Rolling Process for Steel
Rolling Process for Steel
Rolling of steel is perhaps the most important metal-working process. More than 90 % of all the steel go through the rolling process at least one time. Hence, rolled steel products represent a significant portion of the manufacturing economy and can be found in several sectors. Beams and columns used in buildings are rolled from steel. Railway tracks, wagons and coaches are made from rolled steel. The wire used in fences, elevator ropes, electrical conductors, and cables are drawn from rolled steel rods. Several consumer items, including automobiles, home appliances, kitchen, utensils, and beverage cans, use rolled steel sheets. Several parts in automobiles are made by cold, warm, and hot forging using rolled steel bars as the starting material.
Rolling process is plastically deformation of steel by passing it between rolls. Rolling is defined as the reduction of the cross-sectional area of the work-piece being rolled, or the general shaping of the steel products, through the use of the rotating rolls.
Rolling of steel is normally the first step in the processing of steel after it is made and cast in a steel melting shop. The initial rolling of steel is done in a hot rolling mill where slabs, blooms, and billets are rolled down to different rolled products such as plates, sheets, strips, structures, rails, bars, rods, and merchant products. Cold rolling of hot rolled strip is also carried out. Some of these rolled products such as rails, and reinforcement bars etc. are directly used by the consumers while the other rolled products are the starting raw materials for subsequent manufacturing operations for producing different steel products.
In rolling, a squeezing type of deformation is carried out by using two work rolls rotating in opposite directions. The principal advantage of rolling lies in its ability to produce desired shapes from relatively large pieces of metals at very high speeds in a somewhat continuous manner. Since other methods of metal-working, such as forging, are relatively slow, majority of large blooms are rolled into billets, bars, structural shapes, wire rods (for drawing into wire), and rounds (for making seamless tubing). Steel slabs are rolled into plate, strip, and sheet.
Although rolling of metals has been done for some time and has been a very productive means of working large quantities of metals to a variety of shapes and sizes, the state of the technology had been somewhat stagnant until the 1970s, when major innovations started to appear. With the arrival of computer-assisted controls, highly automated, very high-speed rolling mills were installed beginning in the 1970s. The rolling mills are now been operated from climate-controlled pulpits equipped with computerized controls and closed-circuit video monitors. The rolling mills of today feature computer controls which automatically adjust water flow rates, roll speeds, and stock temperatures for meeting the metallurgical requirements. In addition to these developments, computer-aided modelling of the rolling process is now routinely used for design of rolls and optimization of the process parameters. Further, understanding of the materials also has improved considerably, hence permitting development of new products such as high-strength low-alloy (HSLA) steels, which need controlled rolling. In short, considerable developments are happening in this field, which was largely neglected for decades.
Since its development in the 1950s, continuous casting has been widely used for making blooms, slabs, and billets. In continuous casting, liquid steel from the steelmaking operation is cast directly into semi-finished shapes of slabs, blooms, and billets. With the arrival of continuous casting, the casting of ingots and their rolling into slabs and blooms in slabbing mill and blooming mill is phased out.
Blooms, slabs, and billets are rolled into different products such as plate, sheet, pipe, rod, bar, and structural shapes. The definitions of these terms are rather loose and are based on the traditional terminology used in the primary metal industry. For example, the bloom has a nearly square cross section with each side larger than 200 mm, the billet has a nearly square cross section with each side ranging from 80 mm to 150 mm, and a slab has a cross-section with a width of more than twice the thickness. Plates are normally thicker than 6 mm, whereas sheets are thinner-gauge materials with very large width-to-thickness ratios. Sheet material with a thickness of less than half a millimeter is referred to as black plate which is used for the production of tin plate.
Rolling of blooms, slabs, billets, plates, and structural shapes is normally done at temperatures above the recrystallization temperature, that is, in the hot-forming range, where large reductions in height or thickness are possible with moderate forming pressures. Sheet and strip are frequently rolled cold in order to maintain close thickness tolerances. Fig 1 shows a typical rolling mill stand and rolling sequences for producing bar, shape, plate and strip / sheet.
Fig 1 Rolling mill stand and rolling sequence
During rolling, steel work-piece is subjected to high compressive stresses as a result of the friction between the rolls and the surface of work-piece being rolled. The work-piece is plastically deformed by the compressive forces between two constantly rotating rolls. These forces act to reduce the thickness of the steel and affect its grain structure. The reduction in thickness which is the difference in the thickness before and after the reduction is known as draft. In addition to reducing the thickness, the rolls cause feeding of the material as they rotate in the opposite direction to each other.
Friction is a necessary part of the rolling process, but too much friction can be detrimental for a variety of the reasons. Since level of friction is to be controlled in the rolling process, lubrication is an important factor during rolling. For the work-piece to enter the throat of the roll, the component of the friction force is to be equal to or higher than the horizontal component of the normal force.
Torque and power are the two important components of rolling. Torque is the measure of the force applied to the rolls to produce rotational motion while power is applied to a rolling mill by applying a torque to the rolls and by means of work-piece tension. In a rolling mill, the power is spent principally in the four ways namely (i) energy needed to deform the steel, (ii) energy needed to overcome the frictional force, (iii) power lost in the pinions and power transmission system, and (iv) electrical losses in the different motors. Sometimes during rolling of steel, tension (force) is applied to the work piece as it is being rolled. The tension can be applied to the front (front tension), can be applied to the back (back tension), or can be applied at both the ends. This technique helps the forces necessary for rolling of the steel.
The primary objectives of the rolling process are to reduce the cross section of the incoming material while improving its properties and to achieve the desired section at the exit from the rolls. The process can be carried out hot, warm, or cold, depending on the application and the material involved. The technical information available for rolling technology, equipment, and rolling theory is extensive because of the significance of the process. Several industrial personnel prefer to divide rolling into cold rolling and hot rolling processes. From a fundamental point of view, however, it is more appropriate to classify rolling processes on the basis of the complexity of metal flow during the process and the geometry of the rolled product. Hence, the rolling of solid sections can be divided into the categories as given below.
Uniform reduction in thickness with no change in width – This is the case with strip or sheet rolling where the deformation is in plane strain, that is, in the directions of rolling and sheet thickness. This type of metal flow exists, when the width of the deformation zone is at least 20 times the length of that zone.
Uniform reduction in thickness with an Increase in width – This type of deformation occurs in the rolling of blooms, slabs, and thick plates. The material is (i) elongated in the rolling (longitudinal) direction, (ii) is spread in the width (transverse) direction, and (iii) is compressed uniformly in the thickness direction.
Moderately non-uniform reduction in cross-section – In this case, the reduction in the thickness direction is not uniform. The material (i) is elongated in the rolling direction, (ii) is spread in the width direction, and (iii) is reduced non-uniformly in the thickness direction. Along the width, material flow occurs only toward the edges of the section. The rolling of an oval section in rod rolling or of an airfoil section is considered to be in this category.
Highly non-uniform reduction in cross-section – In this type of deformation, the reduction in the thickness direction is highly non-uniform. A portion of the rolled section is reduced in thickness, while other portions can be extruded or increased in thickness. As a result, in the width (lateral) direction material flow can be toward the centre. Of course, in addition, the material flow takes place in the thickness direction as well as in the rolling (longitudinal) direction.
