Types Of Passes used In Blooming Mills

Box passes are used in Blooming mills for following reasons.

Box pass can be used for several passings of stock by screwing down the top roll after each passing. This reduces number of grooves required on rolls and enables a large number of passings can be accomplished in one blooming mill stand. Besides, mill can also accommodate various initial and final cross sections of ingots and blooms.

Construction of Box pass for Blooming Mill

Fig. 4.14 Construction of Box pass for Blooming Mill.

Other advantages to using Box poss are:

  • • Roll with box passes is stronger than those with diagonal or square passes, as later sunk diagonally into roll and weaken the roll to a greater extend.
  • • Rolling in box pass ensures uniform draft along width of stock and deformation may proceed with various degree of spread restriction.
  • • Scale is easily removed from side of stock during rolling inbox passes.

Size and Shape of Pass

Height of Pass (H)

Height of pass (H) is determined by number of passing required and total amount of reduction takes place in that pass. Depth of cut should be half of the smallest size of stock to be rolled through that pass. Deeper the cut of pass will have the more rolling stability of work piece and better will be the quality and dimensional accuracy of finished product.

The depth of groove for blooming mill fluctuates between 0 to 150 mm.

Width of Pass

The width of the Bullhead pass (В) is taken approximately 200 nun more than the maximum width of slab to be rolled, subject in case both bloom and slab are to be rolled fr om the same mill. The width of Bullhead pass is kept slightly more than the maximum width of ingot, if only bloom is to be rolled in Blooming mill.

Width of pass at highest depth (b) should be so chosen that it should not be less than minimum width of stock by 0-10 mm. It helps in protecting stock from twisting, especially during course of rolling.

Width of pass at separation line of rolling should be more than maximum width of stock to avoid fin formation due to spread.

Taper

The taper of sides of blooming mill passes amounts to 5-30 %, generally taper of around 20 % is provided to avoid collaring of stock and formation of fine seams lap on side surface.

Corner Radii

Pass Convexity

The aim of pass convexity (5) is to provide room for spread in succeeding pass. The pass convexity is generally provided to approximately 5% of pass height. Pass convexity is given in intermediate passes, it is not recommended for final pass. Pass convexity also helps in stability in movement of work piece on roll table.

Roll Collars

Generally, alloy forged steel rolls are used in Blooming mills. The widths of collars are taken atleast 50% of the depth of groove in a roll.

Bottom Pressure

To prevent hitting of work piece to the roll table, which is about 7 to 10 times heavier than the roller of roll table. Bottom pressure is given in Blooming mills, by making difference in size of roll diameter to the extend of 5-10 mm. I11 case of Blooming mill, wherein each roll is driven separately by individual motor, the rpm of bottom motor is generally kept more than the top roll to get the desired amount of difference in roll peripheral speed of top and bottom roll.

Turning of the Workpiece

From the point of view of increasing output, work piece should be turned to least possible number of times during rolling. But from the point of view of better quality and high surface finish of the finished product, the stock should be turned to maximum number of tunes.

Repetitive reduction in one direction causes tension to setup in side surface due to spreading and due to the differential elongation between surface and center. It may ultimately lead to cracking on side surface

It is advisable to turn work piece after eveiy two passing, particularly at the beginning and later turning will depend upon size of groove available and tilting facilities of mill. It is to note that width to thickness ratio of work piece should not exceed 1:3 when rolling in bullhead pass and 1:1.7, when rolling in other passes in case of rolling of blooms.

The bar should be turned after eveiy two passings, in case of rolling of alloy or high carbon steel.

Roll Pass Design Data Sheet of Blooming Mills

Based on the principle discussed above, “Roll Pass Design” of blooming mills can be evolved, from where, there certain basic information are required as given in Table 6.3 below:

Table 4.3

Necessary Information

Example

A

The cross sectional dimension of the ingot which are to be rolled together with weights. This is required to fix the size of the bullhead or first pass width.

  • 810x770 , x 2.5 mt
  • 790x750

В

The smallest bloom, which is required to be rolled and hence the stock length from ingot weight

285x285

C

Any intermediate bloom sizes which may be required

325x325

NOTE: Point В and C control the widths of the edging grooves, It is also desirable to know what are sizes of blooms are required to roll and with what accuracy they must be produced with regard to flatness of sides and corner radii

D

The maximum and minimum width of any slabs required to be rolled. This is required for designing of the width of first pass or of the bullhead pass.

The length of roll barrel available

Max. width - 700 mm

Min. Width- 400 mm 2800 mm

E

  • (a) The diameter of rolls i.e. maximum and minimum diameter
  • (b) Roll Neck Diameter

1180 mm 1080 mm 690 mm

F

The manipulation facilities available at each side of the mill The rolling sequence must make the best use of tilting to have both quality and quantity in production. Availability of tilter one side or both the side Nos of filters

Tilter is only available at entry side

1

G

Distance from roll axis to nearest finger of tilter Manipulator Length of guard

Max working opening of guards

2550 mm 8750 mm 2750 mm

Steel quality to be rolled which together with mill power determine the

Mild steel

max draft

Rail steel

Roll material

Forged steel

Top roll lift

1120 mm

Motor

Motor capacity in kW

2x8000

Motor rpm

Initial temperature of rolling

1200°C

Shear

Max. blade pressure

1000 T

Blade length

1200 mm

Max. size of bloom cut

400x400 mm

Max. size of slab cut

200x1000 mm

Rolling Schemes for Rolling Different Size of Blooms/Beam Blanks

Table 4.4

MS Blooms-280x285, 260x300, 265x340

Pass

No. of Passing

Section

  • 810x770
  • 790x750

Reduction

5

1

715

95/75

2

620x800

95

TILT 90°

3

730

70

4

660

70

5

590

70

6

520

70

7

450

70

8

380x660

70

TILT 90°

I

9

575

85

10

480

95

11

385

95

12

295x420

90

TILT 90°

II

13

350

70

14

295

55

15

265x305

30

TILT 90°

II

13

380

40

14

335x315

45

TILT 90°

5

15

270x345

45

TILT 90°

II

13

345

75

14

275x325

70

TILT 90°

II

15

285x290

40

Beam Blank-450

Pass

No. of Passing

Section

  • 810x770
  • 790x750

Reduction

5

1

715

95/75

2

620x800

95

TILT 90°

3

710

90

4

620

90

5

550

70

6

490

60

7

430

60

8

370x660

60

В

450

TILT 90°

9

580

80

10

510

70

11

440

70

12

370

70

13

310

60

14

255x415

55

TILT 90°

5

15

375

40

16

335x270

40

TILT 90°

В

450

17

195

75

18

145

50

19

95x415

50

Note: Sizes shown in the schemes are hot section, i.e., 285><290 mm hot section will produce a bloom size of 280x285 mm cold section

Design of Pass arrangement in 1150 mm Blooming Mill

Fig. 4.15 Design of Pass arrangement in 1150 mm Blooming Mill.

 
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