Finished Steel Products Produced in Steel Plants
Finished Steel Products Produced in Steel Plants
Steel plants produce a large number of finished steel products. The steel products are produced in mild steel, in plain carbon steel, in micro alloy steel, in low alloy steel, or in high alloy steel quality. These steel products can be broadly divided into four categories namely (i) bar, rod, and sectional products, (ii) plate and sheet products, (iii) coated steel products, and (iv) pipe and tube products. The product mix of a steel plant normally does not include all the finished steel products. A steel plant produces only those products for which it has got installed facilities to produce. The different types of the steel products produced in steel plants are described below.
Bar, rod, and sectional products
Bar, rod, and sectional products are hot rolled products normally produced from continuous cast billets and blooms. These products are bundled or piled and then either tied with wire rods or strapped with steel straps before their dispatch from the steel plants. The different types of bar, rod, and sectional products are described below.
Merchant bar products – Merchant bar products are produced to specified sizes with appropriate chemical limits for meeting a set of properties and where the end use is non-critical. Merchant bar products are being produced as rounds, squares, angles, channels and beams etc. The bar products go through the standard production process in a multi-stand rolling mill without any treatment given to the billets and with the chemical analysis ranges staying within the standard limits for carbon, manganese, phosphorus and sulphur. The bars have liberal tolerance controls. The surface and core defects are wide and not well quantified. The use of the merchant bar products can involve mild bending, hot forming, punching, and welding and in applications where additional machining requirements are negligible. The quality norms in terms of internal porosity, and surface seams are liberal. Merchant bar products are rarely used in applications where heat treatment of steel is needed.
Special bar quality products – Special bar quality products are used for a class of long products engineered for very tough applications. The term ‘special bar quality’ is a term predominantly being used in North American steel industry. In Europe, ‘Special bar quality steels’ are normally known as ‘engineering steels’. The special bar quality products are used to describe steel long products having properties for more demanding processing or end-use applications which cannot be met by merchant bar quality products. Special bar quality products are produced with more precise dimensions and chemistry. The products are to be produced to satisfy the customer’s specific requirements, including those related to chemical composition, malleability, hardness, ductility, and steel surface condition. Special bar quality products are normally machined, forged, or cold drawn during subsequent processing. Hence, these products are required to have superior mechanical properties and defect free surface. Further, these products are at the higher end of the quality scale both in terms of dimensional accuracy and metallurgical consistency. Some of the end users of special bar quality products can need superior surface quality, or special chemical restrictions, metallurgical characteristics, heat treatment, or surface finishes. Fig 1 shows comparison of merchant bar products and special bar quality products.
Fig 1 Comparison of merchant bar products and special bar quality products
Sectional products – Sectional products are those products which have a cross section resembling a shape such as equal angle, unequal angle, channel, tee, beams (with ‘I’ and ‘H’ sections), and piling section etc. These products are rolled normally in box passes or in universal rolling mills. When the cross-section is rectangular, the dimensional limitations apply to differentiate them from wide flats. These products are mostly delivered in straight lengths, rarely in folded bundles, but never in coils. Depending on the cross sectional size, the sectional products can be (i) medium sections, and (ii) heavy sections.
Medium sectional products consist of several sections. These are (i) small channels or ‘U’ sections having a cross-section resembling the letter ‘U’ and having a height which is less than 80 mm, (ii) angles which are with cross-section resembling the letter ‘L’ with the classification of equal angle and unequal angle depending on the ratio of the flange width and have rounded corner of the flange, (iii) Tee sections which are with equal flanges and the product cross-section resembling the letter ‘T’ and have the rounded corners, equal and slightly tapered flanges, and slightly tapered web, and (iv) bulb flats which have a cross-section which is normally rectangular and has a bulge along the full length of a longitudinal edge of one of the wider surface and a width normally less than 430 mm.
Special bar and medium sections are rolled in lengths normally of small cross-section or of very special shape, generally in limited quantities. This class of sectional product includes in particular trapezoids, hollow drill bars, bars for grooved springs, semi-rounds and half-flat semi-rounds, ‘Z’ sections, small ‘I’ and ‘H’ sections of height less than 80 mm, ‘T’ section with unequal flanges, square-edged angle, channel, and ‘T’ sections.
