Flattening, Levelling, Slitting, and Shearing of Coiled Flat Steel Product
Flattening, Levelling, Slitting, and Shearing of Coiled Flat Steel Product
Hot strip mill and cold rolling mill produce steel strips in varying widths and material thicknesses. These plates / sheets are typically coiled for efficient handling, transport, and further processing. Steel coils are processed in several ways namely (i) it can be ‘slit’ into a number of individual coils of reduced width, (ii) it can be sheared into rectangular or irregularly shaped blanks for further processing, (iii) it can be fed directly into steel coating lines, a stamping press, or other equipments for the production of parts.
Steel coiled strips need further processing steps for producing steel products which are suitable for the subsequent production processes. These processing steps ae (i) flattening, or levelling, (ii) slitting, or (iii) cut-to-length. Flattening or levelling is used to correct shape in the coiled sheet, strip, or plate. Slitting divides a wide coil longitudinally into narrow coils of desired width, while cut-to-length is to shear coiled plate, sheet, or strip transversely into flat pieces in specific lengths.
During the rolling of steel, the last step is coiling. Normally tension is used when coiling to avoid producing ‘soft’ or collapsing coils. Coiling induces tensile and compressive stresses into the strip, and these stresses can contribute to blank or part distortion in subsequent processes. Unless sufficient winding tension adjustments are made, the degree of these stresses change throughout the coil. This is since the outer laps of the coil can be of the order of 1,800 millimetres (mm) in diameter, while the inner laps typically are wound on a 500 mm to 600 mm diameter mandrel. In addition, the magnitude of these stresses increases with higher strength products, leading to coil shape imperfections like coil set and cross bow (transverse bow).
Coil set and cross bow are attributed to a residual strained length difference between the top and the bottom surfaces. There is a minor difference in length from surface to surface which causes a curvature at the longitudinal direction (coil set) or at transverse direction (cross bow).
Coil set, similar in appearance to curl, is also a longitudinal curvature along the coil length. Coil set can occur when the steel strip is wound onto a small-inside diameter (ID) arbor with excess tension. The smaller the ID, the thicker the steel, and the softer the temper, the higher is the likelihood of inducing coil set. Since the smallest ID exists at the core, this is where the highest tendency for coil set exists. However, each succeeding wrap of coil is actually wound onto the diameter represented by the prior wrap, thereby reducing the bending stresses wrap-by-wrap and reducing the tendency for coil set wrap-by-wrap. Hence, coil set, within the coil where it is first formed, is highly likely to be the most severe at the coil ID and decreases to a lower value or to zero at the coil OD (outside diameter).
Cross bow is a curvature across the full width of the steel which renders it somewhat canoe-shaped, or gutterlike, along its length. It is a bow condition perpendicular to the rolling direction, and curves downward in the same direction as the upper outside lap of an over-wound coil, with the centre portion of the sheet is raised a measurable quantity above the sheet edges (Fig 1a). Coil set is a bow condition parallel with the rolling direction, and curves downward in the same direction as the upper outside lap of an over-wound coil (Fig 1b). Here, the top surface of the coil or strip is stretched more than the bottom surface, and typically becomes more severe as the coil is processed and the lap diameter decreases. Fig 1 shows cross bow imperfections.
Fig 1 Coil shape imperfections
Ther other four types of shape defects in coiled sheet, strip, or plate are (i) camber, (ii) wavy edge, (iii) buckles within a sheet or strip such as centre buckle or quarter buckle, and, (iv) twist. Camber, wavy edges, and buckles are considered to be a result of a longitudinal length differential from edge to edge across the width, e.g., the edges of the strip or sheet longer than the centre causes the wavy edges, while a centre buckle is generated when the centre is longer. A combination of residual strained length difference in both surface-to-surface and edge-to-edge causes a twist in a strip.
Buckles are localized pockets or regions of excessive length or surface which rise and fall from the nominal plane of the strip which are to be accommodated by the surrounding steel. They look like a series of waves or domed areas and are sometimes called ‘oil-canning’ in reference to the domed, snap-through bottom of metal oil cans. Buckles can occur anywhere across the rolled width and tend to repeat along the length.
Buckles which occur in the central portion of the strip are referred to as centre buckles and those which are off-centre are called quarter buckles. The term full centre refers to a centre buckle which has edge-to-edge coverage and extends for some distance along the length of the strip. Centre buckles, quarter buckles, and full centres can be created during cold rolling as a result of mechanical mis-alignment or of cross-sectional irregularities in hot rolled pickled coils. Wavy edges consist of buckle like distortions which exist whenever the edge is longer than other portions of the strip and normally occur at the immediate edge.
Camber is a side-wise curvature along the length of a strip. Although camber can be apparent in full-width material, it is frequently an issue in narrower material. It results from incoming strip shape defects, rolling residual stresses, excessive slit burr at one side edge of a slit strip, or excessive tension on one side edge of a slit strip exiting the slitter. The result, however, is always a strip with one edge longer than the other. In general, the narrower the width and the thicker the steel, the higher is the tendency for camber to develop.
