particularly clean and high strength steel.

Wire rod for music wire – These wire rods are often used for production of high strength bead wire, Low relaxation pre stressed concrete (LRPC) steel wire and music wire. This quality of wire rod is made from high carbon steel with excellent filamentary drawability, high strength and fatigue resistance.

Wire rod for high strength steel (HSS) – This pre-stressed concrete steel rod is used in reinforcing the concrete for telegraph poles and piles. Structural carbon steel containing traces of boron, or a large amount of silicon, undergoes high frequency heat treatment to produce this quality of wire rods. Compared with mild steel, this type of steel has higher elastic limits and lower relaxation.

The wire rods for special purposes require unique characteristics necessary for the specific application. Some of these wire rods are made to standard specifications, while the others are made to proprietary specifications which are mutually acceptable to both the producer and the user.

Wire rod for music wire, valve spring wire, and tire cord wire is rolled and conditioned on cooling conveyor to ensure the lowest possible incidence of imperfections. Surface imperfections are objectionable because they lower the fatigue resistance which is important in many of the end products made from these wires. Internal imperfections are objectionable since they make the wire rod unsuitable for cold drawing to high strength levels and the extremely fine sizes required. Wire rods for concrete reinforcement are produced from steel chemical compositions selected to provide the mechanical property requirements as per relevant standards. This quality rod is produced in coils. Wire rods for telephone and telegraph wire is produced by practices and to chemical compositions intended for the production of wire having electrical and mechanical properties which meet the requirements of the various grades of this type of wire.

Alloy steel wire rods

Alloy steel wire rod for machine structural use – Wire rods made from alloy steel is mainly used for the joints of mechanical parts, such as high strength bolts and nuts. It is also used for the delivery of driving force. It is normally alloy steel having high strength due to the addition of alloying elements such as chromium, nickel and molybdenum etc.

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Free cutting carbon steel wire rod – This type of wire rod is made from free cutting steel where free cutting elements such as phosphorus, lead, bismuth, and sulphur are added. This steel type of wire rod is used in precision parts of automobiles and in components of home appliances.

Stainless steel wire rod – Stainless steel wire rod is used primarily to produce stainless steel wire. The stainless steel wire is then used to produce downstream products such as industrial fasteners, springs, medical and dental instruments, automotive parts, and welding electrodes.

Special requirements for steel wire rod

Some of the qualities of the wire rods have special requirements for the production and the testing of the wire rod. A few of the more common requirements are described below. For some applications, it can be necessary to add one or more special requirements.

Macro-etch test – It is deep-etch testing to evaluate internal soundness. A representative cross section is etched in a hot acid solution.

Fracture test – In fracture testing, a specimen is fractured to evaluate soundness and homogeneity.

Austenitic grain size requirements – For applications involving carburizing or heat treatment, austenitic grain size for killed steels is to be specified as either coarse (grain size 1 through 5) or fine (grain size 5 through 8 inclusive).

Non-metallic inclusion testing – It comprises a microscopic examination of longitudinal sections of the rod to determine the nature and frequency of the non-metallic inclusions.

Decarburization limits – These are specified for special applications when required. A specimen is polished so that the entire cross-sectional area is in a single plane, with no rounded edges. After etching with a suitable etchant, the specimen is examined microscopically (normally to 100 magnification), and the results are reported in hundredths of a millimeter. The examination includes the entire periphery, and the results reported are to include the amount of free ferrite and the total depth of decarburization.

Mechanical property requirements – The mechanical property requirements of the wire rods as per the relevant standards are to be met by controlling the rolling and control cooling parameters.

In wire rod mills, because of the controlled cooling facilities, the intra coil variation is kept to a minimum. The finishing temperature, cooling of water, cooling air, and conveyor speed all are balanced to produce rod with the desired scale and microstructure. This structure, in turn, is reflected in the mechanical properties of the rod and permits the rod to be drawn directly for all but the most demanding applications. The primary source of intra coil variation on the wire rod mills is the overlapping of the coiled rings on the conveyor. These overlapped areas cool at a slower rate than the majority of the ring.

Wire rod defects

During the rolling of the wire rod, steel is heated above its recrystallization temperature and is passed through several grooves in the rolling mill. There are some common defects which occur in wire rods which can be mostly seen by naked eyes or by magnifying glass after being etched. Surface defects produced during wire rod production results into the rejection of either at wire rod stage or during the further processing of the wire rods. Hence, the defects of wire rods are to be reduced as much as possible if they cannot be eliminated. There are a number of common types of defects in the wire rods. Some common defects are laps, fins, cracks, roughness, slivers, rolled in scale, shearing, scratches, scabs, roll marks, fire cracks transfer marks, mechanical damage, decarburization, banding, segregation, and inclusions etc. These defects are described below. Wire rod defects are shown in Fig 4 and Fig 5.

