Metallurgy of Rolling mill Rolls
Metallurgy of Rolling mill Rolls
Rolls are implements or tools used in rolling mills to reduce the cross section of the material being rolled. They are highly stressed tools and are subject to wear. They are needed both for the rolling of sectional as well as flat products. In recent past, rolling technology has improved and changed dramatically, but the rolls have always remained the most critical part of the rolling mills. The weight of rolls can vary depending upon the type and size of the mill and the type of the roll. During rolling the roll is under high load and the contact area between the roll and material being rolled suffers wear.
Depending upon the profile of the rolled product, the body of the roll can be either smooth (plain) for rolling sheets (plates or strips) or grooved for the rolling of the shaped material (sections). Rolls have two main components namely (i) roll body and (ii) roll neck. There are two necks, one on each side. The body is the part which comes into direct contact and deforms the metal of the work piece. A pair of grooved roll showing roll barrel and roll neck is at Fig 1.
Fig 1 A pair of grooved rolls
Rolls are required to take all type of stresses, loads from normal and abnormal rolling conditions in the rolling mill and the stresses which are changing with roll wear during rolling. Rolls are required to carry out the heavy work of reduction during hot and cold rolling.
Roll design is required to take care of two absolutely different requirements into considerations. These requirements are (i) maximum strength for taking care of the separating forces, torque and high pressure between the rolls, and (ii) maximum wear resistance in the contact area between the roll and the material being rolled. Rolls are not to break, spall, or wear and are required to give good performance without causing any problem. Rolls are regularly machined for rebuilding of the desired roll profile and to eliminate the worn, fire cracked, and fatigued surface.
There are specifications for rolls, but the ultimate measure of the roll quality is the performance of the rolls. The cost of rolls per ton of the rolled steel is a decisive factor.
Various types of rolls and the types of rolling mills are shown in Fig 2.
Fig 2 Rolls and types of rolling mills
Besides normal rolling load and the frictional forces, rolls experience several changes in the rolling conditions under normal rolling of the material. Every time the material enters the rolls, it creates an impact. In addition, rolls are to face several abnormal rolling conditions which may arise due to the (i) fault of the mill operator, (ii) defective materials with internal defects entering the rolls, (iii) power failures, (iv) mechanical problems of material transportation to the rolls, and (v) problems in the water cooling systems. During such abnormal rolling conditions, which are more or less very common in the rolling mills, roll damage often takes place affecting both the mill and the rolled product.
The stresses and their distribution in a roll due to the loads of the rolling process are very complex and vary widely. Some stresses have high gradients perpendicular to the roll surface like Hertzian pressure or thermal stresses during rolling hot material and good roll cooling, while some stresses are simply to be considered as a result of static load like torque from the driving motor. However all types of stresses can lead to roll damage. Wear of the roll is another important concern for the roll.
The important aspect connected with rolls is to optimize the different properties such as strength, wear resistance and safety against fire cracks as well as all sorts of damage which usually takes place during rolling abnormalities. The process of optimizing basically includes (i) choosing of the right composition, heat treatment, and the manufacturing process. The soundness of the roll and safety against any roll failure is important and this means developing of the right micro-structure and the controlling of the level of the residual stresses.
Development of materials for rolls
During the nineteenth century, non-alloyed grey iron identified by various C (carbon) equivalents and different cooling rates (sand casting or chill casting) and forged steel were used as materials for rolls. The cast iron grades varied from mild-hard, half-hard, and clear-chill. In the clear-chill rolls the barrel had a white iron (free from graphite) layer while the roll core and the neck were of grey iron. Later cast steel rolls were developed with a C content of upto 2.4 %, with and without graphite.
Around 1930, ‘indefinite chilled double pour’ (ICDP) rolls were developed for hot rolling in the flat mills. ICDP roll grades were modified and enhanced in the late 1990s with carbide improved roll performance. Around 1950, nodular iron material for rolls was developed. The nodular iron material being either non-alloyed or often alloyed with Cr (chromium), Ni (nickel), and Mo (molybdenum) so as to get good wear resistance as well as the strength at the same time. The use of high Cr iron (C- 2 % to 3 %, Cr- 15 % to 20 %) and later on high Cr steel (C- 1 % to 2 %, Cr- 10 % to 15 %) for rolls resulted into the use of new materials with high wear resistance.
In 1985, high speed tool materials have found use for rolls. These materials have evolved as the so-called ‘semi tool steel grades’. For the rolling of wire rods, high tech sintered tungsten carbide material was developed for the rolls. For the cold rolling of steel, forged steel rolls were also developed to provide higher hardness penetration after heat treatment by increasing the Cr content from 2 % to 5 % and with the use of induction heating. Work rolls are being chrome plated after grinding and shot blasting for getting higher life of the necessary surface roughness.
At present, the different groups of grades of roll materials used as per microstructure includes (i) hypo-eutectoid steel, (ii) hyper-eutectoid steel (ADAMITE), (iii) graphitic hyper-eutectoid steel, (iv) high alloyed steel such as high Cr etc., (v) nodular iron, (vi) indefinite chill cast iron, ICDP, and (vii) special materials such as sintered carbides, ceramics etc.
Production of rolls
Rolls can be produced by (i) casting, (ii) forging, (iii) sintering or hot isostatic pressed, and (iv) other methods. All the methods have their advantages, disadvantages, and limits for production. These limits may be caused by (i) roll dimensions, (ii) roll composition, (iii) required hardness or wear resistance, and (iv) production costs.
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 usually depends on the cost of the rolls per ton of steel rolled. Low priced rolls may not be better and can be ultimately counterproductive.
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 of (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 adjust to the change in technology of rolling as well as the technology roll manufacture.