The above shows that, except in strip rolling, material flow in rolling is in three dimensions (in the thickness, width, and rolling directions). Determinations of material flow and rolling stresses in shape rolling are very important in designing rolling mills and in setting up efficient operations during production. However, the theoretical prediction of material flow in such complex cases is extremely difficult. Since the 1980s, finite-element models for the analysis of material flow in shape rolling have been developed. Such models have been shown to be fairly accurate in the prediction of material flow, rolling loads, and torques.
Strip rolling theory – The most rigorous analysis was performed by Orowan and has been applied and computerized by different people. Studies from the 1970s consider elastic flattening of the rolls and temperature conditions which exist during rolling. The roll-separating force and the roll torque can be estimated with different levels of approximations by such mathematical techniques as the slab method, the upper bound method, or the slip line method of analysis. Computerized numerical techniques are also being used to estimate material flow, stresses, roll-separating force, temperatures, and elastic deflection of the rolls.
Simplified method for estimating roll separating force – The strip-rolling process is shown in Fig 2. Because of volume constancy, the relations which hold is indicated by the equation w x H0 x V = w x H x V = w x H1 x V1 (equation 1) where ‘w’ is the width of the strip, ‘H0’, ‘H’, and ‘H1’, are the thicknesses at the entrance, in the deformation zone, and at the exit respectively, and ‘V0’, ‘V’, and ‘V1’, are the velocities at the entrance, in the deformation zone, and at the exit respectively. In order to satisfy equation 1, the exit velocity ‘V1’, is to be higher than the entrance velocity ‘V0’. Hence, the velocity of the deforming material in the ‘X’ or rolling direction is to steadily increase from entrance to exit. At only one point along the roll / strip interface is the surface velocity of the roll, ‘VR’, equal to the velocity of the strip. This point is called the neutral point, or neutral plane and is indicated by ‘N’ in Fig 2.
Fig 2 Representation of strip rolling
The interface frictional stresses are directed from the entrance and exit planes toward the neutral plane since the relative velocity between the roll surface and the strip changes its direction at the neutral plane. This is considered later in estimating rolling stresses. An approximate value for the roll-separating force can be achieved by approximating the deformation zone, shown in Fig 2, with the homogeneous plane-strain upsetting process.
The stress (roll pressure) distribution in strip rolling – The maximum stress is at the neutral plane ‘N’. These stresses increase with increasing friction and length of the deformation zone ‘XD’. Tensile stresses applied to the strip at entrance or exit have the effect of reducing the maximum stress and shifting the position of the neutral plane. The stress distribution can be calculated by using the equations derived in majority of textbooks of rolling process or by following the theory presented by Orowan. However, these calculations are quite complex and need numerical techniques in order to avoid an excessive number of simplifying assumptions. These calculations can also be carried oud with the help of a computerized programme.
For a numerical / computerized calculation of rolling stresses, the deformation zone can be divided into an arbitrary number of elements with flat, inclined surfaces. The element is located between the neutral and exit planes since the frictional stress ‘t’ is acting against the direction of metal flow. When this element is located between the entrance and neutral planes, ‘t’ acts in the direction of metal flow. The stress distribution within this element can be obtained by use of the slab method, as applied to plane-strain upsetting. If the element shown is located between the entrance and neutral planes, then the sign for the frictional shear stress ‘t’ is to be reversed.
The stress boundary conditions at exit and entrance are known. Hence, to calculate the complete stress (roll pressure) distribution and to determine the location of the neutral plane, the length of the deformation zone ‘XD’ is divided into ‘n’ deformation elements. Each element is approximated by flat top and bottom surfaces. Starting from both ends of the deformation zone, that is, entrance and exit planes, the stresses are calculated for each element successively from one element to the next. The calculations are carried out simultaneously for both sides of the neutral plane. The location of the neutral plane is the location at which the stresses, calculated progressively from both exit and entrance sides, are equal. This procedure has been computerized and extensively used in cold rolling and hot rolling of sheet, in plane-strain forging of turbine blades, and in rolling of plates.
Roll-separating force and torque – The integration of the stress distribution over the length of the deformation zone gives the total roll-separating force per unit width in strip rolling. In the deformation zone, the frictional force is in the rolling direction between entry and neutral planes. It changes direction between the neutral and exit planes.
Elastic deflection of rolls – During rolling of strip, especially at room temperature, a considerable quantity of roll deflection and flattening can take place. In the width direction, the rolls are bent between the roll bearings, and a certain quantity of crowning, or thickening of the strip, occurs at the centre. This can be corrected by either grinding the rolls to a larger diameter at the centre or by using back-up rolls. In the thickness direction, roll flattening causes the roll radius to ‘enlarge’, increasing the contact length. There are several numerical methods for calculating the elastic deformation of the rolls. A method for approximate correction of the force and torque calculations for roll flattening entails replacement of the original roll radius ‘R’ with a larger value ‘R0’. A value of ‘R0’ is suggested by Hitchcock and is referred to extensively in the literature.
Mechanics of plate rolling – In rolling of thick plates, material flow occurs in three dimensions. The rolled material is elongated in the rolling direction as well as spread in the lateral or width direction. Spread in rolling is normally defined as the increase in width of a plate expressed as a percentage of its original width. The spread increases with increasing reduction and interface friction, decreasing plate width-to-thickness ratio, and increasing roll-diameter-to-plate thickness ratio. In addition, the free edges tend to bulge with increasing reduction and interface friction.
Lateral spread frequently leads to undesirable features such as double bulging at the plate edges, which is associated with inhomogeneous deformation. It is also widely acknowledged that the issues such as edge cracking, centre splitting, and alligatoring have their origins in the non-homogeneous deformation accompanying lateral spread. Estimation of lateral spread in rolling has been the subject of considerable investigation over the past several decades. The three-dimensional (3-D) material flow which occurs in plate rolling is difficult to analyze. Hence, majority of the studies of this process have been experimental in nature, and several empirical formulas have been established for estimating spread. Attempts have been made to predict elongation or spread theoretically. Once the spread has been estimated, the elongation can be determined from the volume constancy, or vice versa.
Empirical method for estimating spread – Among the different formulas available for predicting spread, Wusatowski’s formula is used most extensively. The empirical formula for predicting spread gives reasonable results within the range of conditions for the experiments from which the formula has been developed. Other notable early empirical studies have been those of Sparling, and Helmi and Alexander. In a more recent study, Raghunathan and Sheppard proposed an equation for lateral spread in plate rolling. Although there are several empirical formulas available, there is no single formula which makes accurate predictions for all the conditions which exist in rolling.
Hence, it is frequently necessary to attempt to estimate spread or elongation by theoretical means. The theoretical prediction of spread involves a rather complex analysis and needs the use of computerized techniques. A modular upper-bound method has been used to predict material flow, spread, elongation and roll torque. The principles of this method are described below. Fig 3 shows the coordinate system, the division of the deformation zone into elements, and the notations used. The spread profile is defined in terms of a third-order polynomial ‘w(x)’ with two unknown coefficients ‘a1’ and ‘a2’. The location of the neutral plane ‘xn’ is another unknown quantity. The kinematically admissible velocity field, initially suggested by Hill is used. Also, the upper-bound method can be applied to predict spread. A computer programme can be used for some steps in the analysis. Fig 3 shows rolling of thick plate in a plate mill.
