roll pass design.pdf

8/9/2019 roll pass design.pdf

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ROLL PASS DESIGN

IN CONTINUOUSBAR MILLS

Department of Metallurgicaland Materials Engineering

INDIAN INSTITUTE OFTECHNOLOGYKHARAGPUR


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The basic five different cross-section

shapes used in roll pass design.


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Passes & Bars•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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The roll pass design for any product dependson the following:

• Starting size and Material Grade.• Mill layout.• Mill stand sizes.• Mill motor power.• Production Requirement.• Product size and shape.


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Typically a pass design calculation has threeparts:

• Pass design and groove details• Pass schedules.• Power calculation.


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Pass Design and Groove Details: This calculation

gives the following parameters for each pass:

Roll groove dimensions. Roll gap. Filled width in pass. Filled area. Area reduction.

Bite angle.


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Pass Schedules: Pass schedule consists of thefollowing for each pass:

Bar length Rolling speed

Rolling time Idle time Loop or tension value between stands


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Power Calculation :Power Calculation works out

for each pass:

Bar Temperature Rolling load Rolling torque Rolling power


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Throughout the mill•Continuous rolling process -the long axis of the bar isbrought between the rollsand is rolled into a shape

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

reduction must be appliedafter a 90-degree rotation ofthe bar at each stand.


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

•To be precise, there is onedeformation that needs specialtreatment: the square-into-oval. It needs rotating thesquare by 45°, which can beobtained (if we don't want to

use twister guides) with a slightaxial displacement of one rollin the stand that produces thesquare.


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The Mills•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 schematizedas a succession of passescentered on the z-axis (when x,y isthe plane containing the roll axes).


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Billet Size Area FinishedSize Area offinished bar

Co-efficient

ofelongation No. ofpasses

150 22500 12 113.1429 198.8636 20.78453

150 22500 16 201.1429 111.8608 18.52503

150 22500 20 314.2857 71.59091 16.77243

150 22500 22 380.2857 59.16604 16.02385

150 22500 25 491.0714 45.81818 15.01982

150 22500 28 616 36.52597 14.12972

150 22500 32 804.5714 27.9652 13.08094

150 22500 36 1018.286 22.09596 12.15586150 22500 40 1257.143 17.89773 11.32834

Number of passes required

No. of Passes= log of co-eff of elongation/log(1.29)


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•First LawThe purpose of the rolling process is to start from a

relatively 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 the volu m e (orthe weigh t) is a co ns tant : from a 1/2-ton billet youwill obtain a 1/2-ton coil. Cross sectional area timesbar length is a constant (this is not strictly true for

CBMs: some weight will be lost with scale and cropends; but we can afford to neglect that loss.)

Laws of Rolling


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•When rolling, we can identify one action and tworeactions.If we focus on a horizontal stand of a continuous mill for

rounds, we see:- that the rolls apply a 'reduction' (vertically);- that this reduction produces a wanted 'elongation';- that reduction produces a 'spread' (sideways).

Action & Reactions


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The Dimensions to be taken for aligningrolls and adjusting roll pass for Box

groove & flat oval groove


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•Square Dimensions A 90° square with sides 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 as90° for s > 45 mm, 91° down to 25 mm and 92° for s


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Oval Pass


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•Oval Radius• An oval pass is made of two circular arcs with facingconcavities. 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)

Important Formulae


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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) (4)

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

R=(maxw^2+H^2)/(4*H) (5)

Important Formulae


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Shape rolling of initial billet withinitial cross section 100x100 mm2to 30x30 mm2 consisting of

sequential passes of square-oval-


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Shape rolling of Cylindrical Bar


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