Construction technology has evolved rapidly over the past few years. Right from the techniques and technology deployed in construction, to the materials used – a lot has changed to become better and better.
With competitive price and timely delivery, Xingtai Steel sincerely hope to be your supplier and partner.
However, this advancement in the building construction domain has left the builders and their clients more confused than ever. And right so – there are so many options to choose from, with little knowledge about what works the best for specific needs.
But worry not! In this post, we talk about one of the most important components of building construction – steel bars, and tell you about their different types their features and utility. So let’s get started!
This is one of the simplest types of steel bars which is being used in building construction for a long time now. Mild Steel Plain Bars, as the name suggests, is characterized by plain outer surface unlike the ribbed structure displayed by other kinds of bars), and have relatively less tensile yield strength (which stands at around psi).
This kind of steel bars is typically used in the construction of small building projects, which do not require a lot of steel strength. Further, these bars are cheaper, and hence the best suited from the economic point of view.
Hot-rolled deformed bars are undoubtedly one of the most common types of reinforced steel bars used in R.C.C structures. These bars have high tensile strength (of about psi) and are suited for all kinds of constructions. The steel provides great strength to the building construction.
In fact, the holt rolling process through which such steel bars are manufactured leads to the creation of deformities on the surface of the bras called ribs), which help create a storing bond with the concrete. This is a feature peculiar to the Hot Rolled Deformed Steel Bars and is not present in the type we discussed before.
Well, TMT steel is the most widely used variety of steel bars in modern-day building construction. Along with unmatched tensile strength, TMT steel offers many features and properties that other steel bars do not. TMT steel is anti-corrosion, earthquake resistant, lightweight, and easy to assemble.
In fact, the very composition of the TMT steel and its manufacturing process make it the best that we have in the market today. Further, TMT steel comes in various sub virtues with varied features and strength, and you can choose the one that suits your needs the best. If you want to know the price of tmt steel in hyderabad then you can visit here.
Cold Worked Steel bars are another type of reinforced steel that is obtained on the further processing of the hot-rolled bars. In this process, the hot-rolled bars are made to undergo the cold working process, during a cold-worked.
A reinforcement bar is obtained by letting the hot rolled steel bars undergo cold working. In the cold working process which is performed at room temperature, the bars will undergo twisting and drawing, which also leads to a reduction in the ductility of the steel.
So if you are looking for steel bars with that peculiar feature, Cold Worked Steel bars should be the ideal choice for you!
Prestressing steel bars are based on a different technology than all other kinds of steel bars. In this, steel bars are modelled as strands, which when deployed together in a construction frame, give rise to the Prestressing action. Strands of Prestressing steel bars are made of multiple wires, with a typical strand constituting as many as 2 to 7 wires. The tensile strength that these strands provide lies in the range of – psi, coupled with great flexibility.
Steel bars come in various types with varied features. However, if you have to choose the best and most reliable amongst them all, we would suggest you go with TMT Steel Bars only.
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Rebar is yet another name for steel bar. These steel bars are being used to reinforce and hold tension in reinforced concrete and masonry structures. Steel bars are a type of construction material used to build structures such as buildings, bridges, dams, RCC roads, and towers. It can be used in concrete to continue providing tensile strength because the tensile strength of concrete is significantly lower than the compressive strength. It will be strong enough to support the structure. To withstand the structural load, it is embedded in concrete fibre.
There are various applications and types of steel bars. Details will be given in this article. There are 5 types of steel bars used and they are as follows: –
Prestressing Steel Bar
Prestressing Steel Bar Steel bars that are pre-stressed use the latest techniques than steel bars that are not pre-stressed. Steel bars are represented as strands in this model, which when deployed in a construction frame produce the Prestressing action. Prestressing steel bar strands are composed of several wires, with a typical strand containing anywhere from 2 to 7 wires. The tensile strength of these strands is between and psi, and they have a lot of flexibility.
