Specific Features of Continuous Mill

In case of continuous mill, the work piece is rolled simultaneously in two or more stands at a time i.e., volume/second passing through each stand of a continuous mill is a constant.

As shown in Fig. 5.11

Where V and F are peripheral speed of roll and area of metal of various stands of a continuous group.

Principle of a Continuous Mill

Fig. 5.11 Principle of a Continuous Mill

or, F x D x N = C, if forward slip is negligible.

Here, C = Rolling constant D = Rolling diameter of roll and N = Roll rpm = n/i, where n is motor rpm and i is a reducer ratio of pinion stand,

F = Area of metal and S is forward slip.

Advantages of Individually Driven Stand Over Group Combined Driven Stand

In a combined drive rolling mill, the selection of the metal section and rolling diameter of rolls at different stands of a continuous group is a very difficult task. The constant i.e., volume per second passing through each stand can only be varied either with roll diameter or with roll speed. In a combined drive of a group driven mill, the flexibility of doing speed maneuvering to change the rolling constant will not be possible. For, only remedy available, if roll diameters can suitably altered to meet the requirement. Roll diameter, in fact can only be varied within a limit of close tolerance.

Thus, group drives imposes limitation on product mix of the mill, while individual driven stand of a group can offer a wide range of speed regulation. Group driven continuous mills are still in use, where mill is required to roll only one specific section, thus considerably reduces the mill manufacturing cost.

hi an individual driven continuous mill, loop growth and tension are completely eliminated by the adjustment of the speed of individual stand of the continuous group. This makes the simplification of technology and production of better quality products within close tolerance range.

Pass Design Details of Billet Mill

Open Box System

Fig. 5.12 Open Box System.

Box passes are used in the roughing group of billet mill, but it is seldom used in finishing train of billet mill because of its limitation of rolling finished billets within the close tolerance.

Diamond-Diamond System

Diamond Pass With Rounded Corners

Fig. 5.13 Diamond Pass With Rounded Corners.

The diamond-diamond system sequence is applied directly after the box passes in open train billet mills, continuous billet mill and roughing stands of medium section mills. This sequence is generally used for rolling high quality steels.

The distinctive advantages of this system are:

  • • The possibility of obtaining a square cross-section from each diamond pass, by turning 90 degree between two passes of one pair of rolling stands.
  • • The possibility of obtaining square billets of several sizes from the same pass by varying the clearances between rolls.
  • • Simple and easy roll adjustments, when changing over of rolling to a different section or steel of different gr ades.
  • • Stability of stock in diamond pass is higher than other passes. Stock is seldom twisted during entry and rolling in diamond pass.
  • • No complex guides are required in rolling in diamond pass. This simplifies the working of rolling crew, as only side guides are used.

The diamond-diamond sequence has the following disadvantages.

  • • In comparison with other breakdown passes, diamond passes are sunk deeper into the rolls, therefore, weakens it to a greater extend.
  • • A square billets rolled in diagonal pass is of octagonal in shape. It has a detrimental effect, when such billets are loaded in and moved along in heating furnace.
Diamond - Square Sequence

Fig. 5.14 Diamond - Square Sequence.

• Furnace scales are not properly removed from the bar during rolling in diamond pass, leading to formation of various defects. Diamond passes are never employed for initial breakdown passes for this reason. A combination of box and diamond pass is applied and is proven more effective.

Diamond-Square Sequence

In this sequence, diamond passes are placed alternate with square passes. After being delivered from a large square pass, the bar is turned to 90° and entered into a diamond pass. After delivery from the diamond pass, the bar is turned to 90°again and so forth. The diamond-square system usually applied for rolling square of comparatively angle varies from 100° to 125° and square angle varies from 90° to 96°, depending upon the placement of stand.

Different Pass design in Diamond-Square System

Fig. 5.15 Different Pass design in Diamond-Square System.

The sequence is designed with following principles:

(д) The sequence is designed so that width (horizontal diagonals) of any given pass is equal to the height (vertical diagonals) of the preceding pass [Fig. 5.15 (д)]. In this case diamond pass is intermediate between two square passes.

With angle at the apex of diamond pr/ = 120°,the maximum co-efficient of elongation of diamond-square sequence will be

(b) The sequence is designed so that the width (horizontal diagonals) of any given pass is larger than the height of preceding pass. (Fig. 5.15(b)). hi this sequence the coefficient of reduction will be a function of apex of diamond pi/ and co-efficient k, where к will be

Advantages of Diamond-square System

  • • Possibility of rolling squares of several sizes due to presence of different square in rolling sequence.
  • • Possibility of getting geometrically exact square with correct angle.
  • • Approximately even deformation over the whole width of cross-section.
  • • Shape of pass protects against formation of cracks, especially at comers, as comers of bar, the whole cross-section cools less extensively than the diamond-diamond system.

Disadvantages of Diamond-Square Sequence

  • • The use of diamond-square system makes the roll weaker, as this sequence has more depth of cut, in comparison with box passes; This sequence is always advised to be used in finishing train, with the use of the box sequence in roughing train.
  • • Quicker wear out of pass due to wide difference in the peripheral speed along the width of the pass.

Oval-Square System

In this system, the stock after its delivery from preceding square pass enters flat down into the following oval pass. After leaving the oval pass, the stock is turned to 90° and then enters to the next succeeding smaller square pass. The oval-square pass sequence is the most effective and heavy reduction sequence of the break down sequences. It is preferred mainly for rolling smaller square sections and wire rod mill. This is rarely used for billet production. However, this system can be recommended only for billet rolling, when there is a shortage of stands in Billet mill and there is no limitation from motor power side for giving higher reduction.

Oval-Square System

Fig. 5.16 Oval-Square System

Advantages of Oval-square System

As shown in Fig. 5.16, due to the continuous renewal of the comers and turning of bar in each pass of oval-square system, it makes to ensure the uniform distribution of temperature across the cross-section of the stock and it enables to achieve uniform properties of finished billets. Besides this, comer renewal avoids tensile stress concentration at the comer of stock.

Disadvantages of Oval-square System

  • • The shape of pass combined with the chances for scale to enter at the roll opening. It gives rise to conditions, where scale breaks out from oval and got pinched in the bottom fillet of square pass and causing impression along the length of finished billet.
  • • Rapid and uneven wear out of passes, especially in oval pass in this system.
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