Camber is a measure of curvature, which can be described as part of a circle with camber as a square function of the radial distance (L) over which it is measured. Camber measured over different lengths can be expressed as C2 = C1 x (L2/L1)square, where C1 is the camber measured over a length L1, and C2 is the camber of the same strip piece measured over a different length of L2.
Twist is a condition wherein a transverse axis held in the plane of the strip rotates around the longitudinal axis when moved along the strip. Such a condition is evident in a short length of material if, when the material is freely placed on a flat surface, only three of the four corners touch the surface. Fig 2 shows types of shaped defects in coiled flat products.
Fig 2 Types of shaped defects in coiled flat products
Some other shape defects in coiled sheet, strip, or plate are (i) herringbone, (ii) longitudinal corrugation, and (iii) curl. Herringbone (Fig 3a) can occur during cold rolling and is recognized as a series of long, overlapping waves running at varying angles between the rolling direction and the transverse direction. A herringbone pattern normally covers the central portion of the metal width and results from application of insufficient tension for the cross-sectional area and strength level of the steel being rolled.
Longitudinal corrugation (Fig 3b) is a longitudinal condition similar to transverse bow except that the sense of the curvature changes sign at least once across the width of the steel strip. Corrugation is normally caused by the application of excessive tension, to which very thin material such as foil is especially susceptible during continuous annealing.
Curl appears as a longitudinal curvature or sweep along the length of coiled steel strip (Fig 3c). Curl can be induced during rolling, or by passing the steel over a small-diameter roll, such that the combined tensile and bending stresses exceed the yield stress of the steel, leaving a degree of permanent curvature in the steel.
In practice, curl and coil set cannot be easily distinguishable although resulting from different causes. Curl can have a higher degree of curvature than coil set. The curvature of the latter is normally no less than the radius of the coil as the material wraps. Fig 3 shows some other shape defects in coiled sheet, strip, or plate.
Fig 3 Some other shape defects in coiled sheet, strip, or plate
Flatteners and Levellers – The first operation when unwinding a coil, is some type of shape correction to ensure flatness before further processing. There are two main types of equipment used to create a flat coil namely (i) a flattener also called straightener, and (ii) a precision leveller. While these two types of equipment are similar, a precision leveller has additional capabilities. Both bend the coil back and forth over a series of work rolls to alternately stretch and compress the upper and lower surfaces as shown in Fig 4. It is also noted that the terminology used for different shape correction equipments is not standardized.
Fig 4 Alternate stretching and compressing of the upper and lower sheet surfaces
Removing coil set needs permanent yielding in the outer 20 % of the top and bottom surfaces of the steel. The central 80 % of the thickness remains unchanged. Flatteners are appropriate for this type of shape correction. Only end bearings support the simplest flatteners, with no backup rolls used. Closing the entry roll gap risks deflection of the unsupported centre, potentially leading to creating edge waves in the coil.
Critical equipment parameters include roll diameter, roll spacing, backup rolls, roll material type, gear design, backup rolls, overall system rigidity, and power requirements. The quantity of force needed to relieve the residual stresses is a function of the sheet thickness and yield strength. Equipment sufficient for shape correction in conventional grades cannot be sufficient to completely flatten the advanced steel grades available now and in the future.
Flatteners and levellers – they are widely used to correct shape defects in coiled sheet, strip, and plate to achieve a needed flat-shape condition. These equipments are widely used in stamping, roll forming, cut-to-length lines, and slitting lines, in addition to the coil process lines. These equipments are used for shape correction and control. However, their designs and hence effectiveness and scope of usage are different.
Flatteners and levellers have a series of rolls which progressively flex the strip to remove the residual stresses. Each successive roll pair has an adjustable gap to deform the sheet to a targeted quantity with the objective of resulting in a flat strip once the steel passes through all the rolls. The entry end has the smallest gap, putting in the maximum deformation. The last pair of rolls has the largest gap, normally set for material thickness. The gap profile varies based on thickness, yield strength, and equipment. Several equipment manufacturers have generated tables to guide the operator as to the best settings for various yield strength / thickness combinations. Fig 5 shows schematics of roll set-up in flatteners and levellers.
Fig 5 Schematics of roll set-up in flatteners and levellers
The flattener for sheet, strip, and plate, incorporates a series of parallel upper and lower work rolls in a staggered position. The entry side and exit side of the upper frame housing the upper set of work rolls can be independently adjusted up and down. In some flatteners, the upper frame can also be adjustable for tilting within a small range along its longitudinal centre-line. The work rolls can either have no backup rolls, as in a two-high flattener (Fig 6a), or be backed-up by a set of backup rolls, as in a four-high flattener (Fig 6b). The two-high flatteners typically have five to nine work rolls and their work-roll diameter is normally sufficiently large to avoid roll deflection. Two high flatteners can be either a self-driven or a pull-through style. The four-high flattener allows the use of small-diameter work rolls since work-roll deflection is restrained by backup rolls. The four-high flatteners are typically self-driven and have seven to thirteen work rolls. Fig 6 shows schematics of types of flatteners and levellers.