Hot Rolled Deformed Bar
One of the most common types of reinforced steel bars used in R.C.C structures is hot-rolled deformed bars. These bars have a great specific strength and can be used for a range of purposes.
The holt rolling procedure is used to manufacture such steel bars results in the formation of ribs on the surface of the bras, which aid in the formation of a storing bond with the concrete. It is a feature that is only found in Hot Rolled Deformed Steel Bars, not the type we previously discussed.
Worked Steel Bar
Worked in Cold Steel bars are a form of reinforced steel that is created from hot-rolled bars that have been further processed. The hot-rolled bars are made to go through a cold-working process in this process. Allowing hot rolled steel bars to undergo cold working produces a reinforcement bar. During the cold working process, which itself is finished at room temperature, the bars will twist and draw, reducing the steel’s ductility.
Mild Steel Plain Bars
The above are among the most basic types of steel bars, and it has long been used in building construction. Mild Steel Plain Bars, as the name implies, have a plain outer layer (as opposed to the ribbed structure seen in other types of bars) and have lower tensile yield strength.
This sort of steel bar is frequently used for the construction of small structures that do not require a great deal of steel strength. Moreover, these bars are less expensive, making them the most cost-effective option.
TMT Steel
TMT steel is the most common type of steel bar used in modern building construction. TMT steel has many features and properties that other steel bars lack, including unrivaled tensile strength. TMT steel is corrosion-resistant, earthquake-resistant, lightweight, and simple to put together.
The TMT steel’s composition and manufacturing process make it the best available on the market today. TMT steel is also available in a variety of sub-virtues, each with its own set of characteristics and strengths, from which you can select the one that best meets your requirements.
Here are some of the applications of Steel Bar: –
Steel bar is well-known for its numerous applications. These bars are used in shipbuilding, defence, automotive, textile, paper and pulp, fabrication, cement, heavy earth moving equipment, and construction, to name a few industries.
Steel bars come in all sorts of shapes, including flat, round, hexagonal, and square, and the shape of the bar largely determines its application space. Steel bars are used in a variety of ways depending on the type of bar required. They also come in a variety of sizes.
Flat Bar: –
Hot rolled stainless steel flat bars have such a large thickness, high strength, and corrosion resistance. They’re used in places like lifts, escalators, and subway stations where the steel isn’t exposed to the elements. Stainless steel flat bars have a variety of applications and use. The vast majority of flat bars are used as base plates and brackets in the construction industry.
Steel posts
Steel posts are another form of steel bar applications. They can be used for strengthening structures like concrete structures Brisbane, which helps retaining walls keep the soil in between the land where it’s placed. The strength of steel helps give the retaining walls strength so it can maintain it’s position while further weight is added due to water weight.
Round Bar: –
One of the most key players in the machining industry is the stainless steel round bar. Stainless steel round bars are used in manufacturing industries that produce fasteners and machinery because of their excellent corrosion resistance. Our Fitmach bars that are a highly specialized product are also used in the machining industry. These calcium-treated bars are of free-machining grades which mean they cut faster and last longer. These specialty round bars are widely used in the manufacture of various machine parts due to their precision straightened and mirror finish surfaces.
Hexagonal Bar: –
Hexagon bars can be used for a variety of purposes. Their final application is in the construction industry. They’re made to carry a lot of weight.
Square Bar: –
Square bars are treasured for their hardness, strength, and resistance to wear. These are generally used to produce medium to large-scale industrial equipment components. These are from one of the most popular categories that are widely used in the manufacturing industry.
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In metalworking, rolling is a metal forming process in which metal stock is passed through one or more pairs of rolls to reduce the thickness and to make the thickness uniform. The concept is similar to the rolling of dough. Rolling is classified according to the temperature of the metal rolled. If the temperature of the metal is above its recrystallization temperature, then the process is known as hot rolling. If the temperature of the metal is below its recrystallization temperature, the process is known as cold rolling. In terms of usage, hot rolling processes more tonnage than any other manufacturing process, and cold rolling processes the most tonnage out of all cold working processes. [1][2] Roll stands holding pairs of rolls are grouped together into rolling mills that can quickly process metal, typically steel, into products such as structural steel (I-beams, angle stock, channel stock, and so on), bar stock, and rails. Most steel mills have rolling mill divisions that convert the semi-finished casting products into finished products.
