Roll Speed

The speed of the rolls varies during the pass when heavy ingots are rolled in reversing blooming or slabbing mills. A typical diagram showing the variation

Variation in Roll Speed and Torque in a Blooming Mill during Rolling

Fig. 4.9 Variation in Roll Speed and Torque in a Blooming Mill during Rolling.

in roll speed for a two-high reversing mill is shown in Fig. 4.9. The rolls bite the bar at the speed nl which is then increased to the maximum value n2 and, at the end of the pass, is reduced to ?/3. After the bar leaves the rolls, the motor is stopped and reversed for a pass in the opposite direction.

The following period are distinguished in the diagram :

  • (a) Acceleration of the mill without the ingot, in which the roll speed increases front 0 to Wj;
  • (b) Acceleration of the mill with the ingot, in winch the roll speed increases from ?/, to 77,;
  • (c) constant speed, in which the rolls maintain a constant speed of //,;
  • (d) deceleration of the mill with the ingot in which the roll speed dr ops from 77, to riy and
  • (e) deceleration of the mill without the ingot, in which the roll speed drops from 7;з to 0. If fj denotes the tune for mill acceleration with the ingot, t2 - the time for the constant speed period and t}- the tune for mill deceleration with the ingot, then the time for the pass with equal:

The time for the period of mill acceleration and deceleration without the ingot is part of the interval between passes.

Main Drives of Motor

The main drive motor of a Blooming or Slabbing should have the following:

  • (1) Reversible duty with frequent starts and stops (upto 600-1000 reversals per hour).
  • (2) Minimum reversal time (from rated speed in one direction to rated speed in the other direction) which will reduce the operative time per bar rolled and increase the mill output.
  • (3) Large overloads wiien metal is entered and is rolled with high accelerations. It leads to considerable dynamic current loads in the armature and high torques.
  • (4) Speed variation in a wide range.

These requirements are fully satisfied with a DC drive motor.

Modem Blooming mill with individually driven rolls 1100 to 1200 nun in diameter are pow'ered with motors of 4000-8000 kW. Blooming mill with rolls of 1100 nun in diameter or less are also equipped with more powerful motors. At present, drive through pinion stand are also used in many cases.

Recent practice is to increase the acceleration and deceleration of blooming and slabbing mill motors, in addition to increasing their capacity.

The acceleration of modem blooming and slabbing mill designing at the present time ranges from 80 to 120 rpm per second and deceleration -from 80 to 130 tpm per sec.

Roll Tables, Manupalaters of Mill

Blooming and Slabbing mills are equipped at front and back with roll tables, whose purpose is to advance ingots to the rolls. It is in the process of rolling, where convey the bar to the shear and from there to the yard. In accordance with operations performed by them, roll tables are classified as receiving, mill and conveying roll tables.

Principle of Blooming Mill Manipulator

Fig. 4.10 Principle of Blooming Mill Manipulator.

The bar moves on the mill roll tables from pass to pass along roll barrel on either side of the stand by manipulators. The manipulator can also do the straighten bars, if they become bent in rolling.

Shears for Primary Mill

Shears (with parallel blades) are used to cut hot bar rolled in blooming or slabbing mills. Cutting force decides size of cross-section to be cut. The blade length is taken equal to 2 to 2.5 times of the maximum width of bloom for blooming mills or 100 to 200 mm more than maximum slab width for slabbing mills.

1-Upper Blade, 2-Lower Blade, 3-Hold-Down, 4-Table Roll, 5-Bar. Fig. 4.11 Cutting Principle of a Parallel-Blade Shear.

The size of cross-section cut by such shears depends upon their designation and the maximum cutting force. The blade stroke is selected so that a bar of maximum cross-section passes freely under the power hold-down. The blade length is taken equal to:

  • («) 2 to 2.5 tunes the maximum bloom width for Blooming mill shear.
  • (b) 100 to 200 nun more than the maximum slab width for Slabbing mill.

The cross-section of blade is usually based on the ratio = h/b = 2.5 to 3.

hi selecting the size of the shear, the maximum cutting force can be determined from the formula;

Where Kx - a coefficient accounting for the increase in the cutting force upon dulling of the blade and an increase in the clearance between them.

K-, - ratio of the shear strength to the tensile strength

F - cross-sectional area of the bar being cut, sq. mm

eb - Tensile strength of the metal at the temperature at which, it is cut, kg/mm2.

Blooms and slabs are delivered to the storage yard through cooling bed. It has the provision of stamping, marking and weighing facilities.

Scale Disposal for Mill

Much attention is to be paid to the scale disposal in Blooming and Slabbing mill. Out of the various methods of scale disposal, the most popular is hydraulic method, in which the scale is washed away with the flowing water. Generally, flight conveyors are deployed below the roll table, towards the direction of scrap way for the disposal of both fine as well as large pieces of scrap.

Large pieces of scraps slide down to inclined grates onto the flight conveyors, which deliver them into the boxes placed in pits of scrap bay and dumped on rail road cars.

Fine scale drops through the slots in the inclined grates and falls into inclined channels through the continuously flows of water. From channels, the scale is washed into large settling pit in the scrap way. From here it is removed by a clamshell crane.

 
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