Rolling Mills Rolls
Rolling Mills Rolls
Rolling had assumed importance in the industrialized world during the nineteenth century. Rounds, profiles, and flats are hot rolled while some flat products are also cold rolled. Rolls are the tools used in the rolling mill. They are highly stressed parts of the rolling mill and are subject to wear. They are used both in the flat product rolling mills as well as in the long product rolling mills. The rolls are the most critical part of the rolling mills and the performance of the rolling mill depends heavily on the quality and performance of the rolls.
Rolls are changing parts of a rolling mill which are used to reduce the cross section and change the shape of the material being rolled. They execute specific and demanding functions under severe conditions of heat and pressure. Since, they operate in severe conditions, their application demand an optimum combination of several properties such as wear resistance, and toughness etc. During rolling, rolls are under high load and the contact area between the roll and material being rolled suffers wear. Also rolls are to be capable to withstand both mechanical and thermal fluctuations to which they are normally exposed during rolling. Hence, rolls have a limited campaign life. After the campaign life is over, rolls are required to be changed for continuation of rolling. The state of the surface is one of the criteria determining the roll change. Rolls which are removed from rolling mill are dressed in roll turning / roll grinding shop and are made ready for another rolling campaign in the mill. Rolls are discarded when their diameter reaches the level of the minimum discard diameter.
Rolls do the most important work in a rolling mill. They constitute a very important component of the running cost of the rolling mill. Hence, it is necessary that optimum performance is obtained from the rolls. Rolls come in a wide variety of sizes, the smallest roll weighs only a few kilograms, the heaviest around 250 tons a piece, and the variety of grades used is also wide, from ductile iron (spheroidal graphite iron) to tungsten carbide, covering all kinds of tool steels and special steels, used only for rolls. Roll properties include blend of hardness and strength as well as resistance to thermal cracking, shock loading, and wear.
Rolls are needed to carry out the heavy work of reduction of the cross section of the steel being rolled. They are to take all kind of stresses, loads from normal and abnormal rolling and which are changing with the roll wear during a rolling campaign. Rolls are regularly redressed to rebuild the desired shape and to eliminate the worn, fire-cracked and fatigued surface, and they never last as long as the desire of the roll users.
Rolls are not to break, spall, or wear. They are expected to give excellent performance without causing any problems. Under the conditions of rolling, the contact area of the roll which comes in contact with the steel suffers wear while other parts of the roll body and roll necks do not experience plastic deformation or fatigue but are under high loads. In the recent past, rolling technology has improved and changed but rolls have always remained the critical part of the rolling mills. Hence the development of roll quality and roll making technology have continued to remain important since these factors determines the roll performance in the rolling mill.
Rolling mills are increasingly demanding rolls which are capable to maintain the shape and profile much longer with the aim to extend the length of the rolling campaigns. Normally, life of rolls of a rolling mill is limited by planned roll discard diameter. However, despite careful attention given by the roll supplier and also given during the operation of the rolling mill, abnormalities and roll failures do take place in service. A roll failure is a big catastrophe in rolling mill which not only leads to partial or total loss of the rolls, also necessitates removal of resulting cobble in the mill, causes mill stoppage and damage to rolling mill equipment. All these affect the mill performance adversely. Hence, roll failures are to be avoided.
Depending upon the profile of the rolled product, the body of the roll can be either smooth (plain) for rolling flats (sheets, plates or strips) or grooved for the rolling of the rounds and shaped material (sections). Rolls have three main components (Fig 1) namely (i) roll body, (ii) roll necks, and (iii) wobblers. The wobblers are for driving or rotating the rolls, the necks are to support the roll in the mill housings and the barrel portion is the working portion of the roll where actual rolling takes place. The body is the part which comes into direct contact and deforms the metal of the work piece.
Fig 1 Components of roll
Presently, there are roll grades available which provide good performance down to discard size without abnormal stock loss for all established rolling operations if standard rolling conditions prevail. Of course, correct handling of the rolls is necessary including campaign length and adequate dressing practice with appropriate non-destructive testing. In addition, measurement of additional features such as wear profile and work hardening can also be beneficial.