There are many types of rolling processes, including ring rolling, roll bending, roll forming, profile rolling, and controlled rolling.
The invention of the rolling mill in Europe may be attributed to Leonardo da Vinci in his drawings. [3] The earliest rolling mills in crude form but the same basic principles were found in Middle East and South Asia as early as 600 BCE. Earliest rolling mills were slitting mills, which were introduced from what is now Belgium to England in . These passed flat bars between rolls to form a plate of iron, which was then passed between grooved rolls (slitters) to produce rods of iron. [4] The first experiments at rolling iron for tinplate took place about . In , Major John Hanbury erected a mill at Pontypool to roll 'Pontypool plates'—black plate. Later this began to be rerolled and tinned to make tinplate. The earlier production of plate iron in Europe had been in forges, not rolling mills.
The slitting mill was adapted to producing hoops (for barrels) and iron with a half-round or other sections by means that were the subject of two patents of c. .
Some of the earliest literature on rolling mills can be traced back to Christopher Polhemin in Patriotista Testamente, where he mentions rolling mills for both plate and bar iron. [5] He also explains how rolling mills can save on time and labor because a rolling mill can produce 10 to 20 or more bars at the same time.
A patent was granted to Thomas Blockley of England in for the polishing and rolling of metals. Another patent was granted in to Richard Ford of England for the first tandem mill. [6] A tandem mill is one in which the metal is rolled in successive stands; Ford's tandem mill was for hot rolling of wire rods
Modern rolling practice can be attributed to the pioneering efforts of Henry Cortof Funtley Iron Mills, near Fareham, England. In , a patent was issued to Henry Cort for his use of grooved rolls for rolling iron bars. [7] With this new design, mills were able to produce 15 times more output per day than with a hammer. [8] Although Cort was not the first to use grooved rolls, he was the first to combine the use of many of the best features of various ironmaking and shaping processes known at the time. Thus modern writers have called him "father of modern rolling."
The first rail rolling mill was established by John Birkenshaw in , where he produced fish bellied wrought iron rails in lengths of 15 to 18 feet. [8] With the advancement of technology in rolling mills, the size of rolling mills grew rapidly along with the size of the products being rolled. One example of this was at The Great Exhibition in , where a plate 20 feet long, 3 ½ feet wide, and 7/16 of inch thick, and weighing 1,125 pounds, was exhibited by the Consett Iron Company. [8] Further evolution of the rolling mill came with the introduction of Three-high mills in used for rolling heavy sections.