Fig 4 Defects in wire rods

Laps – Laps are discontinuities in the material which generally run more or less at an angle to the surface of the wire rod. Laps always run longitudinally on the wire. Laps are discontinuities in the material which generally run more or less at an angle to the surface of the rod. Laps can also be defined as longitudinal crevices at least 30 degrees off radial, created by folding over, but not welding, material during hot working. Laps are usually quite long and of uniform shape.

Laps can be detected by parallel double lines which are running longitudinally along the direction of rolling. Laps occur during the overfilling of passes, in misaligned entry guides, during guide failures of entry guides to hold and feed the bar centrally. They nearly always run longitudinally in the rod, one or more similar defects occurring uniformly distributed at the periphery of the wire rod. Sometimes, they also appear as parallel double lines.

Laps can be detected with the naked eye or with a low magnification. Laps show ragged, longitudinal, and occasionally curved appearance. Microscopic examination shows the slanted nature of the defect, with generally more decarburization on upper portion. Determination of depth, length, orientation, and shape gives the most precise indication of lap origin. Upset tests of lapped material cause a ragged longitudinal split. Torsion tests can detect laps using a small number of cycles.

Pass overfilling is the most frequent cause of laps when the material forced out into the roll gap folds over and is rolled into the rod surface during subsequent passes. Passes are overfilled when too large a reduction is attempted, or the wrong pass design is used. Laps can also occur when the roll pass is not adequately filled so that the ‘lean’ place in the section falls over in the roll pass. Laps on only one side of the section can also be caused by badly aligned guides. Laps are not particular to any type of steel, but are generally a result of poor workmanship. Incoming billet section quality is an important factor in preventing this defect. Any abrupt longitudinal discontinuity can become a lap upon subsequent rolling. Overfill, scratches, segregation, inclusions of extraneous matter are typical sources which are to be eliminated. Corrective action includes more uniform heating, selecting less severe roll pass sequences, or reducing inter-stand tension.

Fins – Fins are protrusions on one or both sides of a bar caused by the section being too large for the pass. If the defect is on one side only, it is normally referred to as ‘off the hole’. Fins generally occur when a groove is overfilled. Overfilling can occur when the rolls are not set properly or the reduction is too large. This defect normally occurs along the entire wire and occurs due to the due to the overfilling of finishing pass. Fins can be noticed by protruded portion formed at the side of the wire rod and along with it. Fins can be detected visually.

Cracks – Cracks are discontinuities in the material which penetrate the wire rod vertically or at an angle to its surface. They vary in length and are normally rectilinear. Occasionally, however, they run at an angle to the longitudinal axis. Large and medium size cracks can be detected with the naked eye or with a low magnification when the surface of the wire rod is chemically or mechanically descaled. Depth of crack can be determined by filing or grinding. More precise results can be obtained from microscopic observation since the etched section also provides indications of the origin of the crack. Cracks can also be revealed without prior descaling by means of torsion tests, using a small number of reversals. Cold and hot upsetting tests or eddy current methods can also be used to detect cracks. The causes of cracks in hot rolled wire rod can be found at any point in the production cycle from steelmaking to where the product leaves the wire rod mill.

Roughness – Roughness is sometimes mistaken as pitting. Roughness consists of continuously recurring, irregular depressions and elevations on the surface of the wire rod. Surface roughness on descaled specimens can easily be detected with the naked eye or with a low magnification. The degree of roughness can be determined by microscopic examination or with roughness depth meters. A rough wire rod surface is normally caused by severe roll groove wear, particularly in the last two forming stands. Even after rolling, the initially smooth surface can still become rough if the wire rod scales too severely because of cooling too slowly. If the rod is stored for lengthy periods in a damp or corrosive atmosphere then surface roughness is caused because of corrosion.

Slivers– Slivers are elongated pieces of metal attached to the base metal at one end only. They normally are hot worked into the surface and are common to low strength grade steels, which are easily torn, especially the grades with high sulphur, lead or copper contents. Slivers are generally detected visually. Micro examination reveals other details such as non metallic inclusions, slag pits, etc. Straightening can expose a sliver to the surface and make it more prominent. Slivers often originate from short, rolled out point defects. Fins and deep ridges from billet conditioning can also cause slivers and are to be avoided. Slivers seem to occur more on mills with higher rolling speeds. Slowing down the mill operation helps in minimizing of the defect.

Rolled in scale – Scale is oxide layer of varying thickness and colouring on the surface of the wire rod which can cling loosely or adhere firmly. Rolled in scale forms an irregular impression in the surface of the wire rod and is caused by incomplete descaling after the heating operation. It can cause a pitted surface on the wire rod. The defect results in wire rod surface irregularity. It can be detected visually. By rubbing with an abrasive tool to remove uniform depth scale, the underneath surface can be revealed. The occurrence of rolled in scale is most dependent on the adherence and not the amount of primary scale produced. Scale adherence is a function of steel composition, furnace heating practices, and prior surface condition of the incoming billets. Modifications sometimes are made to roll pass designs to promote more complete scale breaking and removal. Slab or box passes are the best roughing passes for scale removal.