A good quality roll is required to have several properties which include (i) high wear resistance for longer life and economy, (ii) resistance to fracture to withstand increasing rolling load, (iii) resistance to fire cracking to overcome the susceptibility of rolls to fire cracking due to the steep temperature gradient between rolling temperature and the roll temperature, (iv) spalling resistance to resist the premature failure of rolls due to very high thermal and pressure gradients between the stock and rolls, (v) good surface finish to produce high quality surface in the products, and (vi) good biting properties. The main parameters which control the properties of a roll are (i) mono or compound roll (roll design), (ii) chemical composition of the roll material, (iii) casting (mould design, temperatures, weights, inoculation, and down cooling), and (iv) heat treatment.
The right selection of roll material is vital for good rolling practice. Besides the chemical composition, physical and mechanical properties (micro structure, hardness, yield strength, and ultimate tensile strength etc.) of the materials of the rolls, other properties which are important, are young’s modulus, poisson’s ratio, co-efficient of thermal expansion, heat conductivity and co-efficient of heat transmission. Hence, a roll designer has to select the best material out of those available to suit the requirements. Normally it can be said that the harder the rolls, better is the wearing properties and lesser is the toughness and vice-versa.
There are areas which overlap, where rolls made by different technologies are available but there is no general rule that the rolls made from one technology is better than the roll made from other technology. The final decision on the choice of the rolls normally depends on the cost of the rolls per ton of material rolled. Rolling mills value the rolls by the cost of uses and the excellent quality of the obtained surface on the rolled product. Low priced rolls are not always better and can be ultimately counter-productive.
In the nineteenth century basically non alloy grey iron and forged steel was used for rolls. The cast iron grades varied from mild hard to clear chill, where the barrel showed a white iron layer with grey iron core and necks. These rolls were used for flat rolling without any roll cooling in the sheet mills. Later cast steel rolls were developed. These rolls are still being produced. Around 1930, ICDP (Indefinite chill double pour) rolls were developed for hot rolling. In late 1990s, ICDP enhanced with carbide rolls were developed. Around 1950, nodular (spheroidal graphite) iron was invented and introduced into roll manufacturing. The nodular iron was without alloys and sometimes alloyed with chromium, nickel, or molybdenum. These rolls were having good wear resistance and strength at the same time. The use of high chromium iron (carbon – 2 % to 3 %, and chromium – 15 % to 20 %) and later high chromium steel (carbon – 1 % to 2 % and chromium – 10 % to 15 %) introduced high wear resistance in the rolls. In early 1985, high speed tool steel material was introduced into the rolls. After initial problems were overcome, rolls from these materials gave much better performance. For the production of wire rods, sintered tungsten carbide (hot isostatic pressed, HIP) rolls were developed and development of ceramic rolls are under process of development.
For cold rolling mills, forged steel rolls were improved to give higher hardness penetration after heat treatment by increasing the content of the alloying element. Basically chromium increased from 2 % to 5 %. Chromium plating of work rolls was also introduced to improve surface roughness.
Classification of rolls
Rolls are classified in several ways. Based on applications, rolls can be for (i) longitudinal rolling for the rolling of flats or sections, (ii) transverse rolling, (iii) thread rolling, (iv) ring rolling, (v) tube piercing for seamless pipe rolling, and (vi) skew rolling as shown in Fig 2. In case of longitudinal rolling, rolls are (i) plain smooth rolls for the rolling of flats, and (ii) grooved rolls for the rolling of sections, rounds, and reinforcement bars .
Fig 2 Application of rolls
Based on the production method for the rolls, rolls are produced by (i) casting, (ii) forging, (ii) sintering, and (iv) other methods. Cast rolls are classified as (i) single pour rolls, (ii) double pour rolls, and (iii) centrifugally cast rolls. All the roll production methods have their advantages, disadvantages, and limits for production. These limits can be caused by (i) roll dimensions, (ii) roll material composition, (iii) required hardness or wear resistance, and (v) production costs.
Based on the microstructure of the roll materials , the rolls are classified as (i) hypo eutectoid steel rolls, (ii) hyper eutectoid steel or ‘Adamite’ rolls, (iii) graphitic hyper eutectoid steel rolls, (iv) high alloyed steel rolls, (v) spheroidal graphite iron rolls, (vi) indefinite chill cast iron rolls, and (vii) Special materials such as sintered carbide and ceramic rolls.