Hot rolling is a metalworking process that occurs above the recrystallization temperature of the material. After the grains deform during processing, they recrystallize, which maintains an equiaxed microstructure and prevents the metal from work hardening. The starting material is usually large pieces of metal, like semi-finished casting products, such as slabs, blooms, and billets. If these products came from a continuous casting operation the products are usually fed directly into the rolling mills at the proper temperature. In smaller operations, the material starts at room temperature and must be heated. This is done in a gas- or oil-fired soaking pit for larger workpieces and for smaller workpieces induction heating is used. As the material is worked, the temperature must be monitored to make sure it remains above the recrystallization temperature. To maintain a safety factor a finishing temperature is defined above the recrystallization temperature; this is usually 50 to 100 °C (90 to 180 °F) above the recrystallization temperature. If the temperature does drop below this temperature the material must be re-heated before more hot rolling. [9]
Hot rolled metals generally have little directionality in their mechanical properties and deformation induced residual stresses. However, in certain instances non-metallic inclusions will impart some directionality and workpieces less than 20 mm (0.79 in) thick often have some directional properties. Also, non-uniform cooling will induce a lot of residual stresses, which usually occurs in shapes that have a non-uniform cross-section, such as I-beams. While the finished product is of good quality, the surface is covered in mill scale, which is an oxide that forms at high temperatures. It is usually removed via pickling or the smooth clean surface process, which reveals a smooth surface. [10] Dimensional tolerances are usually 2 to 5% of the overall dimension. [11]
Hot rolled mild steel seems to have a wider tolerance for amount of included carbon than does cold rolled steel, and is therefore more difficult for a blacksmith to use. Also for similar metals, hot rolled products seem to be less costly than cold-rolled ones. [12]
Hot rolling is used mainly to produce sheet metal or simple cross sections; such as rail tracks. Other typical uses for hot rolled metal includes truck frames, automotive wheels, pipe and tubular, water heaters, agriculture equipment, strappings, stampings, compressor shells, railcar components, wheel rims, metal buildings, railroad hopper cars, doors, shelving, discs, guard rails, automotive clutch plates. [13]
Rolling mills are often divided into roughing, intermediate and finishing rolling cages. During shape rolling, an initial billet (round or square) with edge of diameter typically ranging between 100–140 mm is continuously deformed to produce a certain finished product with smaller cross section dimension and geometry. Different sequences can be adopted to produce a certain final product starting from a given billet. However, since each rolling mill is significantly expensive (up to 2 million euros), a typical requirement is to contract the number or rolling passes. Different approaches have been achieved including empirical knowledge, employment of numerical models, and Artificial Intelligence techniques. Lambiase et al. [14] [15] validated a finite element model (FE) for predicting the final shape of a rolled bar in round-flat pass. one of the major concern when designing rolling mills is to reduce the number of passes; a possible solution to such requirement is represented by the slit pass also called split pass which divided an incoming bar in two or more subparts thus virtually increasing the cross section reduction ratio per pass as reported by Lambiase. [16] Another solution for reducing the number of passes in the rolling mills is the employment of automated systems for Roll Pass Design as that proposed by Lambiase and Langella. [17] subsequently, Lambiase further developed an Automated System based on Artificial Intelligence and particularly an integrated system including an inferential engine based on Genetic Algorithms a knowledge database based on an Artificial Neural Network trained by a parametric Finite element model and to optimize and automatically design rolling mills. [18]
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Cold rolling occurs with the metal below its recrystallization temperature (usually at room temperature), which increases the strength via strain hardening up to 20%. It also improves the surface finish and holds tighter tolerances. Commonly cold-rolled products include sheets, strips, bars, and rods; these products are usually smaller than the same products that are hot rolled. Because of the smaller size of the workpieces and their greater strength, as compared to hot rolled stock, four-high or cluster mills are used. [2] Cold rolling cannot reduce the thickness of a workpiece as much as hot rolling in a single pass.
Cold-rolled sheets and strips come in various conditions: full-hard, half-hard, quarter-hard, and skin-rolled. Full-hard rolling reduces the thickness by 50%, while the others involve less of a reduction. Cold rolled steel is then annealed to induce ductility in the cold rolled steel which is simply known as a Cold Rolled and Close Annealed. [19] Skin-rolling, also known as a skin-pass, involves the least amount of reduction: 0.5–1%. It is used to produce a smooth surface, a uniform thickness, and reduce the yield point phenomenon (by preventing Lüders bands from forming in later processing). It locks dislocations at the surface and thereby reduces the possibility of formation of Lüders bands. To avoid the formation of Lüders bands it is necessary to create substantial density of unpinned dislocations in ferrite matrix. It is also used to break up the spangles in galvanized steel. Skin-rolled stock is usually used in subsequent cold-working processes where good ductility is required.
Other shapes can be cold-rolled if the cross-section is relatively uniform and the transverse dimension is relatively small. Cold rolling shapes requires a series of shaping operations, usually along the lines of sizing, breakdown, roughing, semi-roughing, semi-finishing, and finishing.
