Characteristics of Steel Wire Rods
Characteristics of Steel Wire Rods
Steel wire rods are the key product of steel industry with multiple uses. Wire rods are hot rolled steel products produced from a semi-finished steel normally a billet. They are having a circular, rectangular, hexagonal or other cross-section. Majority of the wire rods rolled are round in cross section. Round wire rods are normally produced in nominal diameters of 5 mm to 15 mm, advancing in increments of 0.5 mm. ISO 16124:2015 gives diameters of round steel wire rods/ rounds ranging from 5 mm to 60 mm, advancing in increments of 0.5 mm up to 20 mm and thereafter in the increments of 1 mm. As the wire rod comes out of the rolling mill, it is formed into coils. Also, thermo-mechanically treated (TMT) reinforcement bars of 6 mm, 8mm and 10 mm basically needed for the building construction are produced in the wire rod mills.
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. The end products are used in several cases as vital parts in various industrial fields. Wire rods are normally drawn down to a specific diameter before being subjected to forging or other forming operations in secondary processing. In several cases, the size of the wire rods before being subjected to these forming operations is to be less than the minimum size of 5 mm which cannot be supplied as rolled from the wire rod mill.
Unlike cold-rolled sheets, heavy plates, pipes, sections, and other steel products, wire rods are seldom used as hot rolled for the production of the final products, but they are manufactured into machine parts after undergoing one or more stages of so-called post-processing such as heat treatment, forging, and wire drawing at specialized plants. For this reason, every product of wire rods is developed with due attention to its behaviour at the post-processing stages. The necessary requirement from wire rod products is good processibility and fulfillment of required properties after the processing. Also, since the costs of the post-processing is sometimes several times the price of the hot-rolled steel material, it is increasingly important to reduce the total integrated manufacturing cost from the steel material to final product.
Wire rods are used for several products. They are the raw material for the wire drawing units and hardware manufacturers. They are used for the components needed for the automotive industry, chemical industry, power stations, and machine engineering. They are used for the production of fasteners, gears, springs, bearings, wire ropes, chains, cables, wire meshes, wire fencings, tyre cord, electrodes, barbed wire, steel reinforcement for aluminum conductor and pre-stressed concrete, reinforcement in railway sleepers, and other basic components of safety-related parts typically such as automobile engines, drive train systems, and chassis.
Depending on the capabilities of the wire drawer’s equipment and machinery, the requirement of coil weight can be restricted. Wire rods are normally sold in as rolled condition. The user of wire rods prepares the wire rods for further processing by cleaning and coating, or by heat treating. Some of the products of the wire rods are shown in Fig 1.
Fig 1 Some of the products of the wire rods
As the wire rod comes out of the rolling mill, it is formed into coils. 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 normally it is in the range of 1,100 mm to 1,300 mm. The coil weight can vary from mill to mill and normally it is in the range of 600 kg to 2.5 tons. Coil weights which exceed the capabilities of the rolling mill sometimes can be obtained by welding two or more coils together. The standard tolerances for wire rods are +/- 0.3 mm on the diameter for sizes up to 10 mm and +/- 0.4 mm for sizes 10 mm to 15 mm. The standard tolerances for out-of-roundness are 0.48 mm for sizes up to 10 mm and 0.64 mm maximum for sizes 10 mm to 15 mm.
These days, the user of wire rods demands closer dimensional tolerances and improved metallurgical properties in wire rod products besides the three basic requirements which are (i) to eliminate or simplify one or more of the secondary or tertiary processing to improve productivity and reduce processing cost, (ii) to improve the functionality, i.e., to extend the service life, to reduce the weight, or to bring other advantages to the final product for which the wire rod is used, and (iii) to eliminate the use of environmentally harmful substances during post processing processes.
The coils are secured either by tying with a wire or strapped with a strapping band. In each coil, wire rod is continuous without any break. 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. Fig 2 shows the wire rod coils, the method of coil securing (strapping / tying), and the dimensions defining the coil.
Fig 2 Wire rod coils
Wire rod products, in the present global scenario, are characterized by (i) wire rod product grades which are widely varied from standardized ordinary grades for general applications to high grades for high-end applications according to specifications individually defined for each customer, mainly for the automobile and other manufacturing industries, (ii) the high-end wire rod products which are for safety-related applications typically such as automobile engines, drive train systems, and chassis, are used as functional materials and as such, are required to have highly demanding properties such as high strength and long fatigue life, (iii) wire rod products are semi-finished products, which are seldom used as hot rolled, and they are manufactured into final products after secondary and even ternary processing such as forging, heat treatment, and machining, (iv) final industrial products for which wire rods are used are subject to tough competition in the market, and (v) industrial products for which wire rods are used are required to minimize the load on global environment throughout the life cycle.
