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7/30/2019 presentation2-128039794841-phpapp02

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Presented by

ANSAR HUSSAIN RIZVI

Manager (Production)

Roll Pass Designing in Continuous Bar Mill


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When we talk about a stand, we are actually referringto a pair of rolls.

Rolls are supported by bearings located in the

chocks. The chocks slide within the housing and can be

opened or closed by turning the screws.

Two facing grooves form a roll pass, or simply a pass..

The distance between the barrels of two rolls is calledthe nominal roll gap, or theoretical roll gap.



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Definite passes thosehaving two equal axes inan x, y plane (Squares,Rounds)

Intermediate passes those having one axislarger than the other one(Rectangles box,Diamonds, Ovals)



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A definite bar into oneintermediate pass, or anintermediate bar into onedefinite pass configures adeformation. For example, asquare into an oval pass, or anoval into a square pass. Adeformation can produce anytype of bar

A definite bar into two passes(an intermediate pass

followed by a definite pass,configures a sequence. Asequence only produces adefinite bar.



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Continuous rolling process- the long axis of the bar isbrought between the rollsand is rolled in to a shape

with equal axes, then thisshape is rolled into adifferent shape withdifferent axes, and so on.

The reduction must beapplied after a 90-degreerotation of the bar at eachstand.



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Traditional mills only usehorizontal stands. The ovalsare twisted to bring the longaxis between the rolls.

To be precise, there is onedeformation that needsspecial treatment: the square-into-oval. It needs rotatingthe square by 45, which canbe obtained (if we don't want

to use twister guides) with aslight axial displacement ofone roll in the stand thatproduces the square.



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Structures and schematizations Continuous bar mill (CBM)

structure consists of a number ofindependent stands. 'Independent'means that each stand has its ownmotor (and kinematic chain),

whose rotational speed can be

freely altered. If you don't want thebar to be twisted you use the HVmill configuration (with definitepasses in vertical stands).

From the roll pass design point ofview, a CBM can be schematized asa succession of passes centered onthe z-axis (when x,y is the planecontaining the roll axes).



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Billet Size Area Finished Size Area of finished bar Co-efficient ofelongation No. of passes150 22500 12 113.1429 198.8636 20.78453150 22500 16 201.1429 111.8608 18.52503150 22500 20 314.2857 71.59091 16.77243150 22500 22 380.2857 59.16604 16.02385150 22500 25 491.0714 45.81818 15.01982150 22500 28 616 36.52597 14.12972150 22500 32 804.5714 27.9652 13.08094

150

22500

36

1018.286

22.09596

12.15586

150 22500 40 1257.143 17.89773 11.32834



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A CBM can have either an even or an odd number of standsand, like a Russian matryoshka, it contains three distinctmills:

- the roughing mill - the stretching mill - the finishing mill

These three mills are roughly identified by three groups ofrolls: from furnace down, these groups show decreasingbarrel diameters, increasing surface hardness anddecreasing yield strength - core materials going from steelto 'steel base' to cast iron.



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First LawThe purpose of the rolling process is to start from arelatively short bar with a large section area, aiming toobtain a very long product with a small section area.

Then, the first law to remember is that thevolume (orthe weight) is a constant: from a 1/2-ton billet you willobtain a 1/2-ton coil. Cross sectional area times barlength is a constant (this is not strictly true for CBMs:

some weight will be lost with scale and crop ends; butwe can afford to neglect that loss.)



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Second LawThere is another, important law to remember: the flowis also a constant. Say that the exit bar from stand 1has cross sectional area = 3467 sq mm and the finished

round has cross-sectional area = 113 sq mm (hot bardimensions). If the finished stand delivers at a speed of12 mps, then stand 1 must 'run' at 0.39 mps: 0.3 x 3467 =12 x 113. In this case the constant is about 1050, i.e., if you

know the areas, you can immediately calculate the exitspeeds. And, you have no problems in setting the speedat each stand, as each stand has its own independentmotor.



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When rolling, we can identify one action and tworeactions.

