Steel Wire Rods
Steel Wire Rods
Steel wire rod is a semi-finished product which is rolled from steel billet in a wire rod mill and is used primarily for the manufacture of wire. For a steel plant it is a finished product. The steel for wire rod is produced by all the modern steelmaking processes, including the basic oxygen and electric furnace processes. Steel wire rod is usually cold drawn into wire suitable for further processing such as cold rolling, cold heading, cold upsetting, cold extrusion, cold forging, or hot forging.
Although wire rod may be produced in several regular shapes (round, square, hexagonal, and rectangular), most of the wire rods rolled are round in cross section. Round wire rod is usually produced in nominal diameters of 5 mm to 15 mm, advancing in increments of 0.5 mm. ISO 16124:2004 gives diameters of round steel wire rods ranging from 5 mm to 50 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. The coils are secured either tying with a wire or strapped with a strapping band. In each coil, wire rod is continuous without any break. Internal diameter of a wire rod coil usually 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 usually it is in the range of 1100 mm to 1300 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 that 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 +/- 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.
Wire rods are normally sold in as rolled condition. The user of wire rods prepare the wire rods for further processing by cleaning and coating, or by heat treating. These operations are explained below.
Cleaning and coating
Mill scale is cleaned from steel wire rods by pickling or caustic cleaning followed by water rinsing, or by mechanical means such as shot blasting with abrasive particles or reverse bending over sheaves. The chemical cleaning of steel wire rods is always followed by a supplementary coating operation. Lime, borax, or phosphate coating is applied to provide a carrier for the lubricant necessary for subsequent processing into wire. In lime coating, practices may be varied in order to apply differing amounts of lime on the wire rods. Phosphate coated wire rods may have a supplementary coating of lime, borax, or water soluble soap. Mechanically descaled wire rods are generally drawn without coating using only wire drawing soaps, or may be coated in a fashion similar to that used for chemically cleaned wire rods.
The heat treatments normally applied to wire rods, either before or during processing into wire, include annealing, spheroidize annealing, patenting, and controlled cooling. Annealing usually involves heating to a temperature near or below the lower critical temperature and holding at that temperature for a sufficient period of time, followed by slow cooling. This process softens the steel for further processing, but not to the same degree as does spheroidize annealing.
Spheroidize annealing involves prolonged heating at a temperature near or slightly below the lower critical temperature (or thermal cycling at about the lower critical temperature), followed by slow cooling, with the object of changing the shape of carbides in the microstructure to globular (spheroidal) form, which produces maximum softness.
Patenting is a heat treatment usually confined to medium and high carbon (C) steels. In this process, individual strands of rod or wire are heated well above the upper critical temperature and then are cooled comparatively rapidly in air, molten salt, molten lead, or a fluidized bed. The object of patenting is to develop a microstructure of homogeneous, fine pearlite. This treatment generally is employed to prepare the material for subsequent wire drawing.
Controlled cooling is a heat treatment which is performed these days in the wire rod mills. In this process the rate of cooling after hot rolling is carefully controlled. The process imparts uniformity of properties and some degree of control over scale, grain size, and microstructure.
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 commonly specified for specific end uses. C steel wire rods (Fig 1) are produced in the following different grades or compositions.
- Low C steel wire rods having maximum C content of less than or equal to 0.15 %.
- Medium low C steel wire rods with maximum C content greater than 0.15 % but less or equal to 0.23 %.
- Medium high C steel wire rods having maximum C content greater than 0.23 % but less or equal to 0.44 %.
- High C steel wire rods having maximum C content greater than 0.44 %.
Fig 1 Carbon steel wire rods
Usually, 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 may be added to the steel to improve the machinability.
Qualities of C steel wire rod
Steel wire rods for the production of C steel wires are manufactured 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 2.0 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 manufacture 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 essential 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 often 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 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 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 rod is generally 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 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 austenitic grain size and decarburization are to be controlled in steels 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 amounts of decarburization are usually 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 manufacture 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 usually 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 may 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 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 practices to ensure the required degree of internal soundness and freedom from surface defects. Fully killed fine grain steel is usually required 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 – 0.10 % to 0.15 %, Mn – 0.40 % to 0.60 %, 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, tire bead wire, upholstery 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 patented or control-cooled rod. When drawing to sizes finer than 2.0 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 that are mutually acceptable to both the user and the producer. Wire rods for piano wire, valve spring wire, and tire cord wire is rolled and conditioned to ensure the lowest possible incidence of defects. Surface defects are objectionable because they lower the fatigue resistance which is important in many of the end products made from these wires. Internal defects are objectionable because they make the wire rod unsuitable for cold drawing to high strength levels and the extremely fine sizes required.
- 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 that will meet the requirements of the various grades of this type of wire.
Special Requirements for C steel wire rod
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 given below.
- Macro-etch testing – It is deep-etch testing to evaluate internal soundness. A representative cross section is etched in a hot acid solution.
- Fracture testing – In fracture testing, 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 (grain size 1 through 5) or fine (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.
- Nonmetallic 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 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 (generally 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.
Mechanical properties of C steel wire rod
In the older wire rod mills (WRM), where wire rod was coiled hot, there was considerable variation within each coil because of the effect of varying cooling rates from the centre to the periphery of the coil. Hence, as hot rolled wire rod was seldom sold to specific mechanical properties because of the inherent variations of such properties. These properties for a given grade of steel varied from WRM to WRM 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 of water, cooling 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 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 obtain desired effects on the properties.
The various qualities of alloy steel wire rod possess characteristics that are adapted to the particular conditions typically encountered during fabrication or service. Production of these steels usually 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 may be specified for wire rods intended for processing into critical or highly stressed aircraft parts or for similar purposes. Bearing quality alloy steels are generally specified for wire rods intended for processing into balls and rollers for antifriction bearings. Bearing quality alloy steel is usually specified when buying the standard carburizing grades, or the through-hardening, high C-Cr steel grades. The various 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.
- Special cold heading quality alloy steel wire rods – They are used for wire for applications involving severe cold plastic deformation. 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 required. 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.
Special requirements for alloy steel wire rods
Alloy steel wire rod can be produced with special requirements in addition to those requires 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 amounts 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 rods.
- Heat treatment requirements – When the end product must 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 – Most 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 generally 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).
- Nonmetallic Inclusion testing – When the nonmetallic inclusion test is specified, it is usually done on billets. Prepared and polished specimens are examined microscopically at 100 magnifications. Sample locations, number of tests, and limits of acceptability is required to be established in each instance.
- 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 specimens 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 that adheres to the steel along lines of flux leakage. On properly magnetized steel, flux leakage develops along surface or subsurface 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.