Fig 3 Rolling of thick plate in a plate mill
Prediction of stresses and roll-separating force – Once the spread (the boundary of the deformation zone) has been calculated, this information can be used to predict the stresses and the roll-separating force. The computerized procedure used here is in principle the same as the method described for predicting the stresses in strip rolling. The deformation zone under the rolls is divided into trapezoidal slabs by planes normal to the rolling direction and along the stream tubes. As expected from the slab analysis, the stress distributions are very similar to those which are there for strip rolling.
By use of a numerical approach similar to the strip rolling, detailed predictions of stresses, in both the longitudinal and lateral directions, can be made. The stresses are calculated by assuming the frictional shear stress ‘t’ to be constant, as in the case of upper-bound analysis. Hence, the stress distribution at different planes along the width, or ‘Y’ direction, is linear on both sides of the plane of symmetry. The stress distribution in the rolling, or ‘X’ direction, is calculated along the streamlines of material flow. At each node of the mesh, the lower of the ‘sz’ values (Fig 3) is accepted as the actual stress. Hence, a tent-like stress distribution is achieved. Integration of the stresses acting on the plane of contact gives the roll-separating force.
Shape rolling – Rolling of shapes, also called caliber rolling, is one of the most complex deformation processes. A round or round-cornered square bar, billet, or bloom is hot rolled in several passes into relatively simple sections such as rounds, squares, or rectangles (flats), or complex sections such as angle, channel, tee, beam or other irregular shapes. For this purpose, a number of intermediate shapes or passes are used, as shown in Fig 4 for the rolling of angle sections. The design of these intermediate shapes, that is, roll pass design, is based on experience and differs from one organization to another, even for the same final rolled section geometry. Relatively few quantitative data on roll pass design are available in the rolling mill literature.
Fig 4 Rolling of shapes
Basically, there are two methods for rolling shapes or sections. The first method is universal rolling (Fig 4). The second method is caliber rolling. In universal rolling, the mill and stand constructions are more complex. However, in the rolling of I-beams, H-beams or other similar sections, this method allows more flexibility than does caliber rolling and needs fewer passes. This is achieved since this method provides appropriate quantities of reductions, separately in webs and flanges.
For successful rolling of shapes, it is necessary to estimate for each stand, the roll separating force and torque, the spread and elongation, and the appropriate geometry of the roll cavity or caliber. The force and torque can be estimated either by using empirical formulas or by approximating the deformation in shape rolling with that occurring in an ‘equivalent’ plate rolling operation. In this case, the ‘equivalent’ plate has initial and final thicknesses which correspond to the average initial and final thicknesses of the rolled section. The load and torque calculations can be performed for the ‘equivalent’ plate. The results are approximately valid for the rolled shape being considered. With advances in finite-element analysis in the 1980s and 1990s, very accurate predictions of material spread, elongation, rolling loads, and torques can be obtained using finite-element models.
Estimation of elongation – During the rolling of a given shape or section, the cross section is not deformed uniformly. This is shown in Fig 5 for a relatively simple shape. The reductions in height for zones ‘A’ and ‘B’ are not equal (5a). Hence, if these two zones, ‘A’ and ‘B’ are completely independent of each other (5b), zone ‘B’ is much more elongated than zone ‘A’. However, the two zones are connected and, as part of the rolled shape, are to have equal elongation at the exit from the rolls. Hence, during rolling, metal is required to flow from zone ‘B’ into zone ‘A’ so that a uniform elongation of the overall cross section is achieved (Fig 5c). This lateral flow is influenced by the temperature differences which exist in the cross section because of variations in material thickness and heat flow.
For the estimation of the overall elongation, it is necessary to divide the initial section into a number of ‘equivalent’ plates (‘A’, ‘B’, ‘C’, and so forth), as shown in Fig 5. The elongation for an individual section, without the combined influence of other portions of the section, can be estimated by using both the plate-rolling analogy and other techniques. The combined effect can be calculated by taking a ‘weighted average’ of the individual elongations.
Fig 5 Analysis 0f roll stand in rail rolling and non-uniform deformation in shape rolling
Computer-aided roll pass design – Shape rolling converts large, normally square sections, into smaller sections of different shapes. The most frequently used breaking-down sequences are box pass, diamond pass, square-diamond-square passes, and square-oval-square passes. The nature of deformation in shape rolling is three dimensional and is normally quite complex to analyze. Even after several decades of studies, roll pass design still continues to be more of an art with rolling mills heavily dependent on the experience and skill of roll pass designers. The application of computers in roll pass design is a logical extension of the development of rolling. Several studies in the 1980s aimed at developing computer-based analysis programmes for designing and optimizing roll pass sequences. Development of such programmes has been hindered by the lack of availability of generic models to predict lateral spread.
For successful rolling of shapes, it is necessary to estimate the number of passes needed as well as for each pass, the rolling force, torque, roll geometry, spread, and elongation in the roll bite. In an early study, Lendl proposed an empirical procedure for roll pass design of simple square, diamond, round, and oval grooves. In this procedure, the pass cross section is sub-divided into vertical strips and the spread of these strips is calculated using empirical formulas developed by Ekelund. Lendl has also demonstrated the application of this procedure to multi-pass designs using square, diamond, and oval passes.
Several other procedures, based on a similar approach, have been proposed since then. Some of these have been integrated into computer programmes for designing roll pass sequences, some of which are commercially available. Estimation of the number of passes and the roll geometry for each pass is the most difficult aspect of shape rolling. Ideally, to accomplish this, certain factor described below, is to be considered.
The characteristics of the available installation – These include diameters and lengths of the rolls, bar dimensions, distance between roll stands, distance from the last stand to the shear, and tolerances which are needed and which can be maintained. The reduction per pass is to be adjusted so that the installation is used at a maximum capacity, the roll stands are not overloaded, and roll wear is minimized. The maximum value of the reduction per pass is limited by (i) the excessive lateral metal flow, which results in edge cracking, (ii) the power and load capacity of the roll stand, (iii) the requirement for the rolls to bite in the incoming bar, (iv) roll wear, and (v) tolerance requirements. At the present stage of technology, the above factors are considered in roll pass design by using a combination of empirical knowledge, some calculations, and some educated guesses.
A methodical way of designing roll passes needs not only an estimate of the average elongation, but also the variation of this elongation within the deformation zone. The deformation zone is limited by the entrance, where a pre-rolled shape enters the rolls, and by the exit, where the rolled shape leaves the rolls. This is shown in Fig 5. The deformation zone is cross-sectioned with several planes (for example, planes 1 to 5 in Fig 5, 1 is at the entrance, 5 is at the exit). The roll position and the deformation of the incoming bloom are investigated at each of these planes. Hence, a more detailed analysis of material flow and an improved method for designing the configuration of the rolls are possible. It is evident that this process can be drastically improved and made extremely efficient by the use of computer-aided techniques.