In developing a new wire rod steel product in consideration of the above, attention need to be paid to the important issues such as (i) whether it is possible to eliminate or simplify one or more of the secondary and ternary processing to improve productivity and reduce manufacturing costs, (ii) whether the new product improves the functionality, extends the service life, reduces the weight or brings about other advantages to the final product for which it is used, and (iii) whether it is possible to eliminate environmentally harmful substances.
These requirements are not always mutually compatible, and it is important to proceed with the development steps based on good understanding of the operating conditions at the processing stages, the usage condition, and characteristics of the final product for which the wire rods are to be used and the like.
Wire rods are used in a wide range of industrial fields. Hence, they have to meet given combinations of properties such as strength, ductility, cold formability, drawability, and hardenability. In addition, present market demands some additional properties in the wire rods which are (i) ultra- precision dimensional tolerances and ovality, (ii) lower scale loss, (iii) negative tolerances (in sectional weight), (iv) no variation in dimensions throughout the length of the wire rod, (v) uniform physical properties, and (vi) uniform weight with nominal variation between front, middle, and back end.
Wire rod quality has become increasingly important these days. For several application of the wire rods, it is important that the wire rods have surfaces without any marks, scratches, shells, cracks, overfills, and oxide particles.
Quality assurance throughout the whole length of the wire rod after rolling takes time and labour, and for this reason, quality control of billets, the materials before rolling, is of great importance. Billets are normally inspected by automatic magnaflux and ultrasonic inspection equipments and conditioned for the removal of the surface defects.
Wire rods are rolled in wire rod mill. The production of wire rods in wire rod mills is subject to constant change. Since the beginning of the 2000s, the technologies and methods used to produce wire rod have undergone considerable changes. The growing demands on the quality of the wire rods as well as on the flexibility and cost effectiveness of the wire rod mills have necessitated the development of new and innovative technologies and processes. innovative solutions have been introduced to produce increased quality of the wire rod products with considerable energy savings and improved performance.
Because of the vast varieties of the requirements from the wire rods, wire rod mills are normally designed with versatility in mind. Reliable equipment, process consistency, production flexibility, and waste minimization, all are needed from a wire rod mill. Further all size ranges, materials, and alloys need to be rolled efficiently and with high productivity in the wire rod mills. The mill is to maximize productivity through process optimization and through minimized downtime and seamless interlinking of production stages. The complex mill layout of the mill needs to be optimized to ensure the time needed for each process step, the required temperature profiles, and efficient transfer areas. Further fast change-over is the key to the flexibility of the mill. The mill is to be able to convert to the next order while still processing the current one and change the line from one product to another with virtually no downtime.
Modern wire rod mills are high speed mills capable of rolling of smaller dimensions at high production rates, while at the same time keeping investments and operating costs at the reasonable levels. As a rule, wire rod mills are designed for an annual output of between 300,000 tons and over 800,000 tons (two-strand mills). These days four-strand wire rod mills are no longer being built because of various limitations these mills bring into production of the wire rods. The mills are capable of rolling at speeds ranging from 50 meters per second to 140 metres per second. Fig 3 shows rolling of wire rod in a wire rod mill.
Fig 3 Rolling of wire rod in a wire rod mill
The range of materials in which wire rods are produced, comprises low to high carbon (C) steels, cold heading steels, wire drawing steels, alloy steels, spring steels, ball bearing steels, electrode quality steels, reinforcement bars, tool steels and stainless steels. Wire rods are categorized by the ‘quality’ according to the end use. Industrial quality wire rods which are made from low carbon or mild steels, account for the majority of wire rods production and consumption. These wire rods are primarily intended for drawing into industrial quality wire which, in turn, used for the manufacture of such products as garment hangers, wire mesh, barbed wire, nails, and fencing etc.
Carbon steel wire rod – Carbon steels are those steels for which no minimum content is specified or required for chromium (Cr), nickel (Ni), molybdenum (Mo), tungsten (W), vanadium (V), cobalt (Co), niobium (Nb), titanium (Ti), zirconium (Zr), aluminum (Al), or any other element added to obtain a desired alloying effect. In these steels the specified minimum copper (Cu) content does not exceed 0.40 %, the specified maximum manganese (Mn) content does not exceed 1.65 %, and the specified maximum silicon (Si) + Cu content does not exceed 0.60 %. In all C steels, small quantities of certain residual elements, such as Cr, Ni, Mo, and Cu, are unavoidably retained from raw materials. These elements are considered incidental, although maximum limits are normally specified for specific end uses.