Heavy sectional products in the form of beams are either steel joists or parallel flange beams. These products have the characteristics namely (i) the web height is equal to or greater than 80 mm, (ii) the surfaces of the webs are continued by fillets to the inside faces of the flange, (iii) the flanges are normally symmetrical and of equal widths, and (iv) the outside faces of the flanges are parallel. In case of steel joists, the flanges are of decreasing thickness from the web to the edge. Steel joists are also called sloping flanged beams or tapered flanged beams. In case of parallel flange beams, the inner surface of the beams are also parallel and the flanges are of uniform thickness. These beams are also known as universal beams. Beam sections are sometimes slitted in order to obtain two ‘T’ sections.
Beams are normally produced as (i) normal section, (ii) thin section, and (iii) thick section. Normal sections have standard web and flange thickness. Thin sections are manufactured with the same series of rolls as used in producing the corresponding normal section but which, for an approximately equal web height, have thinner web and / or flanges as a result of adjustment of the vertical or horizontal rolls. Thick sections are produced with the same series of rolls as used in producing the corresponding normal section but which, for an approximately equal web height, have a thicker web and /or flanges as a result of adjustment of the vertical or horizontal rolls.
Among ‘I’ and ‘H’ sections following further distinctions are made which are given below.
- ‘I’ sections with narrow and medium flanges – These are sections in which the flange width is equal to or less than 0.66 × the nominal height of the section and is less than 300 mm.
- ‘H’ sections with broad or very broad flanges – These are sections in which the flange width is higher than 0.66 × the nominal height, or 300 mm or over. Sections with flanges wider than 0.8 × the nominal height are sometimes called columns.
- Bearing piles – These are ‘I’ or ‘H’ sections in which the thickness of web and flanges are identical.
- Channels or ‘U’ sections – These are sectional products with cross-section resembling the letter ‘U’. In the normal series, the flanges with tapered internal faces have a maximum width of ‘0.5 x height + 25 mm. There are channel sections which are thinner or thicker than the normal sections, and sections with parallel flanges.
- Sections for colliery arches – These are sectional products with cross-section resembling the letter ‘I’ or the Greek letter ‘omega’. In the first case, these sections are sometimes distinguished from other ‘I’ sections by a greater slope of the inside face of the flanges (of around 30 %). They also have width greater than 0.70 × the nominal height.
- Special heavy section – These are ‘I’, ‘H’ and ‘U’ or similar sections having special features such as unequal or asymmetric flanges and / or non-standard web thickness and height. These products are normally produced in limited quantities.
Railway track and similar materials include (i) hot rolled products used in the construction of railway tracks for example rails, sleepers, fish plates, sole and tie plates, and base plates etc., and (ii) products of a similar shape and use, such as – crane rails, live conductor rails, grooved rails, rails for switches / crossings and special rails such as guide rails, and brake rails etc. Railway track materials are sub-divided namely (i) heavy track materials, which include rails of linear mass 20 kg/m or more, and sleepers of linear mass 15 kg/m or more, and (ii) light track materials which include rails of linear mass up to 20 kg/m, sleepers of linear mass upto 15 kg/m, fish plates, sole and tie plates, base plates and other light materials, and materials for the construction of railway tracks.
Piling is another sectional product. Sheet piling is the product produced by hot rolling or cold forming (drawing, bending, and cold rolling etc.) to a shape such that, by interlocking of the joints or by fitting together on longitudinal grooves or by means of special fasteners, it can be used to form partitions or continuous walls. Sheet piling is differentiated according to its cross-sectional shape or its application such as (i) ‘S’, ‘U’, ‘Z’ and ‘omega’ sheet piling, (ii) flat sheet piling, (iii) fabricated sheet piling (built up from sheet piles, angles, and other sections), (iv) lightweight sheet piling (trench sheeting), (v) interlocking ‘H’ sheet piling, and (vi) box and tubular sheet piling. Tubular bearing pilings consist of tubes of circular or rectangular (including square) cross-section driven into the ground for transmitting the weight of a structure to the soil by the resistance developed at its base and by friction along its surface.