If we focus on a horizontal stand of a continuous mill forrounds, we see:

- that the rolls apply a 'reduction' (vertically);

- that this reduction produces a wanted 'elongation';

- that reduction produces a wanted 'spread' (sideways).



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When the steel is compressed in the rolls it will obviouslymove in the direction of least resistance, so usually there isnot only longitudinal flow but also some lateral flow. Thisis called Spread. it is generally accepted that beyond aratio width/height = 5, spread becomes negligible.

Dh the absolute draught in the passho stock thickness before the pass

R roll radius;f coefficient of frictionThe coefficient of Spread, Beta is the ratio between exit andentry width and is normally > 1

Db=1.15 X Dh2ho

(R XDh- Dh)2f



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1.Reduction (with a coefficient of reduction Gamma)

2.Elongation (with a coefficient of elongation Lambda).

Gamma (defined as ratio between exit and entry height) isalways < 1. If we reduce a 100x10 flat to 8 mm (a 20%reduction), Gamma=0.8.

Lambda (defined as ratio between exit and entry length, but

more often as ratio between entry and exit section area) isalways > 1. In the example above (100x10 reduced to 100x8)Lambda = 1000/800 = 1.25. Note that Beta = 1. (100/100 =1)



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The rolling process can be synthesized by oneequation: Beta x Gamma x Lambda = 1. Which is amathematical way of saying that the volume is aconstant



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Hot & Cold Dimensions Dimensions in mm refer to COLD bars. Corresponding

hot bars have slightly larger dimensions due to thecoefficient of linear expansion of steel, assumed to be

1.013.



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Square DimensionsA 90 square with side s and corner radius r has area:

A=s^2-0.86*r^2 (1)

and actual 'reduced' diagonal:

d=s* (2)-0.83*r (2)Note: Square grooves generally have facing angles alpha = 90 only

for larger squares. Generally, facing angle alpha is taken as 90

for s > 45 mm, 91 down to 25 mm and 92 for s


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Oval Radius An oval pass is made of two circular arcs with facing

concavities. Three dimensions are considered, referringeither to pass or to bar:

i. b1t = theoretical oval width (pass, not physicallymeasurable)

ii. b1r = actual oval width (bar, physically measurable)

iii. maxw = maximum oval width (pass, physicallymeasurable)

If b1r becomes > maxw , pass overfilled will beoverfilled.



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Oval RadiusTo identify oval height, we only need two dimensions:

i. h1t = theoretical oval height (pass, physically measurable)ii. h1r = actual oval height (bar, physically measurable)

To draw the oval groove we need to know its radius R. Theformula is:

R=(b1t^2+h1t^2)/(4*h1t) [Italian notation] (4)

Now, when gap=0 we have b1t=maxw. This means that if the ovalis identified as maxw x h1t, we can put H=h1t-gap andcalculate

R=(maxw^2+H^2)/(4*H) [German notation] (5)



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1. Definite passes are those having two equal axes in an x, y plane.2. In a square-into-oval deformation, the bar needs to be turned at 90

degrees.3. A CBM contains three distinct mills.4. In continuous bar rolling, the volume remains constant but the flow

varies.5. Increase in the width of the bar is called elongation.

6. Hot size of the bar is normally taken as 1.013 times the cold size.7. Squares and rounds are intermediate passes.8. Two facing grooves form a roll pass.9.

A sequence only produces definite passes.10. In a continuous bar mill, it is not necessary that the reduction mustbe applied after a 90-degree rotation of the bar at each stand.



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1. Definite passes are those having two equal axes in an x, y plane.(True)

2. In a square-into-oval deformation, the bar needs to be turned at 90degrees. (False)

3. A CBM contains three distinct mills. (True)4.

In continuous bar rolling, the volume remains constant but the flowvaries. (False)5. Increase in the width of the bar is called elongation. (False)6. Hot size of the bar is normally taken as 1.013 times the cold size.

(True)

7. Squares and rounds are intermediate passes. (False)

8. Two facing grooves form a roll pass. (True)9. A sequence only produces definite passes. (True)10. In a continuous bar mill, it is not necessary that the reduction must

be applied after a 90-degree rotation of the bar at each stand.


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