Based on surface hardness, rolls are classified as (i) soft rolls with BHN (Brinell hardness number) hardness in the range of 150-250, (ii) semi-hard rolls with BHN hardness in the range of 250-400, (iii) hard rolls with BHN hardness in the range of 400-600, and (iv) very hard rolls with BHN hardness in the range of 600-800. Based in the material of the rolls, rolls are classified as (i) iron based rolls, (ii) steel based rolls, and (iii) special materials such as tungsten carbide.
Iron based rolls
Iron based rolls have maximum uses in rolling mills mainly because they impart good finish and posses good wearing properties. Several years ago cast iron or chill cast iron rolls were only used. For increasing the strength, finish, wear resistance and heat resistance properties, a number of alloying elements, are now added to the iron rolls. Also the improved casting techniques and heat treatment have greatly enhanced the properties of these rolls. Iron based rolls are (i) grey ion rolls, (ii) alloy iron rolls, (iii) clear chill or definite chill rolls, (iv) double pour rolls, (v) indefinite chill rolls, (vi) spheroidal graphite rolls, and (vii) centrifugally cast rolls.
Grey iron rolls – These are cast by sand casting and consist of grey cast iron (contains flakes of free graphite). The structure of the roll is uniform throughout and is very resistant to fire cracking. It is to some extent self-lubricating due to the free graphite which is an advantage where thrust collars arc used to resist end thrust during rolling. A typical analysis of the roll material is carbon – 2.5 % to 3 %, silicon – 0.5 % to 1 %, manganese – 0.4 % to 0.8 %, phosphorus – 0.5 % max, and sulphur – 0.1 % max. Phosphorus is allowed up to 0.5 % to increase the fluidity of the metal during pouring but it is advantageous to reduce it if higher casting temperatures are possible as phosphorus is deleterious to the properties of cast iron. Sulphur is kept to a minimum and silicon is chosen to regulate the free graphite content, since the presence of silicon promotes graphitization. Manganese is used to neutralize the sulphur and to deoxidize the metal. Too high a manganese content resists graphitization.
Alloy iron rolls – For improving the quality of the grey cast iron, different alloying elements are normally added to give the roll an increased hardness. These alloy iron rolls have only small quantities of nickel, chromium, and molybdenum. The free carbon is present in the form of flakes. These are sand cast rolls. These rolls give slightly better wear resistance and strength than the straight grey iron. The alloy iron rolls with higher quantities of alloy additions are much harder and wear less, though naturally they are more costly and are required to yield higher rolled tonnage.
The presence of nickel promotes the formation of graphite but as it is in a very finely distributed form, it leads to greater toughness and resistance to fire cracking. Chromium increases the tendency to form combined carbon and restricts graphite formation so giving a much harder but more brittle iron. Molybdenum and tungsten promote the formation of combined carbon. In addition, they add to high temperature strength. With a chromium alloy iron, there is a tendency for collar breakage in section rolls due to the brittleness and in addition a good supply of water is necessary for cooling the rolls to avoid fire cracking.
A typical analysis of the alloy iron roll is carbon – 3 %, silicon – 1%, nickel – 1 % (or molybdenum – 0.5 %), and chromium – 1 %. A common brand of alloy iron roll is ‘Adamite’ iron. The hardness of rolls is a measure of the resistance to wear and it is normally expressed in degrees of Shore scleroscope hardness. The relatively soft grey iron rolls have hardness of 30-40 deg Shore, but these can be increased to the range 38-50 deg Shore in the alloy iron rolls. This latter is a grey iron but the graphite is finely divided and the matrix is harder.
These rolls are useful in section rolling mostly in the intermediate stands where light drafting is possible. The hardness drop from surface to core is gradual. Softer rolls are favoured for the roughing and intermediate stages of rolling and the rolls with the harder alloy grain are favoured for finishing stands.
Clear chill or definite chill rolls – These rolls are used for rolling flat products like sheets and plates and small sections. A clear chill roll has a surface layer of white iron produced by inducing rapid cooling at the surface (by means of a chill in the mould) which restricts the formation of free graphite. The core is of grey iron due to the slower rate of cooling and the intermediate zone is a mixture of white iron and grey iron. The necks and wobblers or spade ends are not to be chilled and hence retain greater toughness. The chilled layer is hard and wear resistant but it is brittle. The thickness of the chill varies. It can be even up to 25 mm thick depending upon its application. Beneath the chill zone, there is a transition zone known as ‘Mottle’ zone where carbon is gradually flaking from a few specks to the full flake. The core portion is grey iron. Hardness drops down if the chill is worn out. The chilled layer thickness can be increased where shallow grooves are needed.