Fig 6 Schematics of types of flatteners and levellers
A roller leveller, normally simply referred to as a leveller, is similar to a four-high flattener in that the design involves four-high small diameter work rolls. Unlike the four-high flattener, however, each work roll in the leveller is supported by a number of narrow backup rolls, instead of straight solid backup rolls (Fig 6c). This arrangement allows small work rolls and a close work roll spacing in the leveller for more capability in shape correction.
A series of backup rolls at the same transverse position for all work rolls under the same frame are called a flight, and they have a common support housing extending from the entry to the exit of the roller leveller. Each flight of backup rolls can be vertically adjusted, independently from other flights, by either a mechanical or a hydraulic mechanism. A deflection of work rolls in the leveller, hence, can be deliberately adjusted in a controlled manner.
Roller levellers have normally seven to nineteen work rolls. Some levellers have the five-high or six- high backup design for high surface finish quality strip. The six-high levellers have two additional rows of straight solid intermediate rolls between work rolls and adjustable backup rolls, each at the top and bottom frames. This arrangement prevents marking on the top and bottom surfaces of the strip but limits the capability to correct poor shape since the adjustable roll flights act on the intermediate rolls.
The five-high leveller has only one row of the intermediate rolls between work rolls and backup roll, normally in the lower frame, while its upper frame contains only work rolls and backup rolls as in four-high levellers. Hence, it gives better capability for shape correction than the six-high leveller, while preventing marking on one side of the strip surfaces. A roller leveller has a certain capability range in strip thickness for a given work roll diameter and roll spacing. Normally, the upper strip thickness limit is 3 times to 4 times that of the lower limit.
Roller levellers have the ability to control the deflection of work rolls so that one portion of the material across the strip width can be subjected to more bending than another portion. The bending causes a tension in the material of the outer layers in the strip and compression in the material of the inner layers near the work roll. At a certain bending radius, the stress in the material at the outer-most layer exceeds the material yield strength and permanent plastic elongated deformation occurs.
Similarly, the material at the innermost layer experience compressive plastic deformation when the stress exceeds the material compressive yield strength. Higher degree bending with a smaller bending radius causes more material to yield and makes plastic deformation shift inward to the neutral plane (Fig 7a).
Fig 7 Bending mechanism and roller setting in a leveller
Multiple inter-changeable roll cassettes are used in a single roller leveller to extend its thickness capability range. Each cassette has its own specified roll diameter and roll spacing different from those in other cassettes.
Cluster levellers have a design with different diameter work rolls housed in clusters and provide a very wide range in strip thickness which the single cluster leveller is capable of levelling. A tension leveller is a roller leveller with a mechanism for applying tension on the strip by bridle or pinch rolls. Simplified roller levellers without independent adjustments on each flight but with a tension mechanism are also sometimes called tension levellers. During the tension levelling process, external tension is applied on the strip as it alternately passes over a series of very small-diameter work rolls, which are backed-up by bigger-diameter rolls. The strip is elongated under applied tension during the process. The range of the elongation in strip during tension levelling is typically 0.1 % to 5 %.
Principle of shape correction – The principle of shape correction by a flattener and a leveller is simple. In each case, a strip or sheet bends alternatively up and down as it passes between the upper and lower sets of work rolls. This causes selective plastic elongation in a portion of the strip or sheet. A local region of shorter length takes more external stress than a longer-length region in a strip, in addition to the fact that its residual stress is tensile.
Hence, the short-length region yields first and undergoes more plastic elongation than the original longer-length region, where the residual stress is compressive and offsets a portion of the external stress. Then, nominal uniform strip length in different regions, surface-to surface or edge-to-edge, is achieved as a result of a series of reversed bending. The residual stresses in the strip are also reduced and redistributed in a more uniform pattern after a series of bends. During tension levelling, an external tensile stress applied on the strip makes the neutral plane shift toward the inner layer (Fig 7a). Higher tension results in tensile stress across the entire thickness of the strip and, finally, plastic tensile deformation across the entire thickness.
Fig 8A shows the principle on stress-strain curves in a simplified case, where curve R represents a short-length region material and curve S represents a longer-length region material. Initially, the difference in length between R and S is ‘di’, and the residual stresses are assumed to be zero. If a tensile stress from bending or an external tension exceeds the material yield strength, plastic deformation occurs, in addition to elastic deformation. When the stress is released after bending, both curves R and S return along the slope of the elastic modulus. It can be seen that the difference in length between R and S at this point, represented by ‘df1’, is smaller than ‘di’ (Fig 8a).
If the materials undergo a reversed bending at the next set of work rolls and there is no external tension, the stress on the materials becomes compression instead of tension until the stress releases while the stress-strain curves R and S continue to the negative side (Fig 8b). If stress is high enough to cause permanent (plastic) deformation, the difference in length between R and S, ‘df2’, at the zero stress becomes even smaller than ‘df1’. This process is repeated as multiple reversed bending continues when the material moves through a series of work rolls and, finally, the difference in length between R and S becomes near zero. It is to be noted that because of Bauschinger effect, the material yield strength in the regions R and S does not increase after plastic deformation during repeat reverse bending if no external tension is applied.
