Mill Setting and Section Control

When setting of Mill for Flats, special attention should be paid to avoid the distortion of metal in the edging passes. Twisting of metal around its axis, general bending of metal along the entire length and bending of front and back ends from all directions should be strictly avoided. This distortion of section in the edging and box passes may be due to the following reasons :

  • 4.1. Under filling of the pass due to insufficient thickness of the bar fed. As a result of this, the flats will be twisted and may get cobble in other stands. Due to this, we will get finished section with diagonal difference.
  • 4.2. Overfilling of the pass on account of excessive thickness of the section fed. This may happen due to excess reduction in the edging pass or feeding of more metal from previous pass. In this case, twisted and distorted section will come out from next stand.
  • 4.3. Non-coincidence of the pass due to axial deviation of rolls, will also produce metal in twisted and distorted condition.
  • (?) The thickness of the bar fed into the box pass should be such that it should be equal to the root of the pass with the deviation of ±2 mm. (//) Overfilling of the edging pass should not be allowed.
  • (iii) hi case of twisting of section on the plain barrel on account of shifting of the rolls in the previous edger. The previous stand roll should be shifted in the axial direction towards the twisting of metal as observed after plain barrel.

When rolling thin flats, entry and delivery rolling tackles with worn out working surfaces should not be used.

  • 4.4. While rolling thin and wide flats, care should be taken to see that roll tackles and the roll passes of intermediate, pre-finishing and finishing stands should not be excessively worn out. One of the signs of excessive worn out pass is that the delivery of flat is not straight, and it will have periodical looping in the vertical plane.
  • 4.5. When thin flats are rolled, the gaps at the ends of roll barrels (collar gaps) should be set with minimum difference. If rolling tackles are fixed correctly (both entry and delivery), bending in different directions can be rectified by pressing the top roll on the side towards which the piece got bent or by pressing the previous stand top roll from the side opposite to bending. Pressing of previous stand is allowed only in case the dimensions of thickness of the bent flats which are uniform along the entire width.
  • 4.6. When thick and narrow flats are rolled from same roll set, special attention should be paid to the twisting of bars around their axis and difference of diagonals and roughness of the surface of the edger stand. Twisting from vertical stand or roll cross in vertical stand gives rise to diagonal difference.

This twisting due to under-filling of the edging pass, because of insufficient thickness of the incoming bar or due to roll cross causes under filling of the opposite diagonals of flats.

  • 4.7. Trapezoidal flats (the top of flat is wider than the bottom or vice-versa) may occur due to the following two reasons:
    • (/') Due to considerable wear out of the roll with plain barrel (upto finishing stand). The spread will be then unequal and will cause production of trapezoid flats.
    • (//') Due to excessive reduction in the edging pass, the piece will get bend up or down.

Inspection and checking of dimensions of flat sections

5.1. When inspecting thin flats on the cooling bed, the width of flats should be checked atleast at three places i.e., on the front, tail and also at the middle of the workpiece.

hi case, variation of dimensions from the end to the middle is observed, then it is necessary to remove the tension of metal in the continuous group of stands.s

  • 5.2. Quality of the edges of flats should be inspected next. Periodic dents on the edges of flats indicate the sticking of metal in the edging passes.
  • 5.3. Scabs on the surface located uniformly also indicate defects of the same nature. In this case rolling should be immediately stopped and necessary steps should be taken.

Defects and its Rectifications

Round Edges (convex)

Convex sides

Fig. 7.11 Convex sides.

This is due to less reduction in edging pass and too much reduction in finishing pass and barrel passes.

Concave Edges

Concave sides

Fig. 7.12 Concave sides.

This is due to insufficient draught in finishing pass and too much draught in edging pass and sometimes due to use of wrong edging pass also.

Dished

Dished

Fig. 7.13 Dished.

This may also due to insufficient draughting in the final edging and finishing

pass.

Dished and Convexed

Dished and Convexed

Fig. 7.14 Dished and Convexed.

This may also be due to too much draught in final edging pass and not adhering to the draughting practices in other stand as per schedule given by roll pass designer.

Wedge

Wedge

Fig. 7.15 Wedge.

Wedge is caused by either tension in the mill

  • • If edging pass too wide
  • • Entry guide not lined up with the edging pass,
  • • Not using a roller guide infront side of edging pass,
  • • Loop height is too high infront of edging stand.

Opposite Diagonals Rounded

This is due to roll cross of vertical stand, twisting of bar from vertical stand.

Rhomboid

Rhomboid

Fig. 7.16 Rhomboid.

Rhomboid is due to if the edging passes are not square or setting of loose roller entry guide on edging pass or there is too much tension in the mill.

Sticker Mark – Due to Sticking of Metal on the Edger

Wear out Mark on Top and Bottom Edges– Due to Worn out Flat Pass or Edger.

Lap– due to Fin from edger.

Lap

Fig. 7.17 Lap.

Diagonal Difference

The main reasons for the diagonal difference is either due to twisting from vertical stand or may be due to roll cross of vertical stand when twisting is not able to be detected.

 
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