During the 1970s and 1980s, majority of the organizations which produced shapes computerized their roll pass design procedures for rolling rounds or structural shapes. In the majority of these applications, the elongation per pass and the distribution of the elongation within the deformation zone for each pass are predicted by using empirical formulas. If the elongation per pass is known, it is then possible, by use of computer graphics, to calculate the cross-sectional area of a section for a given pass, that is, the reduction and the roll geometry. The roll geometry can be expressed parametrically (in terms of angles, radii, and so forth). These geometric parameters can then be varied to optimize the area reduction per pass and achieve an acceptable degree of fill of the roll caliber used for that pass.
Modelling of microstructure evolution in hot rolling – Traditionally, rolling process designers have been primarily concerned with ensuring correct material flow during the rolling process. In doing so, they normally make use of their experience as well as empirical and analytical guidelines which has been established over the years. Recently, there has been a growing emphasis on the micro-structure of the rolled product and its properties. New grades of micro-alloyed steel which have been developed for cold and warm forging applications call for precise control over the product micro-structure.
Control over micro-structure needs a good understanding of the effect of rolling mill variables on the resulting micro-structure. Variables such as pre-heating time and temperature, rolling deformation, deformation rate, inter-stand cooling and post-rolling controlled-cooling affect the grain size distribution, recrystallization, and phase transformation kinetics which ultimately determine the final micro-structure and mechanical properties in the rolled product. Micro-structural changes occurring at different stages in the rolling process affect the final micro-structure and properties of the rolled product. A typical thermo-mechanical cycle and microstructural evolution during the rolling process is shown in Fig 6
Fig 6 Thermo-mechanical control rolling
Controlled rolling to achieve desired micro-structure and properties is normally known as thermo-mechanical control process (TMCP). In recent years, TMCP has been effectively used in the hot rolling of plates. More recently, this technique has been applied for the hot rolling of bars, rods, and shapes using an integrated approach. In thermo-mechanical control processes, history of temperature, strain, and strain rates at different locations in the work-piece obtained using finite-element analysis or other analytical methods are used in conjunction with micro-structure evolution models to model micro-structural changes during rolling, specifically, static and dynamic recrystallization, grain growth, and phase transformation. Because of the complexity of the physical model as well as the large number of computations involved, such analyses are typically carried out using specially developed computer programmes.
Rolling mills – Rolling mills are classified by descriptive dimensions which indicate the size of the mill, by the arrangement of roll stands, and by the type of product which is rolled. The dimensions used to indicate size vary depending on the type of mill and the product. However, there are three principal types of rolling mills, referred to as two-high, three-high, and four-high mills. This classification, as the names indicate, is based on the way the rolls are arranged in the housings. A two-high stand consists of two rolls, one positioned directly above the other, a three-high mill has three rolls, and a four-high mill has four rolls, also arranged one on top of the other. Fig 7 shows three types of rolling mills.
Fig 7 Types of rolling mills
Two-high mills can be either pull-over (drag-over) mills or reversing mills. In pull-over-type mills, the rolls run in only one direction. The work-piece is to be returned over the top of the mill for further rolling, hence the name pull-over. Reversing mills use rolls on which the direction of rotation can be reversed. Rolling then takes place alternately in two opposite directions. Reversing mills are among the most widely used in industry and can be used to roll slabs, blooms, and billets to produce plates, squares, rounds, and partially formed sections suitable for rolling into finished shapes on other rolling mills.
In three-high mills, the top and bottom rolls rotate in the same direction, while the middle roll rotates in the opposite direction. This allows the work-piece to be passed back and forth alternately through the top and middle rolls and then through the bottom and middle rolls without reversing the direction of roll rotation.
Four-high mills are used for rolling of flat materials such as sheet, strip and plate. This type of mill uses large back-up rolls to reinforce smaller work rolls, hence getting fairly large reductions without excessive quantities of roll deflection. Four-high mills are used to produce wide plates and hot-rolled or cold-rolled sheets, as well as strip of uniform thickness.
Specialty mills – Two types of specialty mills are used (Fig 8). These are cluster mills and planetary mills. The most common type of cluster mill is the 20-high Sendzimir mill. In a typical Sendzimir mill design (Fig 8a), each work roll is supported through its entire length by two rolls, which in turn are supported by three rolls. The pyramid configuration of the backup rolls transmits the roll separating force along the length of the work rolls, through the intermediate rolls, to the backing assemblies, and finally to the rigid mono-block housing. Since the work rolls are supported throughout their length, any uncontrolled deflection is minimal, and extremely close-gauge tolerances can be maintained across the full width of the material being rolled. Sendzimir mills are used for the cold rolling of sheet to precise thicknesses.
Fig 8 Types of specialty rolling mills
Planetary mills were developed in Germany to reduce slabs to hot-rolled strip in a single pass. This is accomplished by the use of two back-up rolls surrounded by a number of small work rolls (Fig 8b). Planetary mills are capable of reductions of up to 98 % in a single pass and have been designed up to 2,000 mm in width.
Rolls – Of all the components of a rolling mill, the rolls are probably of main interest, since they control the reduction and shaping of the work-piece. Maintaining a uniform gap between the rolls is difficult since the rolls deflect under the load needed to deform the work-piece. Strength and rigidity are important characteristics of the rolls used for steel rolling.
During the process of rolling, large forces act on the rolls. Because of these forces, rolls are subjected to different degrees of deflection. In case of flat rolling, where the widths are large, the effect of deflection is more. The rolls initially are flat. During the rolling operation, the work-piece exerts high force on the rolls towards the centre of the work-piece than at the edges. This causes rolls to deflect more at the centre, and hence gives the work-piece higher thickness at the centre. To overcome this issue, the rolls are ground so that they are thicker towards the centre in such a way so as to offset the deflection which occurs during the process. This extra thickness is called the camber. Camber which is ground into a roll is very specific to a particular width and material of the work-piece and force load. A roll with a camber is also called a crowned roll (parabolic crown). The crowned roll only compensates for one set of conditions, specifically the material, temperature, and the quantity of deformation.
Other methods of compensating for roll deformation include continual varying crown (CVC), pair cross rolling, and work roll bending. CVC 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 have a gap between them which is parabolic in shape, and which varies with lateral shift, hence 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 increases or decreases, hence 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.
There are three main parts of a roll namely (i) the body (the part on which the actual rolling takes place), (ii) the necks (which support the body and take the rolling pressure), and (iii) the driving ends, normally known as wobblers (where the driving force is applied). These parts are shown in Fig 9. Rolls need to have good wear resistance, sufficient strength to withstand the bending, torsional, and shearing stresses to which they are subjected, and, for hot rolling, ability to withstand high temperatures without heat checking (thermal fatigue) and oxidation.
Fig 9 Rolling mill roll
Roll design – Rolls are designed by engineering organizations and builders of rolling mills, except for pass and groove designs on grooved rolls, which are normally are engineered in the roll shop of the user. The proportions of rolls are based on application and mill design. The width of the metal to be rolled, or the length of the slab, bloom, or billet where cross rolling is needed, determines the width of the body face. Body diameter is selected to provide the needed bite and pass angle to accomplish reduction and to provide sufficient mass to resist roll deflection and breakage.