The outermost layer of a thin strip has less elongation than that in a thick strip at the same bending radius. For achieving the same quantity of flattening or levelling results, a higher degree of bending (e.g., a smaller bending radius with a smaller bending wrapping angle) in a thin strip is needed than the degree of bending needed for a thick strip. This needs small-diameter work rolls as well as a shorter work roll spacing and a closer gap (e.g., more penetration) between upper and lower work rolls. However, small-diameter work rolls tend to bend under loading during processing. Hence, backup rolls in four-high flatteners and backup rolls in levellers are used to control work roll deflection.
Fig 8 Principle on stress-strain curves in a simplified case and roller setting in a leveller
Shape correction with flatteners – Both two-high and four-high flatteners can control and eliminate coil set and cross bow but cannot eliminate buckle and wavy edge. A flattener can reduce twist in a narrow strip, but eliminating high-degree twist can also introduce another type of shape defect. Normally, flatteners cannot correct camber. However, a flattener with tilting adjustment ability along the longitudinal centre-lines of its upper frame can reduce and eliminate a small degree of camber in a strip. A deliberate mis-alignment of a pull-through flattener to the line can eliminate a camber in a narrow strip (less than 50 mm). Auxiliary side push rolls near the entry side of a flattener are used for camber correction in a narrow strip (less than 100 mm wide with thickness higher than 0.6 mm).
For a coil set strip, length difference between the top and bottom surfaces diminishes after a series of reverse bending in the strip when it is passed between upper and lower rolls, and hence, coil set is corrected to the flat shape in a properly set flattener. For correcting cross bow, a much higher-degree bending is needed since the reduction in length difference between the top and the bottom surfaces in the transverse direction depends on lateral contraction resulting from longitudinal deformation. High-degree bending needs small-diameter work rolls for a small bending radius, and a closer work roll spacing for a small bending wrapping angle. This can be readily achieved in a four-high flattener.
The gap or penetration depth of work rolls at the entry side of a flattener is to be tight enough for shape correction, and the initial gap setting at the exit side is to be around equal to a strip thickness (Fig 7b). In majority of the cases, especially with more than seven work rolls, the setting at the exit side determines a final coil set direction (zero, down or up linear curvature) when the entry gap is tight enough to remove the original coil set. Excessively tight gap of work rolls in a two-high flattener can cause overloading and hence deflection (bending) of work rolls. This can cause wavy edge in a wide sheet during flattening. The upper limit of flattener capability in material thickness and strength is normally restricted by unintended roll bending, while the lower limit (in material thickness) of flattener capability is determined by a work roll diameter and a work roll spacing.
Shape correction with roller levers – Roller levellers can control and correct not only coil set and cross bow, but also wavy edges and buckles. This is since they have the ability to control the deflection of their work rolls so that one portion of the strip can be subjected to more deformation than another. In the case of the centre buckle or longer centre than the edges, the edges are to be stretched more than the centre until the same length is achieved between them. The backup rolls in the leveller are to be set tighter on the edges than the centre so that bending on the edges has a higher degree of bending than that in the centre (Fig 7c). In contrast, if the strip has wavy or loose edges, the centre is to be stretched more until it has the same length as the edges. The backup rolls are to be set up closer in the centre than on the edges (Fig 8B).
Shape correction with tension levellers – Tension levellers can correct coil set, cross bow, buckle, and wavy edges, as well as twist and camber. As shown schematically in Fig 9a, a tension-leveller applies tension to the strip as it alternately passes over and under a series of very small-diameter, backed-up rolls. When used correctly, tension levelling has proved to be the most versatile tool commercially used for improving shape of coiled material.
Fig 9 Schematics of tension levelling and slitting lines
Tension levelling is the only effective method to correct camber. The set-up parameters of a tension leveller include (i) diameter of work rolls, (ii) roll spacing in case an exchangeable roll cassette is used, (iii) adjustment at the entry side of each flight of backup rolls, (iv) adjustment at the exit side of each flight of backup roller, and (v) tension or stretch elongation.
Slitting – Slitting is a process to cut a single, wide strip, sheet or plate, normally in coil form, length-wise into a number of narrower strips. During slitting, a moving strip passes between a number of circular blades or knives mounted on two parallel rotating arbors. Slitting can be applied to several materials including steel strips, from very thin strips (less than 0.1 mm) to over 25 mm thick plate.
Slitting lines – In simple terms, a slitting line consists of three necessary units namely (i) an uncoiler, also known as a decoiler, a payoff, or an unwinder, for holding the coil and feeding the strip to the slitter, (ii) a slitter with two parallel arbors holding the circular blades and other tooling for slitting the strip, and (iii) a coiler, also known as a recoiler, take-up, or rewinder, for coiling the slit strips (Fig 9b). Other equipments which can be added to the line are for coil handling, strip feeding and guiding, shape correction, gauging, shearing, end joining, edge selvage disposal, packaging, and so forth.
The slitter itself consists of supporting structure and two parallel arbors, one above the other, mounted with shear slitter knives. The distance between two parallel slitter arbors is vertically adjustable to accommodate different diameter knives and different thicknesses of materials. This is normally done during slitter tooling setup or beginning of slitting. However, some computer numerical control precision slitters can perform a continuous minor adjustment as per the conditions of an incoming strip during slitting.