Reinforcement bar – It is also known as rebar or reinforcing bar. It is used as a tension device in reinforced concrete and reinforced masonry structures to strengthen and aid the concrete under tension. It significantly increases the tensile strength of the structure. The surface of the reinforcement bar is frequently ‘deformed’ with ribs, lugs or indentations to promote a better bond with the concrete and reduce the risk of slippage.
Reinforcement bar is used for reinforcement and pre-stressing of concrete and has the cross-section which is normally round but sometimes square with rounded corners. The bar is with the diameter or side which is at least 5 mm. It is produced with thermal treatment to obtain the desired strength and other mechanical properties. It is supplied as (i) bars in lengths with a smooth surface, (ii) bars in lengths with a ribbed, deformed ribbed, or crenellated surface, (iii) rod in coils with a smooth surface, and (iv) rod in coils with a ribbed, deformed, or crenellated surface. Reinforcement bars which are supplied in lengths, can have undergone a controlled cold deformation, for example lengthening or twisting about their longitudinal axis.
Wire rod – Wire rod can be produced in several regular shapes (such as square, hexagonal etc.), but the majority production of wire rod is in round cross-section. The range of materials in which wire rods are produced, comprises low to high carbon steels, cold heading steels, wire drawing steels, alloy steels, spring steels, ball bearing steels, electrode quality steels, reinforcement bars, tool steels and stainless steels.
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 (kgs) to 2.5 tons. A longer and larger cross section billet produces heavier coils. Wire rod is 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 wire rod is normally cold drawn into wire suitable for further processing such as drawing, cold rolling, cold heading, cold upsetting, cold extrusion, or cold or hot forging. It is used for many products. It is the raw material for the wire drawing units.
Plate and sheet products
Plate and sheet products mainly comprise of strip, sheet and plate. These products are identified by their width being many times higher than of their thickness. All the flat products are rolled as rectangular section in which length and width happens to be more than 100 times of thickness. The sheets are flat product in straight lengths in which length and width remain specified along with the thickness.
Sheets can be produced either by rolling separately in straight lengths or by uncoiling strips and cutting them in to straight length pieces of desired length. The sheets are defined as the rectangular sections with thickness upto 5 mm. Strips are produced in large lengths and rolled in to coils in which thickness is specified as upto 12 mm. Section happens to be rectangular and length many times of width so as to finish them in coil form for storing and transportation. By definition strips of thickness of 5 mm and above cut into straight lengths shall be called as plates which are also rolled in straight lengths directly.
Coil and sheet products are normally without any coating or surface treatment. The products which have received a simple coating for the purpose of protection from corrosion or mechanical damage (e.g. passivation, organic coating, paper, oil, lacquer, etc.) are normally treated as uncoated products.
Hot rolled plate products – Plate steel is defined as a flat, as-rolled or heat treated product of thickness of at least 5 mm and width of at least 1,200 mm. Plate steel is widely used steel product. It falls normally in the categories of carbon steel, high strength low alloy (HSLA) steel, and alloy steel. For structural applications, plate steel normally does not exceed 0.3 % carbon and 1.5 % manganese. Steel plates of higher thickness are needed for sky-scrapers, drilling rigs on the high seas, bridges with giant spans, slender wind turbines, pipelines, gasholders with enormous pressures, excavators, mobile cranes, container ships, and luxury liners etc. These applications need high quality and high strength in the plates for reliability.
The most important trend in the steel plate is the demand for plates with increasingly higher strengths, high toughness, hardness, and good weldability. Prime quality in terms of thickness, width, profile, flatness, rectangularity, and a homogeneous microstructure are likewise expected with all steel grades and even extreme dimensions. At the same time, ever closer tolerances are being demanded in respect of dimensional accuracy and flatness. For meeting of these requirements, the plate mills are required to have the capabilities of large range of product mix with a high proportion of high strength plates to be produced, and hence new plate mills are designed for a great variety of production technologies.