If processed by a blacksmith, the smoother, more consistent, and lower levels of carbon encapsulated in the steel makes it easier to process, but at the cost of being more expensive. [20]
Typical uses for cold-rolled steel include metal furniture, desks, filing cabinets, tables, chairs, motorcycle exhaust pipes, computer cabinets and hardware, home appliances and components, shelving, lighting fixtures, hinges, tubing, steel drums, lawn mowers, electronic cabinetry, water heaters, metal containers, fan blades, frying pans, wall and ceiling mount kits, and a variety of construction-related products. [21]
Roll bending produces a cylindrical shaped product from plate or steel metals. [22]
Roll forming, roll bending or plate rolling is a continuous bending operation in which a long strip of metal (typically coiled steel) is passed through consecutive sets of rolls, or stands, each performing only an incremental part of the bend, until the desired cross-section profile is obtained. Roll forming is ideal for producing parts with long lengths or in large quantities. There are 3 main processes: 4 rollers, 3 rollers and 2 rollers, each of which has as different advantages according to the desired specifications of the output plate.
Flat rolling is the most basic form of rolling with the starting and ending material having a rectangular cross-section. The material is fed in between two rollers, called working rolls, that rotate in opposite directions. The gap between the two rolls is less than the thickness of the starting material, which causes it to deform. The decrease in material thickness causes the material to elongate. The friction at the interface between the material and the rolls causes the material to be pushed through. The amount of deformation possible in a single pass is limited by the friction between the rolls; if the change in thickness is too great the rolls just slip over the material and do not draw it in. [1] The final product is either sheet or plate, with the former being less than 6 mm (0.24 in) thick and the latter greater than; however, heavy plates tend to be formed using a press, which is termed forging, rather than rolling.
Often the rolls are heated to assist in the workability of the metal. Lubrication is often used to keep the workpiece from sticking to the rolls. To fine-tune the process, the speed of the rolls and the temperature of the rollers are adjusted. [23]
h is sheet metal with a thickness less than 200 μm (0. in). The rolling is done in a cluster mill because the small thickness requires a small diameter rolls. [9] To reduce the need for small rolls pack rolling is used, which rolls multiple sheets together to increase the effective starting thickness. As the foil sheets come through the rollers, they are trimmed and slitted with circular or razor-like knives. Trimming refers to the edges of the foil, while slitting involves cutting it into several sheets. [23] Aluminum foil is the most commonly produced product via pack rolling. This is evident from the two different surface finishes; the shiny side is on the roll side and the dull side is against the other sheet of foil. [24]
Ring rolling is a specialized type of hot rolling that increases the diameter of a ring. The starting material is a thick-walled ring. This workpiece is placed between two rolls, an inner idler roll and a driven roll, which presses the ring from the outside. As the rolling occurs the wall thickness decreases as the diameter increases. The rolls may be shaped to form various cross-sectional shapes. The resulting grain structure is circumferential, which gives better mechanical properties. Diameters can be as large as 8 m (26 ft) and face heights as tall as 2 m (79 in). Common applications include bearings, gears, rockets, turbines, airplanes, pipes, and pressure vessels. [10]
Controlled rolling is a type of thermomechanical processing which integrates controlled deformation and heat treating. The heat which brings the workpiece above the recrystallization temperature is also used to perform the heat treatments so that any subsequent heat treating is unnecessary. Types of heat treatments include the production of a fine grain structure; controlling the nature, size, and distribution of various transformation products (such as ferrite, austenite, pearlite, bainite, and martensite in steel); inducing precipitation hardening; and, controlling the toughness. In order to achieve this the entire process must be closely monitored and controlled. Common variables in controlled rolling include the starting material composition and structure, deformation levels, temperatures at various stages, and cool-down conditions. The benefits of controlled rolling include better mechanical properties and energy savings. [11]
Forge rolling is a longitudinal rolling process to reduce the cross-sectional area of heated bars or billets by leading them between two contrary rotating roll segments. The process is mainly used to provide optimized material distribution for subsequent die forging processes. Owing to this a better material utilization, lower process forces and better surface quality of parts can be achieved in die forging processes. [25]
