# Classification of Rolling Mills

Fig. 2.30 Classification of Rolling mill.

Rolling mill are classified according to following criteria: Table 2.3

 SI. No Criteria of Classification Example 1 Nos. of roll in mill stand 2 high, 3 high, 4 high mill 2 Nos. of drives in mill stand 1 Drive, 2 drive, 3 drive mill 3 Type of product /function Blooming mill, Billet mill, Rail mill, Plate mill 4 Layout of mill Open train, continuous mill, Tandem mill, cross country mill 5 Direction of rotation of roll Reversible, non reversible 6 Special purpose mill Cluster mill, sandzimir mill

## Classification as per Number of Rolls

Fig. 2.31 Roll Arrangement in 2-High and 3-High mill stand.

Fig. 2.31 (a), shows two high rolling mill, equipped with two rolls. As character of rotation of the mill rolls, one distinguishes the two high reversing mills (rotation of rolls is reversible) and two high non-reversing mills (rotation of roll is non-reversible).

Fig. 2.31 (Z>), shows three high rolling mills, having three rolls in the working stands and rotation of their' rolls is non-reversible.

Fig. 2.32 4-High and Cluster mill.

Four high mills have four rolls, Fig. 2.33 (u), arranged in a vertical plane. Two smaller in diameters are working rolls, while larger one are called Backup rolls. The most important requirement for rolling flat products is to maintain dimensional accuracy throughout at width. Back-up rolls support working roll in operation and minimize the elastic deflection. Smaller work roll reduces the power requirement.

A cluster mill is a multi roll mill to use work roll of a smaller diameter [Fig. 2.32 (b)]. The minimum diameter of roll is limited by the permissible flexural stresses. Therefore, in all design of multi-rolls mills, the small diameter work rolls are supported by large diameter rolls (back-up rolls).

## Classification as per Nos. of Drives in the Mill

Fig. 2.33 Two-Drives Rolling Mill.

Here, classification of rolling mill is based upon the No. of drives in the mill

i.e., one drive, two drive, three drive etc. An example of 2 drive mill is shown in Fig. 2.33.

## Classification as per Products Rolled

There are different types of mills according to products to be rolled or function are placed at Table 3.4 below:

Table 2.4

 Mill Size of Roll mm Barrel Length mm Function A. Primary & Intermediate 1. Blooming 800-1450 - Reduction from ingot to blooms 2. Slabbing 1100 - Reduction of ingots to slabs 3. Billet Mill 450-750 - Reduction of blooms to semis billet of 50x50 to 150x150 4. Rounds 700-800 - Rolling of blooms to semis rounds of size of 70 to 350 mm B. Section 1. Rail & Section 750-900 - Rolling rails, beams & heavy sections 2. Heavy Section 500-750 - Rolling of heavy sections, steel squares, rounds 3. Medium Section 350-500 - Rolling of medium sections, rounds, squares 4. Light Sections 250-350 - Rolling of light sections,rounds,squares 5. Strip (Skelp) 300-400 - Rolling Strip 65-500 mm Wide and 1.5 to 10 mm Thick 6. Rod 250-350 - Rolling Rod From 5 To 10 mm dia. C. Flats 1. Plate - 2000-5000 Rolling plates of size 5 to 50 mm thick 2. Sheet - 700-2500 Rolling sheets of size from 600-2300 mm 3. Hot Wide Sheet - 700-1300 Rolling of thin sheets of 0.2 TO 4 mm thick & 500-1200 mm wide 4. Tube Rolling - - Rolling seamless tube upto 650 mm dia. & more 5. Cold Rolling 6. Sheet - 700-2800 Rolling sheets of 600 to 2500 mm wide 7. Narrow Strip " 150-700 Rolling of narrow strip from 0.2 to 4 mm thick & 20 to 600 mm wide, supplied in coils. 8. Foil - 200-700 Rolling foils from 0.008 to 0.012 mm thick.

(Contd...)

 D. For Special Kinds of Rolling 1. Wheel - Rolling wheels of railway wagons & other purposes. 2. Hoop & Tyre - - Rolling hoops & tyres for wheels 3. Ball - - Rolling balls for ball mill. 4. For Rolling Variable Sections - - Rolling various kinds of sections. 5. For Gear Wheels - - Rolling teeth of gear.

Fig. 2.34 PLATE MILL.

Classification of mills particularly the division of large group of section mills into heavy, medium and light although is widely used but it is not clearly defined. In rolling of steel three types of intermediate mills are:

• 1. Continuous billet mill for rolling billet section from 50 x 50 to 150 x 150 nun.
• 2. Continuous sheet bar mill for rolling sheet bars and also other sections.
• 3. Tube round mills for rolling rounds from 70 nun to 350 nun diameters, which are required in production of seamless tubes.

Main types of section mills are universal mill for rolling wide flange beams, rails, medium section, light section, rod and strip or skelp (for rolling strip of width not greater than 400-500 nun). The strip mills are sometimes also called as skelp mill.

Light section mills are used for rolling finished sections in which the following are most commonly rolled: rounds and squares from 8 to 40 nun, angles fiom 20 x 20 to 50 x 50 nun and other sections like beam and channel of smaller sizes.