The roll casting is made vertically with barrel in chill moulds. The chill surface is very hard and imparts good finish. The addition of molybdenum increases the strength and heat resistance properties. The composition is more or less similar to cast iron but its structure is different. The outer surface of the barrel is chilled (iron carbide, Fe3C) imparting very high hardness.
The analysis is similar to grey iron rolls though the carbon content is higher. Lower carbon gives a lower hardness but it strengthens the roll and reduces the incidence of surface cracking and spalling hence it is used in cases of high stress such as in plate rolling. The surface hardness can be between 55 and 65 deg Shore but the rolls have good resistance to temperature change and fire cracking. A part chill roll is produced by chilling chosen parts of the barrel (e.g. finishing passes) and leaving the rest as grey iron. Alloying elements can be added to chill rolls to give hardness values of 65 to 90 deg Shore and can contain around 4.5 % of nickel with chromium for balancing the tendency to form free graphite. The nickel bearing chill rolls are claimed to have a work hardening tendency and are more suitable for cold rolling due to their susceptibility to fire cracking.
Double pour roll – These are also called composite or duplex rolls. In order to obtain both high resistance to wear and high strength, roll makers have developed a roll making technology wherein the outer shell is made hard and the inner core tough by double pouring of metal of different compositions. The shell composition is maintained to give very high wear resistance properties and the core composition to give more strength. These are very costly type of rolls.
Double pouring is a method of combining a very hard surface with a tough core is to cast the roll by double pouring. The first pour gives a shell of highly alloyed white iron which cools rapidly on the surface in a chill mould after which the second pour (frequently of grey iron) displaces the molten centre of alloy iron and replaces it with a tough core. The shell hardness is 75 – 95 deg Shore.
Indefinite chill rolls – These are alloy iron rolls and are cast in chill moulds. After casting the hard top chill layer is cut off and the remaining part of the roll has hardness practically constant up to a great depth. In fact, drop in hardness from the surface to the core of the roll is gradual up to 100 mm to 125 mm depth compared to chill roll where the drop in hardness is sharp.
With this type of roll, there is a very thin clearly defined white graphite-free chill and no intermediate mix zone. The surface layers contain very small particles of graphite and the structure changes smoothly into the grey core. The hardness decreases slowly at first from the surface at a rate of about 1 deg Shore per 10 mm of depth and then more quickly towards the soft centre. Hence there is a good usable depth. The surface is more resistant to fire cracking and spalling than the definite chill roll and the rolls grip the rolling stock better. An alloy indefinite chill roll with a surface hardness of 55 to 75 deg Shore can contain nickel, chromium, and molybdenum. These rolls can be heat treated to toughen them against shock loadings. An example of this type is the Adamite indefinite chill. These rolls can be heat treated and are resistant to spalling and fire cracking.
Indefinite chill rolls are normally used in the intermediate and finishing stands of section mills where deep grooves are required to be cut to make the needed profiles for the sections to be rolled. These rolls have better resistance to fire cracking and spalling than the chill rolls and also better strength. They are reasonably tough with good wearing properties.
Spheroidal graphite iron rolls – This is another type of alloy iron rolls where the structure is completely different from that of cast iron rolls. The graphite is present in the form of spheroids or nodules which increases the ductility and makes the roll more resistant to fracture. Nodularization is achieved by the addition of calcium silicide and magnesium or cerium. The nodular structure of carbon imparts better tensile strength than that of cast iron along with better wearing properties.
The spheroidal graphite iron has much greater strength and toughness, the former being about twice that of a high duty flake graphite iron and the latter is increased about twelve times. Most of the rolls have a pearlitic structure but the acicular structure is also available giving better wear resistance. A good finish can be achieved on the rolls though care in machining is necessary as noxious fumes are given off. The wear properties of spherical graphite iron rolls are that they wear evenly and at a similar rate to flake graphite iron. They are suitable for use where a normal iron roll is not strong enough and where steel rolls give poor life due to excessive wear but, as they are more expensive than both iron and steel rolls, care in the choice of application is necessary. Hardness can be achieved up to 80 deg Shore or more. These rolls are more prone to fire-cracking and need lot of external cooling.