Carbon steel wire rods are produced in the different grades or compositions consisting of (i) low C steel wire rods having maximum C content of less than or equal to 0.15 %, (ii) medium low C steel wire rods with maximum C content greater than 0.15 % but less than or equal to 0.23 %, (iii) medium high C steel wire rods having maximum C content greater than 0.23 % but less than or equal to 0.44 %, and (iv) high C steel wire rods having maximum C content greater than 0.44 %. Normally, sulphur (S) and phosphorus (P) contents are kept within the normal limits for each grade of steel, while C, Mn, and Si contents are varied according to the mechanical properties desired. Sometimes, S and / or P can be added to the steel to improve the machinability.
Carbon steel wire rods for the production of steel wires are produced with shop controls and inspection procedures intended to ensure the degree of soundness and freedom from harmful surface defects necessary for specific applications. The different qualities applicable to C steel wire rod are described below.
Industrial quality wire rod – It is produced from low C or medium-low C steel and is intended primarily for drawing into industrial quality wire. Wire rods of this quality are available in the as-rolled or heat-treated conditions. Practical limitations for drawing these wire rods are normally (i) low C wire rod of 5.5 mm diameter can be drawn without intermediate annealing to 2 mm by five conventional drafts, (ii) medium-low C wire rod of 5.5 mm in diameter can be drawn without intermediate annealing to 2.65 mm by four conventional drafts.
Chain quality wire rod – Wire rod for the production of wires to be used for resistance welded chain is made from low C and medium-low C steel produced by methods which ensure their suitability for drawing into wire for this end use. Good butt-welding uniformity characteristics and internal soundness are necessary for this application. Wire rod for the production of wires to be used for fusion welded chain can be produced from specially selected low C rimmed steel, but is more frequently made from continuous cast 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 5.5 mm diameter can be directly 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 normally drawn in two steps consisting of (i) reducing to an intermediate size no smaller than 0.9 mm and (ii) followed by annealing and redrawing to final size. Fine wire quality wire rod is normally rolled from steel of grades with C content of 0.08 % maximum and is produced using techniques to provide good surface finish and internal cleanliness. In addition to these precautions, it is preferable to carry out for these wire rods additional tests such as fracture or macro etch tests.
Cold finishing quality wire rod – It is intended for drawing into cold finished bars. The production of such wire rod is controlled to ensure suitable surface conditions.
Cold heading, cold extrusion, or cold rolling quality wire rods – Wire rods used for the processes of cold heading, cold forging, cold extrusion, or cold rolling are produced by closely controlled production practices. These wire rods are subject to shop testing and inspection to ensure internal soundness and freedom from harmful surface defects. Heat treatment as a part of wire rod mill processing is very important in the higher C grades of steel.
For common upsetting, represented by the production of standard trimmed hexagon head cap screws, steel wires drawn from annealed wire rods of steel grades with C content 0.13 % – 0.18 % to steel grade with C content 0.35 % – 0.42 % are suitable. Wire for moderate upsetting, also produced from the same steel grades are to be drawn from spheroidize annealed wire rods or are to be in-process annealed. Wire for severe heading and forging, produced from wire rods of steel grades with C content 0.13 % – 0.18 % to steel grade with C content 0.36 % – 0.44 % are to be spheroidize annealed in process or at finished size. Wire rods of these qualities are not intended for recessed-head or similar special-head applications.
In the production of wire rods for cold heading, cold forging, or cold extrusion killed C steels is used with nominal C contents of 0.16 % or more. Both the austenitic grain size and decarburization are to be controlled in steels used for these applications. Such steels can be produced with either fine or coarse austenitic grains, depending on the type of heat treatment and end-use. The maximum allowable quantities of decarburization are normally defined by the average value for the depth of the layer of free ferrite plus the layer of partial decarburization (the total affected depth).
Wood screw quality wire rod – It includes low C 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.