Basically any forgeable metal can also be forge-rolled. Forge rolling is mainly used to preform long-scaled billets through targeted mass distribution for parts such as crankshafts, connection rods, steering knuckles and vehicle axles. Narrowest manufacturing tolerances can only partially be achieved by forge rolling. This is the main reason why forge rolling is rarely used for finishing, but mainly for preforming. [26]
Characteristics of forge rolling: [27]
high productivity and high material utilization
good surface quality of forge-rolled workpieces
extended tool life-time
small tools and low tool costs
improved mechanical properties due to optimized grain flow compared to exclusively die forged workpieces
A rolling mill, also known as a reduction mill or mill, has a common construction independent of the specific type of rolling being performed:
Slabs are the feed material for hot strip mills or plate mills and blooms are rolled to billets in a billet mill or large sections in a structural mill. The output from a strip mill is coiled and, subsequently, used as the feed for a cold rolling mill or used directly by fabricators. Billets, for re-rolling, are subsequently rolled in either a merchant, bar or rod mill. Merchant or bar mills produce a variety of shaped products such as angles, channels, beams, rounds (long or coiled) and hexagons.
Mills are designed in different types of configurations, with the most basic being a two-high non-reversing, which means there are two rolls that only turn in one direction. The two-high reversing mill has rolls that can rotate in both directions, but the disadvantage is that the rolls must be stopped, reversed, and then brought back up to rolling speed between each pass. To resolve this, the three-high mill was invented, which uses three rolls that rotate in one direction; the metal is fed through two of the rolls and then returned through the other pair. The disadvantage to this system is the workpiece must be lifted and lowered using an elevator. All of these mills are usually used for primary rolling and the roll diameters range from 60 to 140 cm (24 to 55 in). [9]
To minimize the roll diameter a four-high or cluster mill is used. A small roll diameter is advantageous because less roll is in contact with the material, which results in a lower force and power requirement. The problem with a small roll is a reduction of stiffness, which is overcome using backup rolls. These backup rolls are larger and contact the back side of the smaller rolls. A four-high mill has four rolls, two small and two large. A cluster mill has more than 4 rolls, usually in three tiers. These types of mills are commonly used to hot roll wide plates, most cold rolling applications, and to roll foils. [9]
Historically mills were classified by the product produced: [29]
A tandem mill is a special type of modern rolling mill where rolling is done in one pass. In a traditional rolling mill rolling is done in several passes, but in tandem mill there are several stands (>=2 stands) and reductions take place successively. The number of stands ranges from 2 to 18. Tandem mills can be either of hot or cold rolling mill types.
In hot rolling, if the temperature of the workpiece is not uniform the flow of the material will occur more in the warmer parts and less in the cooler. If the temperature difference is great enough cracking and tearing can occur. [9]
In a flat metal workpiece, the flatness is a descriptive attribute characterizing the extent of the geometric deviation from a reference plane. The deviation from complete flatness is the direct result of the workpiece relaxation after hot or cold rolling, due to the internal stress pattern caused by the non-uniform transversal compressive action of the rolls and the uneven geometrical properties of the entry material. The transverse distribution of differential strain/elongation-induced stress with respect to the material's average applied stress is commonly referenced to as shape. Due to the strict relationship between shape and flatness, these terms can be used in an interchangeable manner. In the case of metal strips and sheets, the flatness reflects the differential fiber elongation across the width of the workpiece. This property must be subject to an accurate feedback-based control in order to guarantee the machinability of the metal sheets in the final transformation processes. Some technological details about the feedback control of flatness are given in. [30]
Profile is made up of the measurements of crown and wedge. Crown is the thickness in the center as compared to the average thickness at the edges of the workpiece. Wedge is a measure of the thickness at one edge as opposed to the other edge. Both may be expressed as absolute measurements or as relative measurements. For instance, one could have 2 mil of crown (the center of the workpiece is 2 mil thicker than the edges), or one could have 2% crown (the center of the workpiece is 2% thicker than the edges).