Slitting lines normally have either one or two coilers, although some are equipped with individual coilers for each cut produced. On lines with only one coiler, all cuts are wound on this coiler and metal-separating disks or overarm separators are used to keep the individual cuts separated during winding. A disadvantage of single coilers arises in slitting several cuts since the separator thickness is to be accommodated between adjacent cuts, resulting in a fan-out pattern of the cuts from the tight-line slitter (Fig 10a). This situation can induce undesired camber and rippled edges in the slit metal. To avoid this problem, two coilers are normally used such that each takes up every other cut across the width, hence eliminating fanout, as shown in Fig 10b.
Fig 10 Schematics of slitting line with one coiler and with two coilers
Modern slitting lines can slit a coil at a line speed of 300 metres per minute (m/min) to 400 m/min. Some modern slitter lines for thin strips are able to run up to 900 m/min. These slitting lines are normally designed for high productivity with fully automatic loading and feeding of an incoming coil and unloading of finish slit coils in very short time. Either exchangeable second slitter head or programmable automatic knife-position slitter head can be found in these slitter lines. The set-up of slitter tooling on the second slitter head can be verified and adjusted by off-line trial slitting. The exchange between the pre-setup second slitter head and the in-line slitter head can be completed automatically within two or three minutes. The slitting time of these slitting lines, hence, can be readily above 65 % of total operation time, which includes slitter set up time, incoming coil loading time, slitting time, and unloading time of slit coils.
Slitting lines are normally classified into three broad categories namely (i) pull-through, (ii) driven, and (iii) help-driven. The choice between pull-through and driven lines depends largely on strip shear strength and thickness, number of slit strips, slitting speed, and slit quality requirements. In general, when the steel strip to be slit is less than 0.25 mm thick, a drive or helper-drive slitter line is preferred since thin steel strip is likely to tear if it is pulled through a slitter head.
Pull-through slitting lines (Fig 11a) use a motor-driven coiler to pull the strip through the slitter. The slitter arbors are not driven in this type of slitter. This type of slitting line is normally a tight line, i.e., the strip is under tension from the uncoiler through the slitter to the coiler. However, slack strands (Fig 11b) can occur because of differences in speeds of different strip sections.
Fig 11 Schematics of types of slitting lines and the methods of looping
Driven slitting lines (Fig 11c) have motors which drive the coiler and arbors in the slitter. The uncoiler can also be driven. These motors are synchronized to maintain around constant speed of the strip as it travels through the slitting line. Driven slitting lines are preferred for thin strip since they allow winding of slit strips under lower unit tension. Driven slitting lines can operate with one or two slack strands between the slitter and the coiler, especially for thin strip, to allow for minor speed differences in different slit strips wound on a common winding arbor. The speed variation in the different slit sections is from variations in the slit-coil diameter because of poor shape and / or non-uniform thickness across the width of an incoming coil.
Looping is especially useful in slitting soft metal or thin-thickness material. Synchronization of the motors in a driven system can allow a reduction in the applied tension between two given points in the line. In driven systems, looping allows the strip to sag under its own weight, which allows it to find the path of least resistance. Three types of looping are shown in Fig 11d. In pre-looping, the loop is established between the uncoiler and the slitter, which reduces wrinkling and creasing of the steel as it enters the slitter. In post-looping, the loop is established between the slitter and the coiler, which minimizes distortion and damage to the edges of the slit widths as they scrape against the separators on the coiler.
Double-looping (Fig 11d), a combination of pre-looping and post-looping, offers the advantages of both to ensure minimal strip damage. A precision driven slitting line can be also a tight line or operated in the tight-line mode, especially when equipped with a tension leveller placed before or after the slitter and / or with a slip core coiler.
Help-driven-type slitting lines – In the help-driven type slitting line, the torque applied to the slitter arbors, from the slitter driven motor, reduces the tension on the pulled strip to avoid snagging at the entry slitter knives. The helper torque is insufficient to drive the slitters alone, hence avoiding the speed-match issue in a purely driven-type slitting line. The help-driven-type slitting line is in the tight-line configuration.
Inter-actions between knives and strip during slitting – While dish knives are mainly used for slitting plastic film, paper, and metal foil, rotary slitter knives are mostly used in steel strip slitting. The inter-actions between slitter knives and steel strip during slitting with proper slitter tooling and set-up normally can be divided into four steps described below.
The first step is at the beginning of slitting, when the steel strip moves forward to the slitter knives before a shearing point, plastic deformation occurs on the material by compressive force at knife contact surface. This can result in edge distortion or ‘roll over’ along the slit strip edge, especially in soft materials.
In the second step, as the steel strip moves further to the knife shearing point, the knives penetrate into the strip and create the flat, shiny or so-called shear or ‘burnished’ surface on the portion of the slit edge. The depth of the burnished shear surface portion depends on the material strength, ductility, and thickness of the strip as well as the slitter knife clearance.