Scrap-less nut quality wire rod – Wire rods to be drawn into wire for scrap-less nuts are produced by specially controlled production practices. They are subjected to shop floor tests and inspection designed to ensure internal soundness, freedom from harmful segregation, and harmful surface defect for satisfactory performance during cold heading, cold expanding, cold punching, and thread tapping. Wire rods for scrap-less nut wire normally are made from low C resulphurized steels. Non-resulphurized steels are also used. These steels normally are furnished only in grades containing more C than the resulphurized grades and with P content 0.035% maximum and sulphur content 0.045 % maximum (heat analysis). It is customary to produce these steels to a specified S range of either 0.08 % to 0.13 % or 0.04 % to 0.09 %. Because of the practice used in making the steel and the degree to which S segregates, the S content at various locations in a billet can vary from the indicated range.
Severe cold heading, cold extrusion, or scrap-less nut quality wire rod – It is used for severe single step or multiple steps 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 practices to ensure the needed degree of internal soundness and freedom from surface defects. Fully killed fine grain steel is normally needed for the most difficult operations. Normally, the wire made from this quality wire rod is spheroidize-annealed, either in process or after drawing finished sizes. Decarburization limits and the steels to which they apply are the same as those described under cold heading, cold extrusion, or cold rolling quality wire rods.
Welding quality wire rods – They are used to make wire for gas or electric-arc welding filler metal. Welding quality wire rods can be made from billets of low C rimmed, capped, or killed steel, but is preferably made from continuous cast steel. It is produced to several restricted ranges and limits to chemical composition. An example of the restricted ranges and limits for low C arc welding wire rod is C – 0.1 % to 0.15 %, Mn – 0.4 % to 0.6 %, P – 0.025 % maximum, S- 0.035 % maximum, and Si- 0.03 % maximum.
Medium-high C and high C quality wire rods – These are wire rods intended for drawing into such products as strand wire, lock washer wire, tyre bead wire, upholstery spring wire, rope wire, screen wire (for heavy aggregate screens), aluminum cable steel reinforced (ACSR) core wire, and pre-stressed concrete wire. These wire qualities are normally drawn directly from patented or control-cooled rod. When drawing to sizes finer than 2 mm (from 5.5 mm wire rod), it is customary to employ in-process heat treatment before drawing to finish size. Medium-high C and high C quality wire rods are not intended for the production of higher quality wires such as piano (music) wire or valve spring wire.
Wire rods for special purposes – In addition to the C steel wire rod products described above, which have specific quality requirements, several other products are produced, each having the characteristics necessary for a specific application, but for which no specific quality requirement is needed. Some of these products are made to standard specifications while the others are made to proprietary specifications which are mutually acceptable to both the user and the producer. Wire rods for piano wire, valve spring wire, and tyre cord wire is rolled and conditioned to ensure the lowest possible incidence of defects. Surface defects are objectionable since they lower the fatigue resistance which is important in many of the end products made from these wires. Internal defects are objectionable since they make the wire rod unsuitable for cold drawing to high strength levels and the extremely fine sizes needed.
Wire rods for concrete reinforcement – These wire rods are thermo-mechanically treated (TMT) reinforcement bars produced by heat treatment process in the wire rod mill. These wire rods are produced from steel chemical compositions selected to provide the mechanical property requirements as per standards. This quality wire rods are produced in coils.
Wire rods for telephone and telegraph wire – They are produced by practices and to chemical compositions intended for the production of wire having electrical and mechanical properties which meets the requirements of the different grades of this type of wire.
Some of the quality requirements discussed above imply special necessities for the production and testing of wire rods. A few of the more common requirements are (i) macro-etch testing, which is deep-etch testing to evaluate internal soundness, consists of etching a representative cross section in a hot acid solution, (ii) fracture testing in which the wire rod sample is fractured to evaluate soundness and homogeneity.
Austenitic grain size requirements – For applications involving carburizing or heat treatment, austenitic grain size for killed steels are sometimes specified as either coarse with grain size 1 through 5 or fine with grain size 5 through 8 inclusive.
Requirements for heat treatment – When the heat treatment requirements are to be met in the customer’s end product, all heat treatment procedures and mechanical property requirements are to be clearly specified.
Non-metallic inclusion testing – It comprises a microscopic examination of longitudinal sections of the wire rod to determine the nature and frequency of nonmetallic inclusions.
Decarburization limits – These are normally specified for special applications when needed. A sample 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 sample is examined microscopically (normally at 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.
In the older wire rod mills, where wire rod was coiled hot, there was considerable variation of properties within each coil because of the effect of varying cooling rates from the centre to the periphery of the coil. Hence, the hot rolled wire rods were seldom sold to specific mechanical properties because of the inherent variations of such properties. These properties for a given grade of steel varied from the wire rod mill to wire rod mill and were influenced by both the type of mill and the source of steel being rolled.