Rolls of smaller diameter result in less spread of the work material and need less rolling pressure, separating force, and power for a given reduction. In designing rolls for shape rolling, deep grooves are to be placed as far as possible from the centre, in a location where the bending moment is at a minimum. The size of a roll is normally designated by body diameter and body length, in that order, e.g., a 600 mm / 1,200 mm roll has a body diameter of 600 mm and a body length of 1,200 mm. For rolls used in processing shapes, the body diameter given is the nominal, or pitch, diameter.
Journal, or neck, dimensions are determined by imposed bending loads and by bearing design. The abrupt change in diameter from roll body to roll neck intensifies bending and torsional stresses at this location. To prevent breakage, the neck diameter is required to be as large a proportion of the body diameter as is feasible. Safe ratios of neck diameter to body diameter vary with type of bearing, type of mill, and conditions of service. In any event, neck diameter is never to be smaller than 50 % of body diameter.
Roll materials – Roll materials vary and are dependent upon the specific rolling process. Common roll materials used are cast iron, ductile iron, cast steel, and forged steel. Forged steel rolls are stronger and more rigid than the cast iron rolls but have complicated manufacturing process. The composition of the roll materials is selected to suit the rolling process. Nickel steels or molybdenum alloy steels are used as material for rolls for certain rolling processes. In some other rolling processes, rolls are made of tungsten carbide which can provide extreme resistance to deflection.
Cast iron rolls are used in the as-cast condition or after stress relief. Some high-alloy iron rolls are heat treated by holding at high temperature, and then they are subjected to several lower-temperature treatments. Cast irons used for rolls are meta-stable and can be white iron or gray iron depending on composition, inoculation (if any), cooling rate, and other factors. Because of the number of elements present, determination of transformation diagrams is complicated. One of the most important factors in determining the quality of rolls is the control of micro-structure and hardness.
Development of proper roll specifications to meet widely varying rolling needs is an extremely complicated, technical process, e.g., when specifying radial hardness penetration, roll producer is required to consider the needs dictated by the design of each particular mill. Because of these factors, each roll is to be more or less tailored for its intended use, and close cooperation between the producer and the user is necessary to get maximum roll life and performance.
Cast iron rolls – These rolls are classified as chilled iron rolls, grain rolls, sand cast iron rolls, ductile iron rolls, or composite rolls. Chilled iron rolls (Scleroscope hardness – 50 to 90 shore hardness) have a definitely formed, clear, homogeneous, chilled white iron body surface and a fairly sharp line of demarcation between the chilled surface and the gray iron interior portion of the body. Clear, chilled iron rolls can be made in unalloyed or alloyed grades. The depth of chill is measured visually as the distance between the finished surface of the body and the depth at which the first graphitic specks appear. Below this, there is an area consisting of a mixture of white and gray iron known as mottle, which gradually becomes more gray and more graphitic, until it merges with the main gray iron structure of the roll interior.
Alloy chilled iron rolls have hardness values ranging from 60 to 90 shore hardness which is controlled by carbon and alloy contents. Customary maximum percentages of alloying elements are 1.25 % molybdenum, 1 % chromium, and 5.5 % nickel. Several different combinations are used to produce desired properties. Rolls of are used mainly for rolling of flats, both hot and cold. The softer, machinable grades are used for rolling rod and small shapes.
Grain rolls are ‘indefinite chill’ iron rolls (shore hardness – 40 to 90) which have an outer chilled face on the body. There is finely divided graphite at the surface, which gradually increases in quantity and in flake size, with a corresponding decrease in hardness, as distance from the surface increases. These rolls have high resistance to wear and good finishing qualities, to considerable depths. The harder grades are used for hot and cold finishing of flat-rolled products, and the softer grades are for deep sections (even with small rolls). Alloying elements such as chromium, nickel, and molybdenum are normally added, either singly or in combination, to develop specific levels of hardness and toughness similar to those of chilled iron rolls.
Sand cast iron rolls (no chill, shore hardness – 35 to 45) are cast in sand moulds, in contrast to chilled iron rolls and grain rolls, the bodies of which are cast directly against chills. In a sand cast iron roll, the metal in the grooves of the body can be mildly hardened by use of cast iron ring inserts set in the sand mould. Sand cast iron rolls are used mainly for intermediate and finishing stands on rolling mills which roll large shapes. They are also used for roughing operations in primary mills.
Ductile iron rolls (shore hardness – 50 to 65) are made of iron of restricted composition to which magnesium or rare-earth metals are added under controlled conditions to cause the graphite to form, during solidification, as nodules instead of the flakes common to gray iron. The resulting iron has strength and ductility properties between those of gray iron and steel.
High chromium cast iron rolls (shore hardness – 75 to 90) were first developed in the 1970s and are now used widely for hot rolling mill rolls. These rolls have 2.5 % to 3 % carbon, 10 % to 20 % chromium, and up to 3 % vanadium and molybdenum and have a matrix made of martensite and are known to have a high wear resistance. They are used in continuous and reversing roughing stands of hot strip mills.
Composite rolls, sometimes called double-pour rolls (shore hardness, bodies – 70 to 90, and necks – 40 to 50), are rolls in which the body surface is made of a richly alloyed, hard, wear-resistant cast iron, and the necks, wobblers, and central areas of the body are of a tougher and softer material. The metals are firmly bonded together during casting to form an integral structure which produces a wearing surface of high hardness, along with a tougher body and neck. Composite rolls are hence better able to withstand impact and thermal stresses. The outer rolling surface can be of either chilled or grain iron. The main application of composite rolls in the rolling of steel has been for work rolls in four-high hot and cold strip mills and in plate mills with the main application has been for rolls for hot break-down and cold reduction of sheet and strip. More recently with the development of new roll manufacturing techniques, outer surfaces of composite rolls are increasingly being made using superior materials such as high-carbon iron and high-speed steel.
Steel rolls – Differentiation between cast iron rolls and cast steel rolls cannot be made strictly on the basis of carbon content. Iron rolls are normally of compositions which produce free graphite in non-chilled portions, cast steel rolls do not show free graphite. The harder cast alloy steel rolls have hardness values equivalent to those of the softer cast iron rolls, and the superior toughness of cast steel rolls frequently makes them preferable to cast iron rolls.
Alloy steel rolls have almost entirely superseded carbon steel rolls in use. Compositions of majority of the alloy steel rolls are within these limits, 0.4 % to 2 % carbon, less than 0.012 % sulphur (normally 0.06 % maximum), less than 0.012 % phosphorus (normally 0.06 % maximum), up to 1.25 % manganese, up to 1.5 % chromium, up to 1.5 % nickel, and up to 0.6 % molybdenum. Higher carbon contents increase hardness and wear resistance. Some rolls have higher alloy contents, but these are normally used for special purposes. Cast steel rolls are graded according to carbon content. Adjustments in carbon and alloy content are normally made to suit individual conditions.
