Rolling Mills Rolls
Rolling Mills Rolls
Rolling had assumed importance in the industrialized world during the nineteenth century. Profiles and flats are hot rolled while some flat products are also cold rolled. Rolls are tools in the rolling mill. The weight of the rolls can vary from a few kilograms to as high as 250 tons. Rolls are required to carry out the heavy work of reduction of the cross section of the steel being rolled. Rolls have to take all kind of stresses, loads from normal and abnormal rolling and which are changing with the roll wear during a rolling campaign. Roll should never 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 that comes in contact with the steel suffers wear, while other parts of the roll body and roll necks does not experience plastic deformation or fatigue but are under high loads. In the recent past rolling technology was 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 had followed the development of rolling technology.
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 produced today. 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 iron was invented and introduced into roll manufacturing. The nodular iron was without alloys and sometimes alloyed with Cr, Ni or Mo. These rolls were having good wear resistance and strength at the same time. The use of high chromium iron (C- 1-2 % and Cr – 15-20 %) and later high chromium steel (C-2-3 % and Cr – 10-15 %) 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 Cr increased from 2 % to 5 %. Chromium plating of work rolls was also introduced to improve surface roughness. Typical rolls for cold rolling are shown in Fig
Fig 1 Typical rolls for cold rolling
Besides the chemical composition, physical and mechanical properties (micro structure, hardness, YS, & UTS 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.
The main parameters which control the properties of a roll, are as follows.
- mono or compound roll (roll design)
- chemical composition of the material
- casting (mould design, temperatures, weights, inoculation, and down cooling)
- heat treatment
According to the microstructure of the roll materials there are the following groups of grades.
- Hypo eutectoid steel
- Hyper eutectoid steel, ADAMITE
- Graphitic hyper eutectoid steel
- High alloyed materials like high chrome and high strength steels etc.
- Nodular iron
- Indefinite chill cast iron, ICDP
- Special materials such as sintered carbides and ceramics etc.
The materials related to one of the first four groups can be controlled/ influenced by heat treatment, however the structure, shape, size and amount of primary carbides in rolls is more or less given after initial forming and is not changed after heat treatment. The maximum temperature during heat treatment is in most cases restricted by the liquidus temperature of core material of compound rolls, most frequently ductile iron. The amount of carbides increases with the extent of carbon and carbide forming elements (Cr, Mo, Nb, V, and W etc.). The material of the rolls in groups 3, 5 and 6 contains graphite which forms during solidification. Graphite has a high impact on the performance of the rolls during rolling due to the following advantages.
- Graphite makes smaller, finer fire crack pattern/network
- Graphite reduces the risk of spall after rolling accidents
- Graphite creates a better and smoother surface of rolls and rolled product
Graphite has no impact on wear. Wear resistance is due to the amount and hardness of carbides.
Rolls can be produced by any of the following methods of production.
- Other methods
All the above methods have their advantages, disadvantages, and limits for production. These limits may be caused by the following.
- Roll dimensions
- Roll composition
- Required hardness or wear resistance
- Production costs
There are areas that 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 may 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 following expertise.
- Understanding of the roll application (load, speed, and roll cooling etc.)
- Choice of optimum material
- Production of sound rolls without having any defects
- Choice of adequate heat treatment (strength, hardness, and residual stresses etc.)
- Ability to machine the roll to meet the requirements of specifications and prints
- 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 should 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. Usually rolls are subject to less stress than their design load would allow. However in the case of rolling accidents, stress may 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 maker has to involve himself 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.
- The roll must have maximum strength to take separating forces, torque, and high pressure between work roll and back up roll etc.
- 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 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. Spin 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 may 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 – Three factors important for thermal breakage are (i) thermal gradient, (ii) strength and integrity of core material, and (iii) residual stresses.
- 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 – A fire crack pattern on the surface of rolls used for hot rolling with water cooling is very common. Fire cracks may be initial cracks which can develop into deeper cracks and spalls.
- Local overloads
- Roll fatigue – fatigue damage can start at the surface or subsurface.
- Spalling – There are two different types of spalls. One starts at an initial surface crack and the other type starts sub surface.
- Damage of steel rolls due to hydrogen – Problems caused by the hydrogen is (i) special fatigue and (ii) hydrogen related delayed brittle fracture.
- Wear – Roll wear depends on roll material, chemical composition and microstructure. The wear of the roll decreases with an increased content of hard carbides.