Cold rolling mills are divided into following groups:

• 1. Sheet mills for rolling short lengths.
• 2. Wide strip mills for coil rolling.
• 3. NaiTow strip mills for rolling strip from 0.02 to 4.0 mm thick and foil (0.002-0.012 mm) from aluminium, steel molybdenum, tungsten and other metals.
• 4. Wire flattening mills for rolling veiy narrow strips from wires.
• 5. Mills for cold rolling tubes.

Fig. 2.35 Block Mill for Wire Rod.

## Classification As Per Layout

17.4.1 Open Train Mill

Most simple form of mill is a open train mill, wherein, stands are positioned in a line side by side as shown in a Fig. 2.36. It is driven through same set of pinions and with a single motor. One most disadvantage of this mill, is that rolls of all stands are run at a same speed, therefore, it is impossible to roll in this mill, a bar at a speed, that increases as length of bar increase.

Fig. 2.36 Classification as per Layout.

17.4.2 Cross Country Mill

When stands are placed one after another in different rolling line parallel to each other, with the use of repeater or chain transfer to facilitate rolling with higher speed and to facilitate in smaller space, then it is called as a cross country mill as shown below:

Fig. 2.37 Cross Country Mill.

17.3.3 Universal Rolling Mill

A modem mill which has a set of combination of horizontal and vertical stand is called as a universal rolling mill. Generally, a modem wide flange beam mill as shown in Fig. 2.38 below is of such type.

Fig. 2.38 View of A Universal Rolling Mill.

17.4.4 Combination Mill

This type of mill may have tandem, open, looping, and cross-country trains and are used in combination with many layouts, eg., an alloy bar and rod mill could commence with a two-stand, 3-high, open roughing train. It is to be followed by a cross-country train (used as a looping train on smaller bars) terminating with a tandem finishing train, as illustrated in Fig. 2.39.

Fig. 2.39 Alloy Bar and Rod Combination Mill Layout

Multi-strand trains are those designed to roll more than one length of stock simultaneously. In these mills, two, three or even four pass line is set up across the roll barrel to roll simultaneously. The practice is adopted in bar and rod mills

## Continuous/Non-continious Rolling Mill

A Continuous rolling is in the broadest sense implies for a process, in which the stock get engaged in atleast two stands simultaneously. A “Continuous Mill” usually implies for the mill, in which, the stock need not reverse at any stage and makes only one pass per stand. The term “Fully Continuous” is used to signify that once entered stock cannot emerge freely until it get finished in finishing stand, due to the closer spacing of the stands. This is in contrast to mills such as wide strip mills with spaced roughing trains where the stock may emerge freely after each roughing stand. The “Semi- continuous mill” is characterized for a mill, where reversing roughing stand precede a tandem finishing train but the same description cannot be given to a mill, which consists of a continuous roughing train followed by a cross-country finishing train.

## Classification wrt Direction of Rolling

17.6.1 Reversible Mill

Where the direction of rolling gets reversed in each passing’s like 2 high reversible Blooming Mill and Slabbing mill.

17.6.2 Non-reversible Mill

Where direction of rolling is not reversed. Examples is continuous Wire mill and cross country Merchant Mill.

# Special Types Mill

## Double Duo Mill

Fig. 2.40 A Double-Duo Mill.

A Double-Duo mill consists of two pair of two high roll set in the same housing, but at the different height. Although, there are 4 rolls are in same housing, 2 rolls are placed in one plane, while other 2 are placed in different plane. In other words, it serves as double 2 high stands i. e., the bar after passing through the bottom pair of rolls; it turns upward or repeated to go through top pairs of rolls. Its advantage over 3 high is the flexibility of roll pass design avoiding common middle rolls of 3 high stands.

## Sandzimar Planetary Mill

Sandzimar Planetary Mill is used for the production of narrow to medium width strips by hot rolling (Fig. 2.41). It consists of 2 large back up rolls, surrounded by a number of planetary work rolls. The planetary work rolls are

Fig. 2.41 Sandzimar Planetary Mill

driven around backup rolls, driven by the motor. The cumulative reduction by planetary make it possible to have as high as 90% of total reduction in a single pass. Feed rollers are located at the entry side of the mill, to force the stock into the mill. Proper arrangements are made at the delivery side to avoid ripple on strips.

# Determination Of Mill Size

The size of the mill is determined in the following ways:

## Where Rolls are Individually Driven

The mill size is determined by the mean or average roll diameter of the stand. This is applicable, where in the top and the bottom roll are driven by individual motor and in between there is no pinion stand. Blooming and slabbing mill, which are driven by individual motor for top and bottom drive, falls under this

Mean or Average dia.,.

Where Z> and D „ , are max. and min. roll diameter and is the roll gap. An 1150 mm Blooming mill, which has a max. and min. diameters as 1180 mm and 1080 nun respectively and roll gap is 20 nun, then Average diameter will equal to

## Where Rolls are Driven by Single Motor Through Pinion Stand

In case of Merchant mill or a wire rod mill, wherein top and bottom rolls are driven by a single motor through pinion stand, the pitch diameter of pinion stand is called as “Mill Size”.

A 350 nun Merchant Mill, the 350 mm is the pitch diameter of pinion stand.

## Flat Rolling

In case of Plate and Sheet mill, where the length of barrel is more significant than the actual diameter. The mill size is determined by the available roll barrel between necks. For example, a 3200 nun plate mill has roll barrel length of 3200 nun.

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