1. Roll quality
The roll has mainly three parts – Wobblers, Neck and Barrel. The wobblers are for driving or rotating the rolls, the necks are to support them in the mill housings and the barrel portion is the working portion of the roll where actual rolling takes place.
Requirements of a roll:
Resistance to breakage: The roll should be strong enough to roll materials without breaking.
Resistance to wear: It should be wear resistant so that maximum rolling can be done with high dimensional accuracy and surface finish.
Resistance to fire cracking and spalling: The roll surface should not develop fire cracks or spall.
Good surface finish: It should impart good surface finish to the rolled product.
Good biting properties: It should be able to bite and grab the rolled material well enough and to avoid slipping.
It is not possible to get all the above mentioned qualities in a single roll material. Therefore, a roll designer has to select the best material out of those available to suit the requirements. Generally, it can be said that the harder the rolls, better the wearing properties, less tough they are and vice-versa.
2. Classification of Roll MaterialsRoll materials fall into three main groups:
(a) Steel base
(b) Iron base
(c) Special materials like tungsten carbide and high speed steel
a. Steel Base RollsThese may be cast or forged.
Cast steel rolls: These rolls have qualities and grain structure like steel although the carbon content may be quite high (upto 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 alloys are used in big size mills such as Blooming Mills. The most used rolls in this group are called ‘Adamite’ rolls. Depending upon carbon content these adamite rolls are graded as A, B, C, D & 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.
Forged steel rolls: Rolls are made by forging. The forged rolls contain less carbon compared to cast rolls because high carbon content would cause cracks during the forging process. The structure of forged rolls is denser than that of cast steel rolls and therefore, are tougher and can take more load. However, because of the lower carbon content the hardness is low and more subjected to wear than cast 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.
b. Iron Base RollsThese are maximum used in rolling mills mainly because they impart good finish and possess good wearing properties. Many years ago cast iron or chill cast iron rolls were only used. To increase the strength, finish, wearing properties and heat resistance, 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. There are a large variety of cast iron rolls available in the market but only a few representative ones are dealt below:
– Chill cast iron: The roll casting is made vertically with barrel in chill moulds. The chill surface is very hard and imparts good finish. These rolls are used for rolling flat products like sheets and plates and small sections. 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 (Fe3C) imparting very high hardness. The thickness of the chill varies. It may be even upto 20 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 called Grey. Hardness drops down if the chill is worn out.
– Nichillite rolls: These are also chill rolls but nickel is added to them to impart high hardness and strength. The high hardness provides good surface finish to the product. The hardness characteristics are same as that of chill rolls. These rolls are used for flat rolling, in the Finishing Stand of Strip Mill.
– Alloy grain rolls: As the name suggests alloy grain rolls are made from iron having carbon more than 3% with nickel, chromium and molybdenum added as alloying elements. The free carbon is present in the form of flakes. These are sand cast rolls and have reasonably good strength and wearing properties. These rolls are useful in section rolling mostly in the intermediate passes where light drafting is possible. The hardness drop from surface to core is gradual.
– 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 upto a great depth. In fact, the hardness drop from the surface to the core of the roll is gradual upto 4″ to 5″ depth compared to chill roll where the hardness drop is sharp. Therefore, indefinite chill rolls are used in the intermediate and finishing stands of Section Mills where deep grooves may have to be cut to make the required 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 and can be used in the intermediate stands of Strip Mill.
– Spheroidal graphite rolls: This is another type of alloy iron rolls where the structure is completely different from that of cast iron rolls. The carbon is present in the form of spheroids or nodules which increases the ductility and makes the roll more resistant to fracture. Nodularisation 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. However, such rolls are more subjected to fire-cracking and need much 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. They can be used in Medium & Light Structural Mills, Sheet, Bar & Billet Mills, Merchant Mill and also in Reversing Mills.
– Double poured 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.
– 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 it more tough. These rolls are superior to double poured rolls as the shell portion is denser giving better properties. Double poured and centrifugally cast rolls are also known as Duplex Rolls.
– Tungsten carbide rolls: The demands of modern high speed Rod Mills having No-Twist Finishing Blocks brought about the development of special roll materials particularly tungsten carbide as an ideal roll material. 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 used in the last few years. The general use of a single carbide grade throughout the entire finishing block is not always the optimum solution. Atleast two or more grades must occasionally be considered. Tungsten carbide roll manufacture is a highly specialised 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. 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 manufac-turers and supplied to the users to make the necessary roll passes. Some Rod Mills also use high speed steel instead of tungsten carbide for rolls of the No-Twist Finishing Block.
3. Economic Utilization of Rolls
Rolls are very expensive item of the Rolling Mill. Therefore, proper care in selection and utilisation of rolls is of extreme importance. If the rolls fail in service or do not give the expected satisfactory life, it adds upto the cost of production. The cost of rolls varies from Rs 70/- to Rs 100/- per kg depending upon type and quality. Wear and breakage of rolls should be avoided as far as possible.
Further, the roll breakage not only means a full loss of an expensive roll, but along with it damage to guides, rest bars, chocks, driving systems, etc. In the event of a roll breakage there is loss in production due to mill stoppage for several hours; if the roll failure occurs in a primary mill it will hold up the Finishing Mills as no steel can be supplied to these mills and also hot ingots may have to be put in stock.
To avoid the above losses the rolls should be properly designed and carefully used.
The roll consumption in mills rolling flat products is always high mainly due to the following reasons:
– high rolling loads.
– flat products require better surface finish and therefore the roll changing is more frequent.
Rolls consumption can be reduced by some of the following actions:
– improving roll quality so that the rolls do not wear fast. In some Rolling Mills the harder rolls have been introduced in Merchant Mill Finishing Stands, Nichillite Rolls in Strip Mill, SG Iron Rolls in Sheet Bar & Billet Mills and Medium & Light Structural Mill (for rolling beams and channels).
– increasing the working range of the rolls by starting with bigger size rolls as done in some Rolling Blooming Mill, Sheet Bar & Billet Mill and presently being contemplated also in Merchant Mill.
– avoiding roll failures through improvement in the roll designs and faulty handling.