The requirements which are normally made on heavy plate are (i) it is to possess the specified dimensions within narrow tolerances and with good flatness (the thicknesses can range from 5 mm to 500 mm and widths from around 1,200 mm to 5,500 mm), (ii) it is to possess the yield strength and tensile strength values needed by the designers (yield strengths ranging from around 235 MPa to above 1,100 MPa), (iii) it is to possess the toughness values needed by the designers even, in many cases, low temperatures toughness, (iv) it is to possess good workability (such as deformability and weldability), and (v) if required, it is to possess resistance to corrosion resulting, for example, from attack by the hydrogen contained in H2S (hydrogen sulphide) -bearing gases (sour gas), or a certain resistance to atmospheric corrosion (weathering resistance).
For the production of the steel plates for line pipes in sour gas service resistance to hydrogen induced cracking is the most important factor. For off shore marine application specially in those regions where temperatures falls very low during winter, steel plates need to have properties such as higher strength, larger plate thickness, and ultra-low temperature toughness (excellent weld-joint characteristics at low temperatures) etc. These properties are, in some cases, contradictory and have become achievable, in an extreme combination of alloying and processing technologies. In the recent past, several technologies for the property control of the steel plates have been developed for the plate mills to meet the requirements of higher strength and toughness of the plates.
Hot rolled strip products – The most common hot rolled strip product is the hot rolled coil. It is the lengthened steel sheet / plate which is produced in coil form for its easy handling and transport. Coils are produced with an inside diameter of 750 mm on the coilers, with an outside diameter of upto 2,400 mm and with the limitations of coil weight upto 220 kg per cm width. The hot rolled coils used for cold rolling, and for strip shearing and slitting. Attributes imparted to the strip by the hot rolled strip mill include surface quality, thickness, width, and flatness control as well as strip profile. The strip is to be reasonably free form skewness, edge cambering, and waviness.
Hot rolled strip as rolled has very slightly convex edges, but it can also be supplied with trimmed edges or slit from wider strip. Hot rolled strip, according to its actual width, can be categorized as (i) hot rolled wide strip with a width of 600 mm minimum, and (ii) hot rolled narrow strip with a width of less than 600 mm.
Hot rolled strip products are normally further processed. Their applications include practically all sectors of industry such as transport, construction, shipbuilding, gas containers, pressure vessels, and energy pipelines etc. Typical thickness ranges between 2 mm to 7 mm and typical width ranges between 600 mm to 2,100 mm.
Hot rolled coils are sheared in shearing lines to produce straight lengths. Strips below 5 mm thickness if cut in to straight lengths are called sheets where as the strips of 5 mm thickness and above if cut in to straight lengths are called plates. In certain cases, hot rolled flat products are given a very light cold rolling pass, normally less than 5 %, known as a skin-pass or dressing pass.
Hot rolled coils are also being produced in pickled and oiled condition. Hot rolled coils from which the iron oxides present at the surface have been removed in a pickling process can be further processed. Application of pickled coil is in virtually all sectors of industry such as transport, construction, shipbuilding, gas containers, pressure vessels, energy pipelines, etc. Typical thickness ranges between 2 mm to 7 mm while the typical width ranges between 600 mm to 2,100 mm
Cold rolled products – Cold rolled products are flat products which during finishing have undergone a reduction in cross-section of at least 25 % by cold rolling without prior reheating. Cold rolled products are obtained by a further thickness reduction of a pickled hot rolled coil. These products are obtained at low temperature in a cold-reduction mill where pickled hot rolled coil is further processed. The cold rolling process consists of substantial mechanical working, (rolling) below normal recrystallization temperature of steel which goes up to 650 deg C.
Besides reduction in thickness, cold rolling is done for improving the surface finish of steels, for improving the thickness tolerances, for offering a range of ‘tempers’, for improving the physical characteristics, and for preparing the strip for surface coating. Typical thickness of cold rolled products ranges between 0.12 mm to 2.5 mm while the typical width ranges between 600 mm to 2,100 mm.