Hardened forged steel rolls are principally used for cold rolling mills. Extremely high pressures are used in cold rolling, and forged rolls have sufficient strength, surface quality, and wear resistance for cold-rolling operations. Forged steel rolls are normally flat-bodied (or plain-bodied) rolls designed to close dimensional tolerances and concentricity. They vary widely in size from a few kilograms to as much as 45 tons. During production, holes are bored through the centres of larger rolls for heat treatment and inspection purposes. New design developments include tapered journals with drilled holes for the accommodation of a special type of roller bearing, and somewhat higher use of fully hardened bearing journals for direct roller-bearing contact. Forged rolls have been specified for work rolls, back-up rolls, auxiliary rolls, and special rolls.
The normally used composition for forged steel rolls, sometimes known as regular roll steel, averages 0.85 % carbon, 0.3 % manganese, 0.3 % silicon, 1.75 % chromium, and 0.1 % vanadium. Around 0.25 % molybdenum is sometimes added to this basic composition, and the chromium content can be varied to get specific characteristics. In Sendzimir mills, the work rolls and first and second intermediate supporting and drive rolls are normally made from high-carbon high-chromium tool steel with 1.5 % or 2.25 % carbon and 12 % chromium (AISI grades D1 or D4). For more severe service, work rolls of M1 grade are used.
The powder metallurgy (P/M) alloy CPM 10V has wear resistance approaching that of cemented carbide, which makes it attractive for some special forged steel rolls. The composition of CPM 10V is 2.45 % carbon, 5.25 % chromium, 10 % vanadium, and 1.3 % molybdenum.
Selection of the proper hardness for the body of the roll is necessary for successful service performance. The hardness range varies with the specific application and is developed with the cooperation of rolling mill operators. Majority of the forged rolls are heat treated to high hardness, but they can be processed to lower values for specific purposes. Because of their high hardness, hardened steel rolls need careful handling in shipping, storage, mill service, and grinding. Hardness of work rolls for rolling thin strip averages around 95 shore hardness. Lower hardness values are used for rolling thicker strip. In temper and finishing mills, work roll hardness is sometimes higher than 95 shore hardness, and for special applications, it is up to 100 shore hardness.
Hardness of back-up rolls varies from 55 to 95 shore hardness. Values on the high side of this range are specified for rolls in small mills. For Sendzimir mills, customary hardness is 61 HRC (Rockwell hardness C scale) to 64 HRC for steel (grade D1 or D4) work rolls and 64 HRC to 66 HRC for high-speed steel work rolls. Normal hardness of intermediate rolls is 58 HRC to 62 HRC. Only the body section of a forged roll is hardened. Journals are normally not hardened, except those for direct-contact roller-bearing designs, for which a minimum hardness of 80 shore hardness is specified. In normal practice, the journals of forged rolls range in hardness from 30 to 50 shore hardness.
Sleeve rolls – Use of forged and hardened sleeve-type rolls in certain hot strip and cold reduction mills has become common since such rolls are more economical. Sleeves are forged from high-quality alloy steel. Compositions of chromium-molybdenum-vanadium and nickel-chromium-molybdenum-vanadium are normally used. Sleeves are heat treated by liquid quenching in either oil or water and are tempered to hardness values of 50 to 85 shore hardness, depending on application. The mandrel over which the sleeve is slipped can be made (i) from a cast roll which has been worn below its minimum usable diameter, (ii) from a new casting made specifically for use as a mandrel, or (iii) from an alloy steel forging.
The outside diameter of the mandrel and the inside diameter of the sleeve are accurately machined or ground for a shrink fit. Mounting is accomplished by heating the sleeve to get the needed expansion and then either slipping the sleeve over the mandrel or inserting the mandrel in the sleeve. This operation is performed with the mandrel in a vertical position. A locking device prevents lateral movement of the sleeve. Final machining is done after the sleeve is mounted. Forged sleeves provide the hard, dense, spall-resistant surface needed for the severe service encountered in hot and cold reduction mills. Another economical advantage of this type of roll is that the mandrel can be resleeved four or five times.
Adamite steel rolls – These rolls have hardness values in the range of 50 – 65 shore hardness. With carbon ranging between 1.5 % and 2.4 %, these rolls are stronger than iron rolls. These rolls can be produced either by the conventional static mono-block casting method or by the centrifugal casting method. These rolls are normally heat treated to get a tempered martensite and bainite matrix. The matrix makes the material resistant to spalling and fire-cracking. Adamite steel rolls are used mainly in the roughing and intermediate stands.
High-speed steel (HSS) and semi-HSS rolls – These rolls are a relatively new development in roll technology. Semi-HSS and HSS are grades with high content of chromium, molybdenum, and vanadium. These grades are characterized by a high tempering temperature (450 deg C to 550 deg C, instead of the normal 150 deg C to 250 deg C). Depending on the alloy content, especially vanadium, the achievable hardness of the rolls can be between 740 HV (Vickers hardness) and 820 HV. Rolls made of HSS and semi-HSS are widely used in hot strip rolling. These rolls are in general more resistant to wear than conventional high-chromium rolls, which makes possible longer rolling campaign and improved productivity. However, because of their metallurgical structure, the friction between HSS rolls and the hot strip can be 10 % to 20 % higher. This increased friction can be countered easily by the use of a rolling lubricant.
Instrumentation and control – Early rolling mills used few, if any, sensing and monitoring devices and were manually controlled by the operators. However, in modern high-speed mills, instrumentation and process controls are necessary for ensuring a correct mill set-up, the proper operation of mills, and an acceptable product quality. At a rolling speed of 1,500 metres per minute, for, example, 1,200 mm wide sheet which is rolled 0.01 mm too thick can result in a loss of one ton of steel every 5 minute. Hence, at each mill stand, instruments are used to measure roll force, drive motor current, roll speed, and roll gap.
In addition, other devices measure work-piece temperature, size, and shape. Continuous feed-back from various sensors is used by highly sophisticated control systems in conjunction with mathematical models to control the operation of rolling mills. In slower mills, the operator frequently acts as the controller, adjusting the mill operation based on feed-back from instrumentation. However, for high-speed mills (bar, sheet, and strip mills), this is best accomplished by computer control. The principal components of a computer-controlled system are (i) mathematical models which adequately describe the process, (ii) instrumentation to measure the needed variables of the system, and (iii) control equipment, including a digital computer, to perform the needed functions for control of the system.
Process models – A computer-controlled system can only follow the given instruction. it is necessary to tell the computer what to do. This instruction is provided by programming the computer as per the mathematical formulations or process models which describe the relationships between the process variables. The mathematical form of these relationships depends on the specific application and can include differential equations derived from theoretical considerations, empirical equations developed from experimental data, statistical analysis, logical decisions, or some combination of these. The chosen treatment of the processing data is required to provide the processing parameters to be controlled and the desired degree of control. In addition to inputting the computational instructions, the computer is to be programmed for the logic to be used, the time sequence of needed events, priorities of control actions under certain circumstances, and other decisions necessary for proper process control.
Instrumentation – A computer-controlled system accepts the quantitative values of the several processing variables and executes its control function based on these values. A prime requisite of such a system is adequate, reliable instrumentation for translating a process variable from its physical or chemical units to a form suitable for use by the computer. Several instruments are presently available to provide rapid on-line measurements of such variables as width, thickness, position, force, temperature, and flow. Instruments which measure other physical and chemical properties of both the input materials and finished products are available. However, this kind of measurement normally involves the taking of a sample and subsequent analysis in an off-line laboratory.
