Pass Design of Blooming Mill
Barrel or Bullhead Pass
The minimum width of barrel pass is determined by maximum width of ingot and slab. Suppose, ingot width is 785 mm, then the minimum width of barrel pass should be 100 mm more than the maximum width of ingot. The width of barrel pass also depends upon the maximum size of slabs to be rolled. Keeping these into account, the minimum and maximum width of barrel pass is kept as 875 mm and 895 nun respectively as shown in Fig. 4.15, above. The maximum size of slab which can be rolled from this pass will be around 700 mm.
The depth of pass is determined by max lift and strength of the roll. The depth of pass should be made as low as possible, otherwise increases the rolling time, as roll has to travel more. The higher depth of barrel pass will affect the strength of roll, which is the most important criteria for deciding the draft. Angle of bite will also get increases with the depth of the pass. Generally depth of barrel pass varies from 0-200 mm. It is taken as 170 nun as shown in Fig. 6.12 above. The max lift of the top roll is 1120 mm. The radius of 30 mm is given at comers in the barrel pass.
Roughening of blooming mill rolls by ragging or knurling are made for increasing friction and may raise angle of bite upto 34°.
The width of first pass (Fig. 4.16) at the highest depth should be almost equal entry width of work piece to avoid twisting and wear out. Max width is determined by considering total spread of work piece in that pass after reversal passings that is why, if the width of work piece entering in pass is around 380 nun, then the minimum and maximum width of pass will be taken as 395 and 455 nun respectively, after considering total spread of work piece in that pass.
Fig. 4.16 First Pass of Blooming Mill.
The height of first pass is taken as 220 nun, which is higher than the barrel pass by 50 nun. It will not only provide better stability of work piece, but also with this depth, roll will have sufficient strength and also in him higher angle of bite that will ensure higher draught in this pass.
The convexity of first pass is kept to 5 nun, to give better stability of work piece on roll table and to reduce the chances of fin.
The radius of 25 nun is given at comers of finishing pass, taken into consideration of all product sizes to be rolled from this pass.
Second or Finishing Pass
The second or finishing pass (Fig. 4.17) width of 285 mm is chosen on the basis of the width of incoming work piece. The max width of 330 nun is selected on the basis of final cross section of bloom. The height of pass is almost kept as same of pass 1 i.e., 210 nun and convexity of pass is also taken as 5 nun.
Fig. 4.17 Second of Finishing Pass.
Fig. 4.18 Beam blank pass for Beam 450 mm.
The size of beam blank is determined by the input requirement of the sizes of beams to be rolled in subsequent structural rolling mills. The depth and flanges of beam blank are accordingly designed. In the Fig. 4.18 above, Beam blank pass for Beam 450 is shown.
Defects in Bloom Rolling
Defects due to Rolling
Generally following defects din ing rolling of blooms may be observed.
1. Skewed or diamonded blooms [Fig. 4.19(«)]
It is due to misalignment of the roll grooves and non-uniform heating of the ingot, the rolling in a pass wider than the entering bloom or when edging of a bar with a ratio of the sides in excess of 1.5 (especially for small cross sections).
Fig. 4.19 Defects due to Rolling.
2. Collaring [Fig. 4.19(Z>)]
It is mainly due to groove misalignment or bar rolled on collars.
- 3. Fins [Fig. 4.19(c)] on opposite sides as a result of deviations from the draughting schedule. It may also be due to insufficient inclination of the sides of the grooves of a box pass when several passes are made in the same hole only.
- 4. Twisted blooms [Fig. 4.19(c/)] are with the result of pass misalignment, non-parallel rolls, entering the bar on the collars and also, due to, non- uniform heating. It also depends upon the amount of pass wall inclination and amount of draught. It may be also caused by excessive draught, faulty setting of entry and delivery guides or lack of uniform temperature across billet cross section.
The twisting of blooms [Fig. 4.19(r/)] is prevented by properly performed roll turning and in particular, precise alignment of the grooves in the top and bottom rolls, by properly entry of the bar into the pass; by proper setting of the rolls in reference to each other and by uniform heating of the ingot. Besides this, the bar should never be rolled in excessively worn passes, and the rolling speed should be reduced, especially when the front end of the bar enters the rolls.
- 5. The reason for lopsidedness of bloom [Fig. 4.19(c)] is because of different clearances between the end collars of both sides of the rolls.
- 6. Bellying of bloom [Fig. 4.19(/)] is if the walls of the pass are excessively worn out, either due to higher reduction or metal is not hot enough for that
7. Under filling: This condition arises, when pass doesn’t get filled up completely. The metal from previous stand is to be enhanced.