Due to greater strength such rolls are used to replace other types of iron base rolls. Their wearing properties are also better than steel base rolls and at times they are also made to replace steel base rolls. The hardness drop in such rolls is minimal. Spheroidal graphite iron rolls are finding use in some rolling mills.
Centrifugally cast rolls – The composition of centrifugally cast rolls is more or less similar to that of double poured rolls. First, metal which gives high hardness and better wearing properties is poured in the mould, which is then revolved at high speed. After sometime, molten metal of different composition is poured in for the core of the roll to make the core tougher. These rolls are superior to double poured rolls as the shell portion is more dense giving better properties. The centrifugally cast rolls are also known as duplex rolls.
Steel based rolls
Steel rolls can be cast or forged. They are much stronger and tougher than iron rolls and hence are used where an iron roll is considered not strong enough. They permit heavier draughts to be used especially where deep grooves are needed. Breakages due to shock loading are much less likely to occur and the properties can be varied considerably by suitable heat treatment. However, carbon steel rolls wear more quickly than iron rolls due to their low hardness.
Cast steel rolls – These rolls have qualities and grain structure like steel although the carbon content can be quite high (up to 2.5 %). The cheapest rolls in this group are plain carbon steel rolls. To get better wearing properties, molybdenum and chromium are added and nickel is introduced for imparting strength and resistance to fire cracking. These rolls with small quantities of alloying elements are used in big size rolling mills such as blooming mills or heavy section mills. The most used rolls in this group are ‘Adamite’ rolls. Depending upon carbon content, the Adamite rolls are graded as A, B, C, D and E. Adamite ‘A’ being the softest but toughest and Adamite ‘E’ the hardest but least tough. All these rolls are cast, heat treated, and machined and have nearly uniform hardness throughout the transverse cross section.
The cast steel rolls can vary considerably according to analysis. The straight carbon roll has from 0.4 % to 0.9 % of carbon and the hardness is from 28 to 36 deg Shore. Heavy mills (blooming, slabbing, and heavy roughing) use the lower grades (up to 0.6 % C) while billet roughing stands use the higher grades. The addition of around 0.5 % molybdenum to this type of roll together with small quantities of nickel and chromium (or higher manganese) gives increased strength and reduces the severity of any fire cracks which can occur. The hardness is 30 – 42 deg Shore. More highly alloyed rolls normally lie within the ranges of analysis having carbon – 0.8 % to 1 %, manganese – 0.6 % to 0.9 %, nickel – 1 % to 2.5 %, chromium – 0.5 % to 1.1 %, and molybdenum – 0.2 % to 0.4 %. A carbon-chromium roll (carbon -1 %, chromium – 1.5 % to 1.75%) is also made. These rolls are normally heat treated, the hardness range is 35 -55 deg Shore and they are normally used as back-up rolls in 4-high rolling mills. An alloy steel containing tungsten and with a hardness of 40 – 50 deg Shore is very resistant to fire cracking and is sometimes used for roughing rolls in hot strip mills. Cast alloy steel base rolls are made also with the analysis ranging carbon – 0.9 % to 2.5 %, silicon – 0.5 % to 1 %, manganese – 0.4 % to 0.6 %, nickel – 0.25 % to 1 %, and chromium – 0.5 % to 1.5 %. The carbon content is in a higher range than in the cast steel roll. The entire carbon is in combined form. The hardness range is 30 – 55 deg Shore, according to carbon content, and the rolls wear well and are strong. The life is in line with the cost. Good water cooling is needed.
Forged steel rolls – Rolls are also made by forging. The forged steel rolls contain less carbon compared to cast steel rolls since high carbon content can cause cracks during the forging process. The structure of the forged steel rolls is denser than that of cast steel rolls and hence is tougher and can take more loads. However, because of the lower carbon content the hardness is low and more prone to wear than cast steel rolls. These rolls are primarily used where they have to withstand high loads as in blooming mills or in heavy section mills. Forged and hardened rolls are also used as back-up rolls in 4-high mills although normally alloy cast steel rolls are used for back-up.