Cold rolling makes the cold rolled product a much improved product. Cold rolled products offer good control of thickness, shape, width, surface finish, and other special quality features which compliment the need for highly engineered end user applications. To meet the various end user requirements, cold rolled products are metallurgically designed to provide specific attributes such as high formability, deep drawability, high strength, high dent resistance, good magnetic properties, weldability, enamelability, and paintability etc.
The mechanical properties of cold rolled products is obtained by heat treatment (annealing) and strain-hardening of cold rolled steel and this makes the cold rolled steel suitable for further uses (forming and bending). Cold rolled products are normally further processed. They are classified into formable steels, high strength formable steels, weathering structural steels, structural steels, and hardenable steels.
Cold rolled products have excellent forming properties, electromagnetic properties, paintability, weldability, and are suitable for fabrication by forming, pressing and bending. Their application includes domestic applications, automotive applications, lighting fixtures, electrical components (stators, rotors) and various kinds of sections roofing applications, profiled sheets, and wall elements etc.
Cold rolled coils are sheared in shearing lines to produce straight lengths. Cold rolled products include the following products.
- Cold rolled sheets – These are the products where the edges are being allowed to deform freely. The product is supplied normally in a square or rectangular shape, with a width higher than 600 mm. The edges can be as-rolled or sheared.
- Cold rolled strip – Cold rolled strip is the flat product which, immediately after the final rolling pass is wound into a coil. Cold rolled strip as rolled has slightly convex edges, but it can also be supplied with sheared edges or slit from wider strip. Cold rolled strip, according to its actual width can be categorized as (i) cold rolled wide strip with width of 600 mm minimum, (ii) cold rolled wide strip with a width of more than 600 mm and sometimes supplied in slit condition with width less than 600 mm, and (iii) cold rolled narrow strip with strip of width less than 600 mm. After decoiling and cutting to length, cold rolled strip can be supplied as cut lengths.
Coated steel products
Coated steel flat products are the flat cold rolled products with a permanent coating on steel substrates. The permanent coatings can be either on both sides or only on one side. In case of coating on both the sides, it can be either equal thickness coating on each side or of different thickness of coating on each side known as differential coating. Fig 2 shows structure of metallic coating with and without passivation.
Fig 2 Structure of metallic coating with and without passivation
The coatings of steel substrates enhance the life and the performance of the steels. They provide the most effective and economical way of protecting steels against corrosion. The coated steel products offer unique combination of properties which include high strength, formability, light weight, corrosion resistance, aesthetics, recyclability, and low cost.
According to the type of coating and type of surface treatment, the coated products are categorized which is given below.
Galvanized sheet and strip products – Galvanized sheet and strip products are coated with zinc. These products include a range of hot dip galvanized and electro-galvanized steel sheet. The zinc coating provides a continuous barrier which does not allow moisture and oxygen to reach the steel. It reacts with the atmosphere to provide the base steel a protection against corrosion resistance which is improved by several degrees of magnitude. The corrosion protection achieved by zinc-based coatings is as a result of the galvanic effect, since zinc is anodic to iron which makes it act a sacrificial metal in an aqueous or humid environment.
Galvanized steel sheet is one of the most used sheets for the roofs in homes, industries, and commercial premises. The roofs are made up of mild steel coated with zinc. Galvanized steel roofing sheet normally is produced in different form of long run roofing such as trapezoidal, trough section / concealed fix, and corrugated. It is also sometimes being produced in the form of flat sheets. The galvanized steel sheets are normally fixed in place using lead-headed nails in order to prevent it from rusting.
The other functions of the zinc layer on the galvanized steel sheets are (i) to retain the steel intact with its full initial strength, (ii) to provide the surface a more pleasing appearance, (iii) to increase the life of any suitable organic finishing system applied over it, and (iv) to protect the steel from corrosive attack in most environments, acting as a continuous and lasting shield between steel and the environment.
The basic properties of galvanized steel sheets are corrosion resistance, good surface appearance, good formability for meeting the deep drawing requirements, good paintability provided proper pre-treatment is performed, and good weldability. Hardness, ductility, and adherence combine to provide the galvanized coating with very good protection against damage during rough handling. The thickness of the coating is proportional to the coating mass.
