Slit Rolling of Bars

Slit Rolling

Fig. 6.32 Slit Rolling.

Why Slitting is Required

Slitting in hot rolling mills started in the beginning of 20th century, when scrapped and rejected rails were slit into web, flange and head by hot rolling process and then it were further rolled to various shapes.

In the late 60’s, the slitting concept was adopted by Swedish engineers to enhance the production in bar mills where increasing the speed of mill was either difficult or was not advised due to design limitations of the mill.

Slit rolling is a method of rolling that allows relatively small size bars to be rolled at low finishing speeds, but still match with the higher reheating furnace capacity.

For example, to produce a 10 mm rebar single strand at furnace capacity of 80 T/H,it requires a finishing speed of > 20 m/sec. This speed is not possible on most of Bar Mills (either due to motor capacity or cooling bed deceleration).By slitting the bar into two strands part way through the process, mill can roll at 80 tones per hour and at the sametime, the finishing speed can also be reduced by half to 10 m/sec. Each strand will then produce half of the furnace capacity, i.e.,

  • 80 / 2 = 40 t/hr Advantages of Slit rolling
  • • Increased production.
  • • Reduced number of rolling stands, (for a particular product range).
  • • Lower rolling speeds with increased productivity.
  • • Increased competitiveness Disadvantages of slit rolling
  • • Increased quantity of guide equipment.
  • • More complex rolling technique.
  • • Stringent maintenance of rolling mill equipment.
  • • Increased operator vigilance required

Sections Produced by Slit Rolling

  • (я) Re-bars : 8 mm, 10 mm, 12 mm, 16 nun, 20 nun and 25 nun.
  • (ft) Squares : 6 nun, 8 nun, 10 nun, 12 nun, 16 nun and 20 nun.
  • (c) Plain Rounds : 10 nun, 12 mm, 12,7 nun, 16 nun and 20 nun.
  • (d) Small Flats : 12 nun x 3 nun, 16x3 mm and 20 x 3 nun.
  • (e) Small Angles : 16 x 16 x 2 nun and 20 x 20 x 3 mm

The Process of Slitting

STAND 1 - OVAL

OVAL

Fig. 6.33 OVAL.

It is to be ensured that the metal should be filled equally on both sides and sharp enough to enter the small radii in the fluted square at stand 2.

STAND 2 - FLUTED SQUARE

Fluted Square

Fig. 6.34 Fluted Square.

It is vital, that this square should be equal, across both the flats end and of diagonals. The entering oval must not be lean.

Stand 3: Dogbone

Dog bone

Fig. 6 .35 Dog bone.

The dog bone pass must be filled completely, so that 4 roller entry guide should be set centrally on the pass with the front rollers being tight on the bar, and therefore it will facilitates the square to enter absolutely flat in slitting pass. Bum the sides to ensure that the pass should be completely filled, thus ensuring equal stock in each strand.

Stand 4: Slitting Pass

Slitting Pass (Entry)

Fig. 6.36 Slitting Pass (Entry).

The pass should have the minimum collar gap. It minimize the amount of steel holding the two bars together, aiding to the slitting. The 4 roller entry guide must be set centrally on the pass with the front rollers holding the dog bone.

To maintain equal stock in the strands, wood burning of the sides should be done.

Slitting Pass Guide (Delivery)

Fig. 6.37 Slitting Pass Guide (Delivery).

The slitting guide must be set centrally on the pass in both the vertical and axial plane, the slitting roller knife points should be touching and be directly above each other. The bearings must be tight, to prevent any axial play in the slitter guide rollers.

Stand 5: Leader Oval

Leader Oval

Fig. 6.38 Leader Oval.

Bum the sides of the each oval to ensure that they are filled equally and are not too sharp

Stand 6: Finishing Passes

Finishing Pass

Fig. 6.39 Finishing Pass.

Check both passes for the wear out of pass and steel grade brands.

Control of Loop Heights between Stands 4 and 5 and 5 and 6

  • (a) The loop seamier should be scanning down to control the highest loop and to help to prevent speed cobbles.
  • (b) The loop heights of the two strands should be the same; this will indicate that the two strands will have the same weight/mt.
  • (c) If there is a small difference between the loop heights of the two strands, then it requires to control the sides and to ensure equal fill with a hint of overfill.

Method of Slitting

In the conventional slit rolling process, a square is used to form the dog-bone, which is fed into the slitting pass to get two bars of equal cross sections. In the slitting pass the material is tom by the rolling forces into two equal parts. For conect tearing of the material in the slit pass, the settings of pass and guides are veiy important besides it requires an accurate pass design.

The square used for slitting has the following difficulties:

  • 1. Turning of the square by 45°, to ensure the con ect entry of the square into the dog bone pass.
  • 2. Balancing the material equally on both sides of the dog bone.
  • 3. Stability of the guides and rolls.
  • 4. Low pass and guide life.

A ty pical 12 mm slit rolling conventional slit roll pass design-Using leader square.

mm Slit Rolling Conventional Design Using Leader Square

Fig. 6.40 12 mm Slit Rolling Conventional Design Using Leader Square.

12 nun Slit Rolling-Using Leader Round

mm Slit Rolling Conventional Design Using Leader Round

Fig. 6.41 12 mm Slit Rolling Conventional Design Using Leader Round.