In the modern wire rod mills, which are equipped with controlled cooling facilities, this intra-coil variation is kept to a minimum. In such mills, finishing temperature, cooling by water, cooling by air, and conveyor speed all are balanced to produce wire rods with the desired scale and microstructure. This structure, in turn, is reflected in the uniform mechanical properties of the wire rod and permits the wire rod to be drawn directly for all but the most demanding applications. The primary source of intra-coil variation on these new 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.
Alloy steel wire rod – Alloy steels are those steels for which maximum specified Mn content exceeds 1.65 %, maximum specified Si + Cu content exceeds 0.60 %, or for which a definite range or definite minimum quantity of any other element is specified in order to achieve desired effects on the properties.
The various qualities of alloy steel wire rod possess characteristics which are adapted to the particular conditions typically encountered during fabrication or service. Production of these steels normally includes careful selection of raw materials for melting, exacting steelmaking practices, extensive billet preparation, and extensive testing and inspection. Sometimes, alloy steel of a special quality is specified for the production of wire rods. Aircraft quality alloy steel can be specified for wire rods intended for processing into critical or highly stressed aircraft parts or for similar purposes. Bearing quality alloy steels are normally specified for wire rods intended for processing into balls and rollers for antifriction bearings. Bearing quality alloy steel is normally specified when buying the standard carburizing grades, or the through-hardening, high C-Cr steel grades. The different standard qualities and products available in alloy steel wire rod are described below.
Cold heading quality alloy steel wire rods – They are used for the production of wires for applications involving cold plastic deformation by such operations as upsetting, heading, forging, or extrusion. Typical applications are fasteners (cap screws, bolts, and eye-bolts), studs, anchor pins, and balls and rollers for antifriction bearings etc. Surface quality requirements are more critical than for cold heading quality. Steel with very uniform chemical composition and internal soundness, as well as special surface preparation of the semi-finished steel is needed. Typical applications are ball joint suspension studs, socket head screws, recessed-head screws, and valves etc.
Welding quality alloy steel wire rods – They are used for the production of wire used as filler metal in electric arc welding or for building up hard wearing surfaces of parts subjected to wear. The heat analysis limits P and S which is 0.025 % maximum each.
Alloy steel wire rod can be produced with special requirements in addition to those needed as per the quality descriptions given above. These special requirements are described below.
Special surface – It entails a product with minimal frequency and severity of seams and other surface defects.
Decarburization limits – These can be specified for special applications. Decarburization limits are the maximum allowable quantities of decarburization as defined by the average value for the depth of the layer of free ferrite plus the layer of partial decarburization (the total affected depth) and the average depth of the layer of free ferrite alone. When limits closer than those specified in standards are needed for the end product, it is sometimes appropriate to incorporate C restoration (recarburization) in the fabrication process. For some applications, the wire rod producer can include C restoration in the mill heat treatment. The method of measuring decarburization is the same as that described for C steel wire rods.
Heat treatment requirements – When the end product is to be heat treated, the heat treatment and mechanical properties are to be clearly defined.
Hardenability requirements – These are normally specified by H-steel designations and hardenability bands.
Austenitic grain size determination – Majority of the alloy steels are produced using fine grain practice. Fine grain steels are useful in carburized parts, especially when direct quenching is involved, and are less sensitive than coarse grain steels to variations in heat treatment practices. Coarse grain steels are deeper hardening and are normally considered more machinable. Austenitic grain size is specified as either coarse grain (grain sizes 1 through 5) or fine grain (grain sizes 5 through 8 inclusive).
Non-metallic Inclusion testing – When the non-metallic inclusion test is specified, it is normally done on billets. Prepared and polished samples are examined microscopically at 100 magnification. Sample locations, number of tests, and limits of acceptability is needed to be established in each case.
Magnetic particle inspection – For alloy steel wire rod and wire products subject to magnetic-particle inspection, it is normal to test the product in a semi-finished form, such as billets (using samples properly machined from billets), to ensure that the heat conforms to the magnetic-particle inspection requirements, prior to further processing. The method of inspection consists of suitably magnetizing the steel and applying a prepared magnetic powder, either dry or suspended in a suitable liquid which adheres to the steel along lines of flux leakage. On properly magnetized steel, flux leakage develops along surface or sub-surface discontinuities. The results of the inspection vary with the degree of magnetization, the inspection procedure (including such conditions as relative location of surfaces tested), the method and sequence of magnetizing and applying the powder, and the interpretation.