In the third step as the knife penetrates further into the strip, the maximum shear strength of the material is exceeded, and fracture takes place, which results in a total separation of the material. The fracture surface on the edge appears dull, in contrast with the shiny, burnished shear area in the second slitting stage. The angle of fracture normally falls between 6-degree and 12-degree depending on the type and mechanical properties of the material.
In the fourth step, a burr is formed on the strip edge on the opposite side of the roll-over. Burr is attributed mainly to a compressive plastic deformation displacement around the edge corner of the knife. Fig 12a shows the exaggerated cross-section view of a single slit edge introduced by these four slitting stages.
Fig 12 Views of slit edge and standard set-ups
Slitter tooling and set-up – Slitters can be equipped with either standard (male / female) tooling or solid tooling set-ups. Solid set-ups can be used when tighter tolerances are needed than those possible with standard set-ups. In the standard set-up, slitter knives of a fixed or standard thickness (width) are used, and spacers are located between the knives to establish the desired slit width (Fig 12b). In this set-up, the slit width is equal to the full spacer width of the female load, and the slit strip has a burr pointing up. A standard slitter set-up for three cuts is shown in Fig 12d, with alternating male and female loads on each arbor. These cuts, from left to right, have edges with burr down, up, and down respectively.
Fig 12c shows basic slitter tooling and one type (male / female) of slit set-up in a single slit strip. The slitter tooling basically consists of knives, spacers, steel or rubber stripper rings over the spacers. It can also include metal or plastic shims of a thickness between 0.127 mm and 0.013 mm for width match, but this practice is being replaced by shim-less tooling. The spacers and stripper ringers in the male side can be simply substituted by a knife or knives, or whole male set of knives, spacers and stripper ringers substituted by a single knife. Wood fingers, a type of set-up used in the past, can still be found in place of stripper rings in some slitters.
Appropriate slitter tooling set-up is necessary to achieve proper slit width, slit edge, and other slit qualities. The slit width is the distance between female knives as shown in Fig 12c. Knife horizontal clearance and vertical clearance (or overlap) are very important for slit quality, especially for edge condition. A large horizontal clearance can cause excessive edge roll-over and burr, while too tight horizontal clearance can cause bad edge as well as premature wearing or damage of slit knives.
When a strip thickness becomes thinner than 0.38 mm, a horizontal clearance tends to be at the lower end of the standard percentage value or even lower, especially when edge quality is critical for burr-free. In majority of the cases, when metal strip thickness is below 0.127 mm, a zero horizontal clearance is used with thin knives. Knife overlap is also called knife vertical clearance or positive vertical clearance. It depends not only on the thickness and strength of material strip to be slit, but also on other factors such as number of slit strips, horizontal clearance, knife sharpness, rubber ring diameter or steel ring pressure, and slitter arbor deflection.
A formula to decide knife overlap is difficult to give. Normally, for the same slitter, slitter tooling, slit material, slit width, and incoming width, the knife overlap increases with strip thickness from near zero for ultra-thin foil to a maximum value for the strip at a thickness between 0.75 mm to 1.6 mm, then decreases with strip thickness from a positive value to the negative side (as in Fig 12c). In practice, it is determined by operator experience using a trial-and-error approach. At the beginning of slitting, the arbors are adjusted to bring them together until all slit strips have clean cuts. Excessive knife overlap can cause excessive knife wear, coil set, camber, and wavy edges.
It is useful to use a stereoscope or common optical magnifier to check slit edge quality. Examination of shear / fracture ratio, appearance, and burr on slit edge and their consistency can determine whether the slitter tooling clearance setup is correct. The selection of proper thickness knives, spacers, and their combination for needed slit widths can be made using manual calculation or readily by a commercial computer software programme. These are typical process parameters which can be applied universally with some mathematical calculations, either by a computer programme or manually. Other process parameters are specific for different slitter machines, different sets of slitter tooling, different materials, different materials thickness, and different width. A slitter can have hundreds of set-up sheets, and different slitters with different sets of slitter tooling can have different process parameters for tooling set-up even for the same material and same size.
Slit quality attributes and trouble-shooting – Quality attributes of slit strip include slit width, burr, edge quality, cross bow, and camber. Slit width is the most important slitting attribute in slitting operations. A slit width tolerance of +0.075 mm can be readily achieved for a slit width less than 75 mm and +0.125 mm for a width less than 175 mm on a modern precision slitter.
Slit width is equal to the distance between female knives if the strip is held flat at the slit knives. Verification of correct slitter tooling setup is to be performed to check whether the width is within the needed tolerance. A loose arbor nut can cause the slit width to become out of tolerance. Excessive arbor deflection, poor slitter arbor maintenance condition, or other slitter machine issues can be the reasons for poor width accuracy. Use of pressure boards or multiple deflection rolls at the entry side of the slitter knife can overcome an inconsistency in slit width because of poor shape of a thin strip.
Burr is normally found to some extent on slit edge. Dull slitter knife or too much horizontal clearance of the knives is a primary cause of excessive burr. Knife vertical clearance, machine rigidity, machine maintenance, and precision of knives also have effects on severity of burr. Checking the actual knife clearance by filler gauges (measuring / checking the distance of a gap by inserting different thick metal ‘sheets’ into the gap), inspecting knife sharpness, and readjustment of slitter tooling setting are among the first things to do for trouble-shooting of excessive burr.