A part of the galvanized steel production is corrugated to produce galvanized corrugated steel sheets. Corrugated galvanized steel sheets are at present the best known, most widely available low cost roof covering material. These sheets are light-weight roofing material made of thin sheets, stiffened by corrugations. Corrugations, such as waves, considerably increase the strength and stiffness of the light-weight material. There is a large variety of qualities of corrugated steel sheets which are available in the market. These sheets are very popular. Most of the corrugated sheets are light-weight and considerably of low cost.
Corrugated galvanized steel sheets are made of thin plain galvanized steel sheets, stiffened by corrugations. The corrugations, such as waves or folds, increase the strength and stiffness of the sheeting material considerably. Without these waves, the sheets have limited load bearing capacity and are highly deformable. The sheets are normally made of mild steel, which is then galvanized to increase the resistance against corrosion and increase the durability or service life. The corrugation adds stability to the sheet or panel materials so they can span larger widths. After zinc coating, the surfaces can be passivated by chromating or phosphating.
Tinplate – Tinplate is a thin steel sheet coated with tin (Sn) metal. The thin steel sheet on which tin coating is done is known as black plate. Low carbon steel with carbon well below 0.1 % is normally used for tinplate production. Tin coating is applied either by hot dipping or electrolytic coating process. Tinplate is used as a food packaging material.
Conventional tinplate is a heterogeneous material with a stratified structure, formed by a low carbon steel sheet coated with tin on both sides. Tinplates are produced with plate thickness range of 0.13 mm to 0.50 mm, width range of 200 mm to 1,067 mm and length range 406 mm to 1,110 mm. In case of electrolytically coated tinplates, the tinplates are produced in coil form. The tin coating mass varies from 0.5 gsm (grams per square metre) to 34 gsm, representing less than 1 % of the steel weight. The tin coating on tinplate is so thin that for practical purposes it can be ignored in considering thickness, so that the specified thickness is essentially that of the steel base.
Tin-free steel – Tin-free steel (TFS) is an electrolytic chrome plated steel consisting of a thin layer of chromium and a layer of chromium oxide deposited on a cold rolled steel base (black plate steel) which gives it a beautiful, lustrous metallic finish on both sides. It is also known as electrolytic chromium coated steel (ECCS). TFS offers outstanding corrosion resistance, lacquer adhesion as well as printability. It is an economical and high quality replacement for tinplate. It is not suitable for soldering and can only be used for welding after edge cleaning. ECCS is required to be lacquered on both surfaces. TFS has been developed to meet economic requirements, and excels tinplate in paintability, paint adhesion, and economy.
Hot dip galvannealed steel sheet – In the production of the galvannealed steel sheet, the hot dip galvanize coating is diffusion alloyed with the steel by additional heating in the tower above the coating bath. Typical metal coating contains 8 % to 11 % of iron. These sheets are normally intended to be painted for most applications. These sheets are characterized by its high hardness and brittle behaviour during forming. The galvannealed sheets are easier to spot weld and paint than the galvanized sheets. Performance with painting is synergistically improved because of the excellent bond formed between the paint and the surface of the coating. Compared to galvanized steel, galvannealed steel sheets normally shows less undercutting corrosion beneath paint at exposed edges, scratches, or other defects in the paint. These steel sheets are used for auto body panels. Coating weight (mass) is normally in the range of 90 gsm to 180 gsm total on both sides.
Hot dip 55 % Al – Zn alloy coated steel sheet – These coated sheets are also known as galvalume sheets. The coating is aluminum (Al) – zinc (Zn) alloy coating which contains around 55 % Al, 43.5 % Zn, and 1.5 % Si (silicon). This coating offers very good barrier protection combined with some galvanic protection. Retention of galvanic protection is an important feature for this coating. This particular combination of Al and Zn causes the formation of a coating microstructure which is very important for good performance. This coating provides a very good balance between galvanic and barrier protection. Si is added to control the alloy layer growth rate. The coating improves adhesion during forming. The coating has a much higher resistance to corrosion than galvanize coatings in most environments. Its long term durability has been demonstrated. Coating weight (mass) is normally in the range of 100 gsm to 210 gsm total both on sides.