Macro-etch testing – Soundness and homogeneity of alloy steel wire rods are sometimes evaluated macroscopically by examining a properly prepared cross section of the product after it has been immersed in a hot acid solution. It is customary to use hydrochloric acid for this purpose.
Wire rods which are produced in wire rod mills are to meet needs for specific applications and quality requirements of the customers. The desired metallurgical properties are imparted by adjusting the chemistry during steelmaking as well as by rolling and cooling practices. The wire rod rolling process determines the rod’s size (diameter) and dimensional precision, depth of decarburization, surface defects and seams, amount of mill scale, structural grain size, and within limits set by the chemistry, tensile strength, and other physical properties. Standards of product quality (e.g., tighter dimensional tolerances, control over residuals, and coil weight) have become tighter in recent times along the entire range of wire rods, largely in response to customer demands for improved performance on the customer’s equipment. Tab 1 gives the quality, end-uses, and important characteristics of wire rods.
|Tab 1 Quality, end-uses, and important characteristics of wire rods
|Electric welded chain
|Butt welding properties and uniform internal soundness
|Cold finishing quality
|Cold drawn wires
|Cold heading quality
|Cold heading, cold forging, cold extrusion products
|Internal soundness, good surface quality, can require thermal treatments
|Non-deformed rods for reinforcing concrete (plain rounds or ribbed)
|Chemical composition and online heat treatment important since they affect mechanical properties
|Insect screen, weaving wire, florist wire etc.
|Wire rods are to be suitable for drawing into wire sizes as low as 0.9 mm without intermediate annealing, internal quality is important
|High carbon and medium carbon
|Wire rope and strand, tyre bead, upholstery spring, mechanical spring, screens, ACSR core, pre-stressed concrete strand, pipe wrap wire
|Needs thermal treatment, it is not intended to be used for piano wire or valve spring wire
|Industrial (standard quality)
|Nail, garment hangers, mesh for concrete reinforcement, fencing, barbed wires etc.
|Can be drawn a limited number of times before requiring thermal treatment
|Piano wire, music spring wire
|Springs subject to high stress, valve springs
|Internal soundness, good surface quality
|Scrap less nut
|Fasteners produced by cold heading, cold expanding, cold punching, and thread tapping
|Internal soundness, good surface quality
|Tread reinforcement in pneumatic tyres
|Restrictive requirements for cleanliness, segregation, decarburization, chemistry, and surface imperfections
|Wire for gas welding, electric arc welding, submerged arc welding, and metal inert gas welding
|Restrictive requirements for uniform chemistry
Since wire rods coils are in majority of the cases transported in an unwrapped condition, they are hence normally 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 rolled product which is subjected to further processing in order that it can 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.
Several 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 ‘loose turns’ of 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 are 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 goods are destined for a fabrication of a more delicate nature, e.g., wire for musical instruments. Special steels wire rod coils are normally protected from corrosion and mechanical stresses (e.g., scratching and buckling) and are normally provided with multilayer packaging using corrosion protection e.g., oiled or VCI (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, chain slings). 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. Non-slip material is 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 latching) and / or by frictional securing (e.g., tie-down latching).
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 esthetical, 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), (ii) magnetite (Fe3O4), and (iii) hematite (Fe2O3) from the inner layer to the outer layer. However, for some steel grades only two layers are substantially present, because of the low quantity 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 needed, so chemical descaling is used.
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
Fig 5 Defects in wire rods
Laps – Laps are discontinuities in the material which normally run more or less at an angle to the surface of the wire rod. Laps always run longitudinally on the wire 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 rolling. Laps are normally 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 normally 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 normally 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 normally 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 rod and occurs due to the overfilling of the 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 achieved 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 normally detected visually. Micro-examination reveals other details such as non-metallic inclusions, and slag pits etc.
Straightening can expose a sliver to the surface and make it more prominent. Slivers frequently 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 the 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 irregularly. 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 quantity 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 frequently 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. The tend to be round or oval shaped and concentrated to only certain billets. Scabs are normally ductile when bent. If material is brittle, it can be rolled in scale.
Roll marks – Roll marks are ‘embossed’ elevations or depressions normally recurring periodically and varying greatly in shape and size. The defect can be normally detected with the naked eye or with a low magnification on the scaled or descaled sample. 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 angle 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 several mechanical auxiliary and conveying installations such as driving mechanisms, reels push-off gear, suspension chains, conveyer belts, hook conveyers, transfers, 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, 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.