Measurement of the distance of a gap is done by inserting different thickness of metal sheet into the gap. Plastic shim used in slitter tooling set-up is not desirable since it can become soft and compressed and, hence, change the horizontal clearance when slitter tooling becomes hot during high-speed slitting. If burr-free edges are needed, a subsequent device such as a deburr roll or file can be installed in the line to roll down the burr or to remove the burr.
Poor edge quality can be caused by too-tight slitter knife clearance when the edge appears rough or there are metal fines or slivers on the strip edge. Too-tight horizontal clearance causes double shear and break. Speed mismatch between the slitter and the recoiler in a tight slitter line can also result in poor edge. Inspection and adjustment of separators near or at the coiler and knife vertical overlap are also desired when poor slit edge is present.
Turnover – Another feature which can accompany a slit edge is called turnover, or pulldown. As shown in Fig 13a, turnover is a slight curvature along the slit edge created by the shearing action of the slitting process. Turnover, normally found only in very soft metal, is almost negligible on the slit edges of precision sheet, strip, and foil.
Edge conditioning – There are several applications which need reorientation or elimination of the edge burr on slit strip to facilitate handling, accommodate tooling clearances, or eliminate cracking during severe forming operations. In order to meet these needs, the slit strip is to be processed through specialized edge conditioning equipment after slitting.
In cases where the intention is merely to reorient or mash down the burr to prevent personal injury or jam ups in processing equipment, the edge-conditioning unit utilizes a pair or a series of rolls which flatten the burr, or turn it 90-degree from a vertical to a horizontal plane, as shown in Fig 13e. This method still leaves the strip with a burr, except that it sticks out instead of down, which is the direction of a slitting burr on an unconditioned edge (Fig 13d). A modification of this method is one in which the burr is turned 90-degree to a horizontal plane and then mashed along the edge of the strip, as shown in Fig. 13f.
Again, since the burr has not actually been removed, the strip cannot be thought of as deburred, although the resulting edge is satisfactory for some applications. A truly deburred edge is one which has been processed through some type of cutting tool which actually removes the burr, resulting in a cross section such as that shown in Fig 13b.
There are some high-performance applications which need edges from which have been removed not only the burr, but also the brittle metal in the fracture zone of the cut, to prevent the possibility of strip breakage emanating from edge cracks. This type of edge, normally called a rounded edge (Fig 13c), is produced by processing the strip through side-mounted shaving, filing, or grinding equipment.
Fig 13 Turnover, edges, and burrs
Cross bow is a transverse curvature across the width in slit metal strip (Fig 1). It can either come from an incoming material strip because of the residual stress or poor shape or be introduced by poor slitter tooling set-up. Normally, high cross bow has the same direction in adjacent slit strips if it comes from the incoming material. If the cross bow is introduced by poor slitter tooling set-up, there is a need to check whether slitter knife horizontal clearance is too tight, slit knife overlap is too much, rubber ring diameter is too large, or steel ring pressure is too high.
Camber is width-wise curvature present longitudinally in the same plane of metal strip (Fig 2). Camber can be introduced by the slitter or be attributed to an incoming material strip itself. For verifying whether the camber comes from the incoming strip, one can simply flip over the incoming strip to observe whether the camber of the slit strip is in the same direction as the one before flipping. If the camber has the same direction from the slitter head, the camber is most likely introduced by the slitter tooling set-up, and the slitter tooling then needs to be reset to correct the issue.
Excessive burr on one edge of the slit strip can also cause camber when the slit strip is wound into a coil. The other slitter-introduced camber is because of the ‘fan-out’ of slit strips from the slitter to the coiler in a tight slitter line when there are several narrow-slit strips. In this case, the slit strips near the edges can have more severe camber than the centre slit strips. Trouble-shooting of reverse direction camber along the same slit strip can need checking the slitter tooling and its set-up, slitter arbors, housing bearings, and slitter design capability as well as an incoming material strip.
Coiling uniformity describes how even the side edge of the wound finish slit coils is. The coiling uniformity depends on strip shape condition, strip thickness uniformity across width, slit burr, residual stress in the material, and coiling tension control during slitting. For an intermediate thickness between 0.4 mm and 1.5 mm, separating plates between each slit cuts at the coiler can be used to force the slit strip being wound to wobble only within a small range move during coiling in order to achieve even coiling uniformity for each slit coil. However, caution is to be taken, otherwise a separating plate can damage the slit edge during coiling.
Cut-to-length lines – Cut-to-length lines are used to cut a steel plate, sheet, or strip transversely to a certain needed length from the coil. Different technologies, such as laser cutting, plasma cutting, and water jet cutting, can be used in cutting plate or sheet to a needed length. However, cut-to-length lines are normally referred to the coil process lines using shearing or knife cutting. Majority of the cut-to-length lines are those which use straight knife shearing. However, some fundamental of line configurations in cut-to-length lines can be applicable when an alternative cutting method is used to replace knife shearing.