Control equipment – The final component of a computer-controlled system is the digital computer system, including hardware and software, and process regulating devices. The computer hardware includes a central processing unit which has the arithmetical and logical capability needed to run the mathematical models, a storage (memory) unit for accumulation of process measurement data and other information, and a computer interface to allow the central processing unit and memory to communicate with the instrumentation, with process regulators such as screw position regulators and speed regulators, with operators, and with other computers, including a business computer system.
Software consists of all the computer programmes needed to accomplish the desired functions of the computer-controlled system. Control based on classical and modern control theories needs an accurate mathematical model of the process to be controlled. Using an accurate model along with continuous feed-back from different sensors during rolling, the desired control can be achieved quantitatively. However, with control systems involving substantial non-linearity, it is very difficult to develop good process models. Lack of accurate process models limits the degree of control which can be achieved. Remarkable progress has been made in (i) fuzzy theory, which can handle ambiguity, (ii) expert systems, which utilize the knowledge of experts, and (iii) neural networks, which are very effective for pattern recognition and learning. These methods have been used effectively in different rolling mill control systems.
Automatic gauge control (AGC) – One of the key measures of quality in strip rolling is the consistency of the gauge thickness. AGC is one of the normally applied control techniques in strip rolling. In AGC, the gauge meter equation is normally used to analyze gauge variation. The gauge meter equation is H1 = S0 + F/Ks, where ‘H1’ is the exit thickness of the rolled product, ‘S0’ is the no-load roll gap, ‘F’ is the rolling load, and ‘Ks’ is the structural stiffness of the mill. Fig 10 shows the gauge meter equation graphically.
Fig 10 Principle of automatic gauge control
In the Fig 10, slope of line 1 is equal to the mill structural stiffness. Line 2 is called the mean deformation resistance of the material being rolled and represents the relationship between the rolling force and the exit thickness for a certain incoming strip thickness ‘H0’. The equation for line 2 is based on deformation theory and is dependent on rolling temperature, rolling speed material factors, and roll size. The intersection of lines 1 and 2 gives the values of the rolling force and the exit thickness. Knowing the value of the rolling force from a mathematical process model, the structural stiffness of the mill, and the desired strip exit thickness, the no-load roll gap ‘S0’ can be determined quite easily using the gauge meter equation.
During rolling, the rolling force and the exit thickness change because of (i) variation in the mean deformation resistance caused by a change in speed, temperature, or the entry side strip thickness, or (ii) variation in the roll gap because of changes in temperature of the rolls, roll wear, and so forth. In other words, a change in the exit strip thickness can be instantaneously detected by monitoring the rolling force (along with temperature and rolling speed). When the rolling force changes, the exit strip thickness can be kept constant by adjusting the no-load roll gap ‘S0’ by the quantity needed to compensate for the rolling force difference as shown in Fig 10.
Materials for rolling – A large number of steel qualities are rolled using the methods and equipment described above, or slight variations of them. By far the largest quantity of rolled material falls under the general category of ferrous metals, or materials whose major constituent is iron. Included in this group are carbon and alloy steels, stainless steels, and specialty steels.
Conventional primary and secondary rolling of steels is normally conducted at high (hot-rolling) temperatures. In a typical hot-rolling operation involving multiple passes through a reversing or multi-stand mill, the temperature of the work metal drops considerably. For carbon steels, the initial rolling temperature is around 1,200 deg C. It can drop to 900 deg C or lower by the final pass. Since the size of recrystallized grains decreases with temperature, hot rolling results in a fine grain size.
The control of grain size and other micro-structural features during rolling is especially important in low-carbon and low-alloy steels. High-carbon and high-alloy steels produced in the form of plates, bars, or shapes frequently undergo subsequent mechanical (for example, forging or extrusion) or thermal (such as hardening or tempering) processing in which the final properties are tailored to the end-use. Two products in which rolling is used almost exclusively to control structure and properties are low-carbon steel (used in automotive and appliance applications) and high-strength low-alloy steels (used in various structural applications).
Low-carbon steel is produced in the form of sheet by a combination of hot and cold rolling. The starting steel is killed. In these steels, it is difficult to get very fine grain sizes by means of controlling the rolling process (Fig 11). This is because of the absence of alloying elements, which can retard the rapid grain growth which occurs between passes. However, grain size and strength are of secondary importance in this product. The most important property is cold formability, since the sheet metal is frequently subjected to drawing operation afterwards into complex shapes at room temperature.
Fig 11 Micro-structure of steel after rolling
High-strength low-alloy steels, in contrast to the low-carbon steels are designed to have high strength and a relatively modest amount of cold formability. These grades are typically produced as hot-rolled sheet, bar, and plate with 0.05 % carbon to 0.1 % carbon and small quantities of niobium, vanadium, and titanium. The correct thermo-mechanical treatment is extremely important in determining the final properties of high-strength low-alloy steels. For these materials, controlled rolling (Fig 6) is used to refine the relatively coarse austenite structure by a series of high-temperature rolling and recrystallization steps. A moderate to heavy reduction is imposed on the material below the recrystallization temperature to achieve the desired fine grain size and the associated properties.
Stainless steels are available in the same product forms as carbon and low-alloy steels. Rolling mills of more rugged construction are needed for the rolling of stainless steels than are needed for plain carbon and alloy steels because of the higher strengths of the stainless steels, otherwise rolling practice is similar to that used for carbon and alloy steels. Stainless steel rolled products are normally obtained in the annealed condition, but strength or hardness higher than that in the annealed condition can be attained by controlled cold rolling.
Heated-roll rolling – Heated-roll rolling is a process which was developed at Battelle Columbus Laboratories to roll difficult-to-work materials. Heated-roll rolling is an isothermal or near-isothermal process in which the work rolls are heated to the same or nearly the same temperature as the work metal. Heated-roll rolling is analogous to isothermal and hot-die forging.
Heated-roll rolling of sheet and strip – In conventional hot rolling of high-temperature materials, such as titanium and nickel alloys, the hot metal is deformed between cold or warm rolls. This causes chilling on the surface of the rolled metal, resulting in higher working loads and stresses than are needed if chilling is to be avoided. Further, chilling limits the maximum possible reduction per pass and the minimum thickness attainable in conventional hot sheet rolling. From a materials point of view, conventional hot rolling frequently needs rolling temperatures which are higher than optimal, which cause more work-piece contamination and can result in micro-structure and property variations in rolled products.
For overcoming some of the issues associated with conventional hot rolling, the techniques of isothermal and near-isothermal hot rolling have been developed. The technique has been demonstrated for sheet rolling on modified conventional rolling mills with either two-high or four-high arrangements. In the two-high arrangement, where the rolls are relatively large, a composite roll design has been frequently used. This consists of rolls with outer sleeves made of a high-temperature super-alloy and with cores of hot-work tool steels (e.g., AISI H13). Such a design satisfies the need for a roll with good hot hardness at temperatures in the range of around 815 deg C at a cost less than that of a solid roll made from an expensive super-alloy. This set-up is improved by induction heating the roll surfaces and by internal water cooling of the core of the rolls and roll bearings. The viability of heated-roll rolling also has been demonstrated on a four-high mill. In this case, the work rolls and the back-up rolls are heated by banks of radiant heaters and has a design maximum operating surface temperature of 815 deg C for the work rolls.