It is typically desirable to have some crown in the workpiece as this will cause the workpiece to tend to pull to the center of the mill, and thus will run with higher stability.
Maintaining a uniform gap between the rolls is difficult because the rolls deflect under the load required to deform the workpiece. The deflection causes the workpiece to be thinner on the edges and thicker in the middle. This can be overcome by using a crowned roller (parabolic crown), however the crowned roller will only compensate for one set of conditions, specifically the material, temperature, and amount of deformation. [11]
Other methods of compensating for roll deformation include continual varying crown (CVC), pair cross rolling, and work roll bending. CVC was developed by SMS-Siemag AG and involves grinding a third order polynomial curve into the work rolls and then shifting the work rolls laterally, equally, and opposite to each other. The effect is that the rolls will have a gap between them that is parabolic in shape, and will vary with lateral shift, thus allowing for control of the crown of the rolls dynamically. Pair cross rolling involves using either flat or parabolically crowned rolls, but shifting the ends at an angle so that the gap between the edges of the rolls will increase or decrease, thus allowing for dynamic crown control. Work roll bending involves using hydraulic cylinders at the ends of the rolls to counteract roll deflection.
Another way to overcome deflection issues is by decreasing the load on the rolls, which can be done by applying a longitudinal force; this is essentially drawing. Other method of decreasing roll deflection includes increasing the elastic modulus of the roll material and adding back-up supports to the rolls. [11]
The different classifications for flatness defects are:
Symmetrical edge wave - the edges on both sides of the workpiece are "wavy" due to the material at the edges being longer than the material in the center.
Asymmetrical edge wave - one edge is "wavy" due to the material at one side being longer than the other side.
Center buckle - The center of the strip is "wavy" due to the strip in the center being longer than the strip at the edges.
Quarter buckle - This is a rare defect where the fibers are elongated in the quarter regions (the portion of the strip between the center and the edge). This is normally attributed to using excessive roll bending force since the bending force may not compensate for the roll deflection across the entire length of the roll.
It is important to note that one could have a flatness defect even with the workpiece having the same thickness across the width. Also, one could have fairly high crown or wedge, but still produce material that is flat. In order to produce flat material, the material must be reduced by the same percentage across the width. This is important because mass flow of the material must be preserved, and the more a material is reduced, the more it is elongated. If a material is elongated in the same manner across the width, then the flatness coming into the mill will be preserved at the exit of the mill.
The difference between the thickness of initial and rolled metal piece is called Draught. Thus if {\displaystyle t_{0}} is initial thickness and {\displaystyle t_{f}} is final thickness, then the draught {\displaystyle d} is given by
{\displaystyle d=t_{0}-t_{f}}
The maximum draught that can be achieved via rollers of radius {\displaystyle R} with coefficient of static friction {\displaystyle f} between the roller and the metal surface is given by
{\displaystyle d_{max}=f^{2}R}
This is the case when the frictional force on the metal from inlet contact matches the negative force from the exit contact.
There are six types of surface defects:
Lap
This type of defect occurs when a corner or fin is folded over and rolled but not welded into the metal. [32] They appear as seams across the surface of the metal.
Mill-shearing
These defects occur as a feather-like lap.
Rolled-in scale
This occurs when mill scale is rolled into metal.
Scabs
These are long patches of loose metal that have been rolled into the surface of the metal.
Seams
They are open, broken lines that run along the length of the metal and caused by the presence of scale as well as due to pass roughness of Roughing mill.
Slivers
Prominent surface ruptures.
Many surface defects can be scarfed off the surface of semi-finished rolled products before further rolling. Methods of scarfing have included hand-chipping with chisels (18th and 19th centuries); powered chipping and grinding with air chisels and grinders; burning with an oxy-fuel torch, whose gas pressure blows away the metal or slag melted by the flame; [33] and laser scarfing.