All cut-to-length lines normally have at least four functions. These are (i) coiled product (sheet, plate, or strip) is first uncoiled and sent to a leveller or flattener, (ii) steel is levelled or flattened by the leveller or flattener for ensuring its flatness before it is fed to a shear, (iii) steel is then cut by the shear to a prescribed length, and (iv) finally, sheared plates or sheets are stacked by a stacker.
The lines normally have feeding or conveyor systems to support these four functions. Some cut-to-length lines can also incorporate a slitter for edge trimming or strip slitting before the shear. In modern lines, the length-measuring or gauging system can be computer numerically controlled.
Stationary shear lines – There are two basic types of shear lines namely (i) stationary shear type, and (ii) flying shear type. Majority of the cut-to-length lines use stationary shears, i.e., the strip is stopped at the shear during the cut. These lines are normally more accurate than other types. There are three types of feeding arrangements for stationary shear lines namely (i) start / stop, (ii) hump table, and (iii) looping pit.
Stop / start cut-to-length lines (Fig 14a) are normally arranged so that the coil is fed into the shear to a prescribed length and then stopped during the cut. Upon completion of the cut, the coil feed accelerates until it stops for the next cut. Close tolerances can be achieved, and line configuration is simple. However, the throughput (meter/minute or sheet/minute) is low. Hence, these lines are normally confined to heavy-gauge requirements. Stopping the coiled sheet in the flattener or leveller prior to shearing can leave a mark on lighter-gauge materials.
Fig 14 Types of cut-to-length lines
Stationary shear lines with hump tables (Fig 14b) consist of an uncoiler, a flattener, and / or a leveller for correcting the strip shape and for feeding the strip over a hump table, a stationary shear, a gauge table with retractable stop, and a stacker which stacks the sheared plates or sheets as they are delivered from the gauge table.
The retractable stop with gauge table, which follows the shear sequence, is used to control the length of the cut sheets. When the uncoiled strip touches the gauge stop, it triggers a limit switch which actuates the shear to cut a sheet. Since the strip continues to flow from the uncoiler, it causes a hump to form above the hump table in front of the shear. When shearing is completed, the gauge stop retracts, and the cut sheet is delivered to the stacker. As the cut sheet is removed, it activates a limit switch which resets the gauge stop. Then, the shear opens, permitting the strip to slide forward through the shear and onto the gauge table against the gauge stop again, and the cycle is repeated.
Looping pit shearing line is shown in Fig 15. Several stationary shear lines have precision measuring feeder rolls just before the shear, instead of hump and gauge tables. In these lines, there is a looping pit below the pass line just before the feed rolls and the shear. The flattener and / or leveller runs continuously. A slitter for edge trimming or normal strip slitting can be added after the flattener or leveller and before the loop pit. The strip can be accumulated in the pit during a shearing cycle. Side guides control the feed angle of the strip for maximum cut squareness as it exits the looping pit and enters the shear.
In some lines, the shear knives can be pivoted to cut trapezoidal blanks. The stacker is equipped at the end of the line to pick up and stack the sheared pieces for packing or subsequent operations. An air cushion can be used in some stackers for high surface finish quality of the cut-to-length products.
Fig 15 Cut to length line with looping pit
A stationary shear line normally provides the best squareness and length tolerance and maximum productivity for the lighter-gauge materials. Because of the nature of the loop, this feeding method is not practical for strip over around 6 mm thick. A hump line can be somewhat less expensive when the costs of installation are included to consider, but tolerance, squareness, and productivity can be sacrificed.
Flying shear lines – There are several different types of flying shear lines such as rocker (flying die), rotary drum, or oscillating types of flying shears. The lines are designed to shear steel strip or sheet in a way to synchronize a speed match with the moving steel. Since the shear action matches the speed of the moving strip, sheet, or plate, an accurate sheared length can be achieved without having to stop the strip and to restart each time of shearing.
This continuous flow results in high line productivity. However, the flying lines normally cost more than stationary shear lines when a certain designed requirement in accuracy is needed. The lines can include an uncoiler, a flattener, measuring rolls, a flying shear, a run-out conveyer, and a stacker. The rocker shear line is the least-expensive type in the flying shear lines. It is best suited to light-gauge sheet or strip shearing at slow speeds. The rotary drum shear is frequently used for light-gauge narrow strip applications. It offers normally a high shearing accuracy at high speeds. The oscillating shear line is normally used for high-quality finished parts with higher speeds than rocker shear lines.
Blanking lines are a special type of shearing line. Blanking lines have the special capability of rapidly cutting and stacking relatively short and dimensionally accurate blanks as final parts.
Cut-to-length line capability – The capability of cut-to-length line in maximum plate / strip thickness is limited by the shear capability but can also be affected by the thickness capability of a flattener or leveller in the line for tight requirements in flatness and dimensional accuracy. Dimensional accuracy of the cut sheet length depends on the line configuration, condition of the equipment, speed and length of sheet, and the condition of the master coil. Majority of the modern lines normally can achieve an accuracy of +0.8 mm, and up to +0.4 mm when equipped with sophisticated devices.