The advantage of using round as Leader to fonn Dog-bone

  • 1. Use of a round to fonn the Dog-bone would eliminate the twisting required in the case of a square.
  • 2. Better control of equal balancing of the material on both sides of the dog- bone.
  • 3. Maintains the stability of rolls and Guides.
  • 4. Pass and Roll life improves.
  • 3-Strand Slitting

4-Strand Slitting

Passes Used For 4 Strand Slit Rolling

Fig. 6.43 Passes Used For 4 Strand Slit Rolling.

Slitting and Slit Guide

Slitting of the material is done by the rolls, whereas the guide bars are supposed to act only to guide the individual strands.

However, in actual practice, it has been observed that the slit guide bars help in the tearing process and in addition to their main function to guide the individual strand.

12.6.1 Slitting Force-Slitting Roll Profile

Pass Design for Providing Slitting Roll Force

Fig. 6.44 Pass Design for Providing Slitting Roll Force.

12.6.2 Typical Slitting Guide

Slitting Guide

Fig. 6.45 Slitting Guide.

Slitting Guide Point of Contact of Slitter Rollers and Dog bone Points on Slitter Knives, Perfectly Aligned

Fig. 6.46 Slitting Guide Point of Contact of Slitter Rollers and Dog bone Points on Slitter Knives, Perfectly Aligned.

The Slitting delivery guide box consists of:

  • (д) A complex nose piece that is accurately cast to fit the dog bone pass.
  • (b) A pair of high grade steel slitting knife rollers.

The nose piece strips the dog bone from the pass and guides it to the center of the slitting rollers. The dog bone is not cutting into two rounds with a slicing or knifing action, but its thrust apart by an axial separating load exerted by the slitting rollers against the inside shoulders of the two rounds.

Therefore, it is important that:

  • (a) The pass should be square, which will be minimizing the material holding the two rounds together.
  • (b) The slitting rollers should be square and their bearings should be tightly fit. The cassette type guide fits into a guide holder to ensure ease and accuracy

of set up during guide changes.

Problems Faced During Slit Rolling and its Solutions

  • 12.7.1 Fails to Slit and Cobbles
  • 1. Change slitter box, check old one for:
    • (a) Worn slitter rollers,
    • (b) Worn or loose slitter box bearings,
    • (c) Scrap builds up in nose piece or slitter rollers.
  • 2. Worn out dog bone pass or passes.
  • 3. Entry guides on dog bones will ensure that the front rollers are tight on bar.
  • 12.7.2 Excessive Side Rib

To check the excessive side ribs, ensure the following:

  • 1. Size of oval against product guide sheet.
  • 2. Weight, size and collar gap on finished bar.
  • 12.7.3 No Side Rib

To have the no side rib, it is required to check the following:

  • 1. Size of oval against product guide sheet.
  • 2. Weight, size and collar gap on finished bar.
  • 3. Tension between stands.
  • 12.7.4 One Sided Side Rib

To check the one sided rib, ensure the following:

  • 1. Line up of the entry guides (finisher).
  • 2. Fill on the dog bone passes.
  • 12.7.5 Transverse Rib Under Tolerance

To get the transverse rib under tolerance, firstly it is required to determine that whether ribs are on both sides of the bar or only at one side.

  • (t) If It is on one side
  • (a) Oval is symmetrical in shape.
  • (b) Oval is entering at an angle due to loop height.
  • (Й) If it is at both sides of bar
  • (a) Size of oval,
  • (b) Oval is not too sharp or too blunt,
  • (c) Oval is leaning in the entry guide.

Mill Setting problem during Slitting

Rib Height below Tolerance on Both the Top and Bottom of The Bar

Reasons are:

  • 1. Oval is too sharp.
  • 2. Oval is too blunt.
  • 3. The oval is leaning on entry into the finishing pass.
  • 4. Width of the oval is too narrow causing insufficient dr aft.
  • 5. Collar gap on the finishing pass too large.

Rib Height Below Tolerance on Either the Top or Bottom of the Bar

Reasons are:

  • 1. Oval is non symmetrical due to unequal fill.
  • 2. The oval is not entering the finishing pass straight or flat due to the loop height, this will cause the oval to be pushed into the ribs on one roll and held away from the ribs on the other roll. It will also cause a poor delivery.

Oval With Unequal Fill Due to Misalignment of the Entry Guide

Small Kinks Every 1 to 1.2 m

These normally occur at the pass brand.

  • 1. Pass brand too deep.
  • 2. Grade brand too deep and situated in the bottom of the pass rather than in the side.
  • 3. The oval not entering the finishing pass straight and flat due to the looper being set too high.
  • 4. Delivery nozzle and tube too large over the finished bar size.

Lap

  • 1. Worn- out slitter pass.
  • 2. Dogborre slitter pass not square.
  • 3. Worn slitter rollers in guide box.
  • 4. Worn bearings in slitter rollers.
  • 5. Slitter rollers crossed.
  • 6. Overfill on one of the dogborre passes.

Variations in Filling Between Strands

If the different strands vary significantly in filling of metal or one of the strands is empty or excessively full. The following checks and adjustments should be immediately earned out:

  • 1. Observe, if the highest loop corresponds to the heaviest and/or fuller finished bar.
  • 2. Confirm that the fluted square is equal across both the diagonals and the flats.
  • 3. Wood bum the sides of both dog bones and confirm that the fuller side corresponds to the heaviest finished bar and the strand with the higher loop.
  • 4. Make very small axial adjustments to the entry guide on the first dog bone that exhibits these conditions and observe the change in looper heights and also confirming they are becoming more equal in height.
  • 5. Confirm that the dog bones are filled equally, and the finished rebar bars from all strands are within ± 1/2% on weight.
 
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