Hot dip Zn -5 % Al alloy coated steel sheet – These coated sheets are also known as Galfan sheets. The galvanic coating contains around 95 % Zn and 5 % Al. This coating provides approximately the same galvanic protection as galvanized and improved corrosion resistance in most environments. This coating is normally used mostly for applications which need good coating ductility (such as deep drawn parts and pre painted sheets) and superior corrosion resistance. Coating weight (mass) is normally in the range of 90 gsm to 700 gsm total on both sides.
Hot dip Zn – Al – Mg (magnesium) alloy coated steel sheet – The coating is a galvanic coating which contains Zn, Al (5 % to 11 %), and Mg (2 % to 4 %). This coating provides superior corrosion resistance in many aggressive environments. The coating is used in such applications as transportation, infrastructure, construction, agricultural, electric power, and automotive. Coating weight (mass) is normally in the range of 60 gsm to 600 gsm total on both sides.
Hot dip aluminized steel sheet – There are two types of aluminized coatings namely (i) Type 1 – Al along with 5 % to 11 % Si, and (ii) Type 2 – pure Al coating. Most common form is Type 1 coating which is used for applications that need heat-oxidation resistance such as furnace parts, small appliances, and exhaust systems etc. Aluminized coating is the best coating on steel sheet for heat-oxidation resistance. It can be applied over stainless steel to offer even better high temperature performance. Pure Al Type 2 coating is used for exterior applications. The corrosion performance of this coating is based on barrier protection with no galvanic protection in most environments. Barrier corrosion protection is very good in this case since Al of the coating forms a stable aluminum oxide film on the surface of the coating. Coating weight (mass) is normally in the range of 75 gsm to 300 gsm total on both sides for Type 1 and in the range of 200 gsm to 300 gsm total for Type 2 on both sides.
Terne coated steel sheet – This coating is a Pb (lead) alloy coating which contains 3 % to 15 % Sn (tin). Sn is added to develop a bond between the coating and steel. The coating is very formable and improves the deep drawing behaviour. Also, the coated sheet can be easily welded. The coated sheet has a very good resistance to gasoline, although use for fuel tanks is decreasing (related to the environmental issue associated with Pb and not related to the performance of the coated sheet). Coating weight (mass) is normally in the range of 75 gsm to 335 gsm total on both sides.
Zinc-nickel alloy coated sheet and strip – In this product sheet or strip is coated electrolytically with a zinc-nickel alloy, with a coating thickness normally between 1 micro meters to 8.5 micro meters per side.
Colour coated steel products – Colour coating is a term used to describe the application of a decorative and / or protective organic coating to steel substrate supplied in coil form. Colour coated steel is also called as pre-painted steel.
Colour coatings are paint coatings and are specialty products, which are used to give the steel a long term protection under a broad range of corrosive conditions, extending from atmospheric exposure to full immersion in strongly corrosive solutions. A colour coating provides little strength to the substrate steel, yet it protects the steel so that its strength and integrity can be maintained.
Colour coating normally refers to the application of liquid paint coat over the substrate in an automatic and continuous process after pre-treatment. The pre-painted colour coated steel is a very high value-added product which combines the best properties of both substrate and organic coating, additionally imparting it an aesthetic finish, high degree of durability, and high corrosion resistance. The thickness of colour coating is normally in the range of 15 micro meters to 40 micro meters and the finishes are smooth, matt, high gloss, textures and printed. For standard colour coated sheet, the thickness of substrate steel normally varies from 0.2 mm to 1.6 mm and width varies from 600 mm to 1,600 mm.
Colour coating is done on various substrates of steels to produce most cost effective, quality assured products with the top coat compatible with environment. The substrate steels normally used are (i) cold rolled steel, (ii) hot dipped galvanized steel, (iii) electro galvanized steel , (iv) galvalume which is also known as zincalume and consists of 55 % Al, 43.5 % Zn and 1.5 % Si, and (v) Galfan which is 95 % Zn and 5 % Al.