In studies conducted at Battelle Columbus Division, rolling was not performed at low deformation rates and hence did not rely on the super-plastic characteristics of the work-piece materials, which are frequently used in isothermal and hot-die forging. Rather, these studies concentrated on determination of the workability of and uniformity obtainable in difficult-to-work and temperature-sensitive alloys in the absence of chilling. Hence, attention was focused on a much wider range of alloys than those used in isothermal forging, including tungsten, beryllium, Ti-6Al-4V, alloy 718, and several alloy steels and oxide-dispersion-strengthened alloys.
The effect of heated rolls on temperature uniformity in rolling of strip from high-temperature alloys is quantified through a series of heat transfer simulations performed on a digital computer. When the rolls are at room temperature, the temperature of the hot strip decreases considerably during rolling, and large temperature gradients are present. Both of these factors can cause workability problems. On the other hand, temperature decreases and thermal gradients through the strip thickness are calculated to be comparatively small when the rolls are heated to a surface temperature of 540 deg C. Experiments at Battelle have also shown that heated-roll rolling lowers roll separating forces and enables larger reductions per pass by eliminating or minimizing the chilling effects.
Similar to the micro-structures achieved in isothermal forging, micro-structures in isothermally rolled sheet have been found to be very uniform. Hence, it has been suggested that in some cases one or more post-rolling heat treating steps can be eliminated. This has been demonstrated in the processing of 8670 alloy steel strips which have been pre-heated to 840 deg C, rolled, quenched upon exit from the mill, and tempered for 2 hours at 175 deg C. At 840 deg C, this steel is totally austenitic. When the rolls are not heated, a coarse martensitic structure at the strip centre, micro-structural non-uniformity near the surface, and rolling directionality are obvious. However, when the rolls were heated to 840 deg C to produce isothermal metal-working conditions, a fine, uniform martensitic micro-structure with no directionality is achieved. Hence, heated-roll rolling of steel sheet and shapes which are to be subsequently hardened can offer the advantage of eliminating the austenitizing and quenching stages.
Heated-roll rolling of shapes – The application of the isothermal and heated roll rolling concept to shapes also has met with success, although the commercialization of the process, similar to that for flat rolling, has yet to be fully realized and accepted. One organization, in cooperation with Battelle Columbus Division, modified an existing two-high production rolling mill with 250 mm diameter rolls into a heated-roll rolling setup. This set-up has been used to produce a structural L-shape to close tolerances from a high-temperature super-alloy. Similar to designs used in Battelle’s works, the organization used a composite roll design consisting of an AISI A9 tool steel core and a super-alloy sleeve. Further, the rolls have been heated by banks of quartz-tube radiant heaters. With this tooling, 150 metre, a production size quantity of the structural shape, have been successfully rolled for each of the two alloys. Compared to conventional rolling practice, the heated-roll rolling of the superalloy needs fewer rolling and other major operations.
Defects in rolling – A number of defects or undesirable conditions can develop in the rolling of flat, bar, or shaped products. Broadly, these problems can be attributed to one of four sources namely melting and casting practice, metallurgical sources, heating practice, and rolling practice.
The major problems associated with melting and casting practice are the development of porosity and a condition known as scabs. Porosity is developed in cast products when they solidify and is of two types namely pipe and blow-holes. Pipe is a concave cavity formed at the top of the cast product because of non-uniform cooling and shrinkage. If not cropped off, pipe can be rolled into the final product to form an internal lamination. These laminations are not immediately evident following rolling, but can become apparent during a subsequent forming operation. The occurrence of laminations is most prevalent in flat-rolled sheet products. Fig 12 shows typical surface defects on hot rolled steel strip.
Fig 12 Typical surface defects on hot rolled steel strip
Blow-holes are normally a less serious defect. They are the result of gas bubbles entrapped in the metal as the liquid steel solidifies. If the surfaces of holes are not oxidized, the blow-holes can be welded closed during the rolling operation.
Scabs are caused by improper pouring during casting, in which metal is splashed against the side of the mould wall. The splashed material, or scab, tends to stick to the wall and become oxidized. Scabs normally show up only after rolling and, and as can be expected give poor surface finish.
Defects such as poor surface finish can also result from a metallurgical source, e.g., non-metallic inclusions. In steels, inclusions are of two types, refractory and plastic. Refractory inclusions are frequently metallic oxides such as alumina in aluminum-killed steels. When near the surface, such inclusions give rise to defects known as seams and slivers. Plastic inclusions, such as manganese sulphides, elongate in the rolling direction during hot forming. The presence of these elongated inclusions (stringers) produces fibering, which cause directional properties. For example, ductility transverse to the fiber is frequently lower than that parallel to it.
In high-strength low-alloy steels, sulphide shape-control elements, such as niobium, are frequently added to prevent the development of such fibering, which is especially undesirable from the viewpoint of subsequent forming operations or service behaviour.
Another rolling defect whose source is metallurgical is alligatoring. This defect is manifested by a gross mid-plane fracture at the leading edges of the rolled metal.
Two rolling defects which stem from heating practice are rolled-in scale and blisters. The development of scale during reheating of slabs, blooms, or billets is almost inevitable, particularly for steels. Sometimes descaling operations involving hydraulic sprays or preliminary light rolling passes are not totally successful, scale can get rolled into the metal surface and become elongated into streaks during subsequent rolling. The other defect, blistering, is a raised spot on the surface caused by expansion of sub-surface gas during heating. Blisters can break open during rolling and produce a defect which looks like a gouge or surface lamination.
Rolling practice can cause defects also. In bar and shape rolling, for example, excessive reduction in the finishing pass can cause metal to extrude laterally in the roll gap, leading to a defect known as finning. Finning in an intermediate pass causes folds or laps in subsequent passes. Excessive reduction in the leader pass (the pass prior to the finishing pass) also can wrinkle open the sides of a bar, which after turning 90-degree in the finishing pass can result in a series of hair-line cracks. In the rolling of plates, two defects which affect yield are fish-tail and over-laps, both of which need to be trimmed off. The former results from non-uniform reduction in the width, and the latter results from non-uniform reduction in thickness. These defects can be reduced by proper design of the rolling sequence.
Several defects, such as wavy edges, centre buckle, herringbone, and quarter buckle can be created in cold rolling of sheets and strips. These defects are mainly because of localized over-rolling, which can occur because of improper roll profiles or variations in the properties or shape of the incoming strip.
On-line detection of surface defects is extremely important for timely detection of rolling mill problems and for reducing scrap. On-line detection of surface defects can be carried out using non-destructive testing methods. The eddy current inspection technique is one of the most suitable and also most widely used techniques for on-line inspection of rolled products. It is a non-contact technique which can be applied to rolled products travelling at the rate of hundreds of metres per minute. It has been proven to be a very reliable inspection technique which can also be easily automated. Other non-contact inspection techniques such as ultra-sonic inspection using electro-magnetic acoustic transducer technology or laser-induced ultrasonics have also been developed and tested successfully in rolling mills.
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