The control of the roll making technology is more crucial than the technology itself. The roll making procedure is always to be under control. The ratio of roll costs to the tons of material rolled is decreasing because of (i) better mill technology, and (ii) better roll performance. It is not due to the lower cost of the rolls. Low priced rolls are ultimately counter-productive.
Rolls and roll material properties
There are several material properties which are of interest to roll manufacturer and roll users. These are described below.
Physical properties – The important physical properties of roll materials needed for stable rolling are (i) Young’s modulus, (ii) Poisson’s ratio, (iii) co-efficient of thermal expansion, (iv) heat conductivity, and (v) co-efficient of heat transmission. The properties of Young’s modulus and Poisson’s ratio are to know the limits for elastic and plastic transformation. Co-efficient of thermal expansion is a function of temperature and it is normally constant number for a small temperature range. Heat conductivity is always for a material of which the roll is made. Coefficient of heat transmission is of high importance and it is influenced strongly by the surface of the rolls and the material being rolled.
Hardness – Hardness measurement is fast and cheap and there are good correlations between hardness and other mechanical properties at least for the same type of materials (same composition, micro-structure). However, in case of rolls, correct hardness readings are difficult to obtain and the linear relation of hardness to other properties is always limited to a certain degree. This is because roll materials have a wide variation of composition and structures. Hence, hardness readings are more confusing than helpful in case of rolls. The views of roll manufacturers and roll users on hardness readings have always being differed from each other.
Only surface hardness can be measured in a roll non-destructively. This two dimension measurement is usually considered to be a representative for the three dimensional volume behind the surface. But in the roll there exists hardness gradient due to macroscopic and microscopic variations caused by casting (decreasing solidification speed with increasing distance from the surface) and heat treatment (decreasing cooling speed with increasing distance from the surface during quenching in relations to time-temperature- transformation curves). Further hardness depths are influenced by the compositions and the heat treatment methods.
Roll surface is also prone to several things. There can be mishandling, oxidation, corrosion, work hardening, local tempering by burning during grinding etc., and decarburization due to heat treatment. All these can cause positive or negative microscopic gradients at the surface of the rolls which in turn has effect on the roll hardness.
Residual stress – Rolls normally do have residual stresses. These residual stresses are two dimensional at the surface and three dimensional in the volume. At the surface the radial stress is zero and the longitudinal stress (axial) is also zero at the barrel edge. At the main part of the barrel, axial and circumferential (tangential) are equal in sign and size. At the centre line, close to the axial area of the roll, tangential and radial stresses are equal in size and sign. Here the relation of longitudinal to tangential/radial stress is given by the relation of roll diameter to length. Which stress exceeds the material strength of the roll, causes a spontaneous breakage of the roll. The fracture can be perpendicular to the axial direction in case the longitudinal stress is too high first, or the fracture can occur in axial direction if the tangential/radial stress is too high first.
Residual stress has a high impact on the strength of the rolls. Compression strength increases the fatigue strength, reduce crack propagation, and reduce shear stress at the roll barrel surface and work hardening. Tensile residual stress may cause roll breakage. Compression and tensile residual stresses in a roll compensate each other over the cross section of the roll. The right level of residual stresses is required to be controlled in rolls.
Fatigue strength – It is important since many failures in rolls occur due to fatigue. Fatigue takes place due to the changing loads as well as due to the rotating and the bending stresses. Notches and fire cracks have big impact on the fatigue strength.
For homogeneous steel, bending fatigue strength has a linear relation to hardness upto a limit and beyond the limit the fatigue strength drops. There are several factors which influence fatigue but the material becomes increasingly brittle and sensitive to notches, and a small increase in the stress helps in initiating a crack which propagates under repeated load. In case of heterogeneous materials like gray iron, graphitic cast steel, or hyper eutectoid steel, fatigue strength is lower than that of homogeneous steel of same hardness.
Rotating bending fatigue values are always higher than the tensile/compression fatigue strength. Hence rotating bending is the most general reason for fatigue failures in case of rolls.
Wear strength – Wear is of principle importance in case of rolls. However, there are a large number of parameters which influence roll wear. These parameters can be (i) material related such as composition, microstructure, and hardness etc., (ii) properties of opposite material being rolled, (iii) wear conditions such as degree of slippage, pressure, speed, temperature, and cooling system etc., and (iv) interactive agents like water, lubricants and all kinds of corrosive agents. All these parameters can vary widely.
In rolling mills, the wear takes place mainly at the areas of highest friction, which is between the roll and the material being rolled. Wear is generally not uniformly distributed on the barrel on one end to the other. Also, the roll surface is influenced by the changing temperatures during each rotation which can cause fire cracks. Another factor which impacts roll wear is the roll cooling. Cooling agents can contain materials which can aid wear.
The wear resistance of the rolls can be improved by the selection of proper grade of the roll material. The influence of hardness of the material of the roll on the wear resistance is only marginal. The contents of C and alloying elements and the microstructures are more important parameters for wear resistance.
Frictional force – In the rolling mill, the roll and the material being rolled are both pressed together by the same load. Hence for the movement of the material being rolled between the roll gap, a force is necessary. The amount of force depends on the surface conditions (shape, and roughness etc.) as well as on the coefficient of friction between the roll material and the material being rolled. The situation in the roll gap is somewhat complicated.
Continuous rolling with no or only minimum tension needs high friction between rolls and the material which is being rolled. In case friction is low, slippage occurs. The critical parameters of rolling conditions to avoid slippages are the bite angle and the rolling speed. The higher the speed, the lower is the bite angle. If the bite angle is too high for the rolling speed then either the rolling speed is to be reduced or the roll surface needs to be modified to increase friction. The ragging of the roll surface or having a fire-crack pattern is useful for improving the friction.