These rolls are forged from a cast steel ingot and the necessary mechanical working results in an improved tougher structure. In the carbon steel form (0.35 % to 0.75 % carbon) they are used for blooming, slabbing and heavy roughing mills in the lower end of the carbon range and for smaller intermediate mills in the higher end of the range. This is somewhat arbitrary and depends on the particular mill conditions. They are normalized before use and the hardness range is 24 – 30 deg Shore. In the alloy steel form they can be heat treated to give a wide range of hardness. In the hardness range of 50 – 55 deg Shore, they are used for large back-up rolls, in the range of around 80 deg Shore for small back-up rolls in cold rolling, and in the range of 90 – 100 deg Shore (fully hardened) for work rolls in cold rolling. A typical analysis is carbon – 1 %, chromium – 1.5 % to 1.75 %, and nickel – 0.5 %. Forged steel rolls in the hot rolling hardness range are highly resistant to shock loading.
After forging process, preliminary heat treatment is normally carried out in order to prevent products from hair cracks and to refine the metallurgical structure of material. After rough machining, quality heat treatment is performed to obtain proper mechanical properties and fine structure.
Tungsten carbide rolls
Tungsten carbide (WC or W2C), is a chemical compound containing tungsten and carbon. Its extreme hardness makes it useful in the manufacture of mill rolls for extended life in applications where long rolling campaigns are required. Tungsten carbide rolls are used in the modern high speed wire rod mills which uses ‘no twist finishing blocks and in some shape rolling mills. In tungsten carbide rolls, tungsten carbide is the major alloying constituent which gives the roll high wear resistance and hardness. The other constituent is the binding material cobalt although nickel has been increasingly being used. The normal use of a single carbide grade throughout the entire finishing block is not always the optimum solution. At least two or more grades are occasionally to be considered.
These rolls need high quality cooling water in a narrow pH range and having limited hardness. Using roll cooling water outside the recommended pH range leeches the binder from the roll causing premature roll surface failure. Various grades of carbide rolls are available based on grain size and binder content and binder composition. The range of application in recent years has extended the use of carbide rolls back into the intermediate mills by using a carbide sleeve mounted on a steel shaft. The mounting is carried out by a mechanical method or by creating a composite roll by pressing and sintering a carbide ring on the shaft.
Tungsten carbide roll manufacture is a highly specialized field involving metallurgical, chemical, and mechanical processes. After selecting the powder composition, the main stages from powder to finishing roll block are pressing, shaping, and sintering. Tungsten carbide in combination with the binder materials in powder form are mixed, milled, granulated, and compacted to near net shape blanks which are finally sintered in a vacuum furnace. Some rolls are then hot isostatically pressed (HIP). The rolls are then ground using diamond grinding wheels or lathe turned using very hard turning tools to the required dimensions. Sintered roll blanks have a maximum outer diameter of 500 mm. Once the sintering process is complete carbide rolls reach their peak hardness and are then more difficult to machine. Mostly they are ground by the manufacturers and supplied to the users to make the necessary roll passes.
Issues related to roll production
In order to make roll making commercially attractive while making the rolls available to the customers at reasonable price, the roll producers need to have the expertise, which include (i) understanding of the roll application (load, speed, and roll cooling etc.), (ii) choice of optimum material, (iii) production of sound rolls without having any defects, (iv) choice of adequate heat treatment (strength, hardness, and residual stresses etc.), (v) ability to machine the roll to meet the requirements of specifications and prints, and (vi) ability to follow the changing requirement of the roll user.
There are some technical issues which need to be overcome by the roll producers. These technical issues are as follows.
- Some rolls while being used in the rolling mill, have premature failure or they are broken before their expected life is achieved. In such case, roll maker and roll user are to have a mutual understanding of the reasons for failures so as to decide who will bear the cost. For this, a roll failure analysis report acceptable to both the parties is needed.
- The design load of the rolls in a rolling mill is never the random but is the mean load. Normally rolls are subject to less stress than their design load allows. However in the case of rolling accidents, stress can be much higher.
- Rolling conditions can be described only in general. In real practice, these conditions are never stable. The conditions can change and frequently from good to worse within a rolling campaign.
- Some rolling mills face roll problems frequently though some very similar mills do not have the same kind of problem. Though such problem may not be a roll problem, still the roll makers have to involve themselves for solving the problem so as to have good understanding between roll user and roll producer.
- Rolls are rated for performance on the assumption that the statistics equalize all the different experiences of the roll during its life.