Shearing – Shearing occurs when a longitudinal strip of the base metal is torn off the wire rod during rolling. The strip is often reattached as rolling continues, although not necessarily to the same rod. Shearing can refer to either the discontinuity caused by detachment or subsequent reattachment. Shearing is visually detected. Microscopic examination sometimes indicates an overheated condition between grains. Excessive rubbing of steel as it rolls through the mill causes overheating, shearing material which is later picked up from mill components on the same or another rod. Mill adjustments can reduce sources of frictional heating. Improved guiding, pass design, and section control can reduce incidents of shearing.

Scratch – Scratches are furrow like depressions which always run longitudinally. They vary from minute sharp almost crack like indentations to large, flat bottomed furrows with partly projecting or overlapping edges according to the source of the defect. Scratches are detected visually and are caused by unintentional contact with build up on mechanical parts and mill components during rolling. Scratches are due to the scoring of the stock by sharp or pointed objects. The defect typically has a more rounded bottom and less scale than a seam or crack. The defect can be detected with the naked eye or with low magnification, even in scaled condition. It seldom opens up in upsetting or torsion tests. Micro examination can distinguish a scratch from a seam, lap or crack. The defect can be caused by the uneven surface of guide parts on which scale or particles of the rolled product have built up. Poorly machined, worn or broken guides can also cause scratches.

Scabs – Scabs are irregularly shaped, flattened protrusions caused by splash, boiling, or other problems from casting, or conditioning. They occur prior to rolling. Scabs have scale and irregular surfaces beneath them. They tend to be round or oval shaped and concentrated to only certain billets. Scabs are generally ductile when bent. If material is brittle, it can be rolled in scale.

Fig 5 Defects in wire rods

Roll marks – Roll marks are ‘embossed’ elevations or depressions usually recurring periodically and varying greatly in shape and size. The defect can be usually be detected with the naked eye or with a low magnification on the scaled or descaled specimen. If the defect occurs as elevations on the surface of the rolled product, it is caused by depressions of various kinds in the rolls themselves or in the pinch rolls. Depressions in the wire rods are caused by elevations on such installations, e.g. chips and remnants of scale.

Fire crack transfer marks – Fire crack transfer marks are patterns of elevations which recur periodically and run at right to the direction of rolling. They can be easily detected with naked eye or with low magnification because of their characteristics. During hot rolling, the surface of the rolls is subjected to continuous heating and quenching. With inadequate cooling and the use of unsuitable roll material, stress cracks can occur in the roll grooves. These crack depressions in the roll surface leads to elevations on the rolled product. Although fire crack transfer marks are smoothed in subsequent passes, they can lead to other surface defects such as cracks and laps.

Mechanical damage – Striations, abrasion and gouging of the rod surface and bruising of the cross section are classed as mechanical damage. Also, due to the mechanical damage, sections of entire coils are kinked or distorted. The defect can be recognized with the naked eye. After leaving the finished stand, the wire rod comes into contact with many mechanical auxiliary and conveying installations such as driving mechanisms, reels push-off gear, suspension chains, conveyor belts, hook conveyors transfer and collecting gear. During all these operations, the wire rod is in danger of being getting damaged.

Decarburization – Decarburization occurs due to the excess heating in furnace and refers to the removal of carbon from the outer surface of the billet due to continuous oxidation. The removal of carbon occurs by partial as well as complete decarburization which determines the total length of decarburized portion of the wire rod. It is not to exceed 1 % of the total diameter of the rod. It is detrimental to the wear life and fatigue life of steel heat-treated components. Decarburization layer can be observed under microscope.

Banding – In the hot rolled low alloy steels, pearlite and ferrite are arranged in the wide layers. In longitudinal section, this arrangement is visible as a banded structure. Banding is the defect observed in the wire rod during the time of cracking where in the inter-ferrite distance increases with thickening of the pearlitic deposition in the rod. It can be detected as the lamellar streaks of ferrite and pearlite observed under microscope. It occurs due to the slow cooling on cooling conveyor. Banding can lead to upset failure.

Segregation – After hot rolling, the presence of segregation in the centre of wire rod can lead to a non-uniform transformation, resulting in bands of martensite in the microstructure. This is considered to be a defect, called centre-martensite. Segregation can be observed at the centre of the cross section of the wire rod. Segregation occurs during continuous casting at the time of solidification of the strand.

Inclusions – Inclusions are a piece of foreign material in the cast part. An inclusion can be a metallic, inter-metallic or nonmetallic piece of material in the metal matrix. Inclusion can be observed along the rolling direction, along through the rod in microscope at the 100 magnification. It can occur due to the entrapping of the impurities in the mould during continuous casting.

Grain size – The grain size of the wire rod is determined by the micro structure. The grains can be smaller or bigger depending on the time and nature of the cooling on the cooling conveyor.

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