When colour coating and galvanized steel are used together, the corrosion control provided is superior to either system used alone. The galvanized coating protects the base steel, supplying cathodic and barrier protection and the colour coating slows down the rate at which the zinc is consumed, greatly extending the life of the galvanized steel.
Pipe and tube products
The term ‘pipes and tubes’ is used to cover all hollow products. These products are long, hollow products, open at both ends, of round or polygonal cross-section. Pipes and tubes are normally finished at the ends either by threading or flaring, or coated on the interior and / or exterior. These products have applications which are almost innumerable, but they are most commonly used as conveyors of fluids (liquid and gases), pneumatic conveying of solids, and as structural members.
Pipes and tubular products have an outside dimension, an inside dimension, and the wall thickness. The steel pipe and tubular products are usually classified broadly as (i) pipe, and (ii) tube. Pipe and tube products are normally produced in cylindrical form. However, they are often subsequently altered by various processing methods to produce square, oval, rectangular, and other symmetrical shapes.
In general practice the designations pipe and tube are almost interchangeable, but in the pipe fitting industry and engineering discipline the terms are uniquely defined. The main difference between a pipe and a tube is the way the diameter of the pipe or tube is designated. Pipe is normally designated by a ‘Nominal Pipe Size’ (NPS) based upon the ID (inside diameter) of the most common wall thickness while the tube is designated by the measured OD (outside diameter).
Pipes and tubes are produced mainly by two distinct production processes which result in either a welded pipe or a seamless pipe. As the production process for the seamless pipes and tubes does not include any welding, seamless pipe is perceived to be stronger and more reliable. Seamless pipe is regarded as withstanding pressure better than other types of pipes.
Seamless pipes and tubes are the products without a seam which are produced from a bloom or by centrifugal casting. They are made by piercing the bloom to obtain tube hollows. These hollows are then transformed into pipes and tubes by rolling, extrusion or drawing over a mandrel. Seamless pipes and tubes can be finished by reducing the cross-section by hot or cold rolling or by drawing.
Welded pipes and tubes are formed from either hot or cold rolled flat products, welded across the abutting edges. The weld can be either longitudinal or spiral. Hollow sections are pipes or tubes used for structural and similar purposes. Hollow special quality bars are seamless pipes or tubes intended principally for machining applications and distinguished by its close tolerances, which ensure clean-up dimensions with minimal machining allowances. Welded steel pipes are produced with either a longitudinal seam or a spiral (helical) seam. The diameters of these pipes range from around 6 mm to 2,500 mm with wall thicknesses in the range of 0.5 mm to around 40 mm.
The two broad classifications of steel pipe and tubular products are subdivided into several named use groups. As an example, the term tube covers three such groups namely (i) pressure tubes, (ii) structural tubing, and (iii) mechanical tubing. Similarly the term pipe covers five such groups, namely (i) standard pipe, (ii) line pipe, (iii) OCTG (oil country tubular goods), (iv) water well pipe, and (v) pressure pipe. There are also pipes for special applications, such as conduit pipe and tubular piling which do not fit any of these classifications.
Pipes and tubes are frequently designated by their method of manufacture such as (i) ERW (electric resistance welded) pipe, HFI (high frequency induction) welded and electric fusion welded (EFW) pipe, (ii) SAW (submerged arc welded) pipe such as LSAW (longitudinal submerged arc welded) and DSAW (double submerged arc welded) pipe, (iii) spiral welded pipe and SSAW (spiral submerged arc welded) pipe, (iv) pipe produced by UOE process, (v) seamless pipe produced by the various processes such as ‘pierce and pilger’ rolling process, ‘plug’ rolling process, ‘continuous mandrel’ rolling process, ‘push bench’ process, ‘pierce and draw’ process, ‘tube extrusion’ process, ‘cross rolling’ process, ‘Assel’ rolling process, and ‘Diecher’ process, (vi) cold drawn pipe, (vii) cast pipe and centrifugally cast spun pipe, and (viii) fabricated pipe.