The design of the rolls has to take the following two absolutely different requirements into consideration namely (i) the roll has maximum strength to take separating forces, torque, and high pressure between work roll and back up roll etc., and (ii) maximum wear resistance in the contact area between roll and rolling stock. It is not easily possible to achieve these properties from a single material. In case of cold rolled flat rolling the solution is reached by surface hardening of the barrel of the forged steel rolls and by use of high tech material. This ensures hardened zone deep enough to reach the scrap diameter of work roll avoiding need of re-hardening. The clear chill rolls also provide differential properties; one is in the surface area of then barrel and second is in the core and neck area. The chilled barrel of high carbon cast iron solidifies as white iron of high carbides / cementite content, while the rest forms almost carbide free grey iron with lamellar graphite. This leads to a high wear resistance barrel surface area and relatively strong core and neck material.
For providing a high wear resistant barrel and high strength material in other parts of the roll, various technologies have been developed. The common method for all these is to use different materials for the working zone of the barrel and the rest of the roll. The popular method of producing such rolls is by double pouring. Centrifugal casting is the most popular method for the production of double poured rolls
Factors affecting roll quality
The roll quality is heavily dependent on the roll material, its composition, and its microstructure. However the following two factors influence roll quality very much.
Roll hardness – Hardness of the material of a roll is to be optimum. Higher hardness improves wear resistance but increases the risk of roll failure. Higher hardness also creates issues during machining and grinding of the rolls. It is not a fact that everything improves in the roll if hardness is higher. In fact opposite is valid.
Residual stresses – Residual stress act as a pre-stress and has a high impact on the strength of the roll. Compression stresses increase the fatigue strength, reduce crack propagation and reduce shear stress at the roll barrel surface as well as work hardening. Tensile residual stresses can cause roll breakage. Compression and tensile residual stresses in a roll compensate each other over the roll cross section. The right level of residual stresses (high compression versus low tensile) is to be controlled.
Reasons for roll failures
The failure of the rolls can be due to any or combination of the following reasons.
Thermal breakage – In case of thermal breakage, the barrel is broken showing radial oriented fracture lines whose origin is at or near to the axis of the barrel. The fracture is perpendicular to the roll axis and normally occurs close to the centre of the barrel length. The thermal breakage is related to the maximum difference of temperature between surface and axis of the roll barrel. Three factors important for thermal breakage are (i) thermal gradient, (ii) residual stresses, and (iii) strength and integrity of core material.
Torsional breakage of driven roll necks – This happens mostly during the rolling accidents. Fatigue torsional failure of the roll necks is observed very rarely.
Fire cracks – Fire cracks are thermo-shock cracks which form under a very sharp cooling rate on the roll surface. A fire crack pattern on the surface of rolls used for hot rolling with water cooling is very common. Fire cracks can be initial cracks which can develop into deeper cracks and spalls.
Local overloads – Local overloading is one of the most common reasons for premature failures of the rolls.
Roll fatigue – Fatigue damage can start at the surface or sub-surface. Rolls are also damaged because of fatigue. The damage due to fatigue can start at the surface or the sub-surface. The problem of fatigue in the rolls can arise due to high loads in the mills. Corrosion fatigue can also be a problem. With corrosion fatigue, there is no safe operation at all, and there is no fatigue limit.
Spalling – Spalling can be another reason for roll failure. There are two different kinds of spalls in the rolls. One starts at an initial surface crack while the other kind starts at the sub-surface. Surface cracks are normally caused by local overload, and all types of rolling abnormalities including abnormal rolling conditions. There are five types of spalls. These are (i) saddle spalls, (ii) pressure cracks and ribbon fatigue spalls, (iii) shell / core interface-bond related spalls, (iv) spalls due to insufficient shell depth, and (v) barrel edge spalls.
Damage of steel rolls due to hydrogen – Hydrogen can cause two types of problems in steel rolls. One is special fatigue shown by starting of one or more round of cracks perpendicular to the longitudinal direction of rolls and growing conically into both the directions. It takes a long time until this fatigue becomes evident.
Wear – The roll wear takes place mainly at the areas of highest friction, which is between the roll and the material being rolled. Roll wear is normally not uniformly distributed on the barrel on one end to the other. Roll wear depends on roll material, its chemical composition and microstructure. The wear of the roll decreases with an increased content of hard carbides.