Rolling metalBangkit RH22/04/2014Rolling*

Rolling

Two High Rolling Mill*Rolling is a bulk deformation process in which the thickness of the work is reduced by compressive forces exerted by two opposing rolls. The rolls rotate to pull and simultaneously squeeze the work between them.

Introduction to Rolling

Two High Rolling Mill

*Introduction to RollingThe basic process shown in the previous figure is Flat Rolling, used to reduce the thickness of a rectangular cross section. A closely related process is shape rolling, in which a square cross section is formed into a shape such as an I-beam.

Shape Rolling

Flat Rolling Shape Rolling

Rolling*Introduction to RollingAfter casting, ingots are rolled into one of three intermediate shapes called blooms, billets, and slabs:

Blooms have square cross section 6 x 6 or larger. They are rolled into structural shapes.

Billets have square cross section 1.5 x 1.5 or larger. they are rolled into bars and rods.

Slabs have rectangular cross section 10 x 1.5 or larger. They are rolled into plates, sheets and strips.

Rolling

Rolling*As any other metal forming process, rolling can be performed hot (hot rolling) or cold (cold rolling).

Most rolling is carried out by hot rolling, owing to the large amount of deformation required.

Hot-rolled metal is generally free of residual stresses, and has isotropic properties. On the other hand, it does not have close dimensional tolerances, and the surface has a characteristic oxide scale. Moreover, cold rolled metals are stronger.

Introduction to Rolling

Rolling

*Types of RollingBased on work piece geometry : Flat rolling - used to reduce thickness of a rectangular cross section Shape rolling - square cross section is formed into a shape such as an Ibeam Based on work temperature : Hot Rolling most common due to the large amount of deformation required Cold rolling produces finished sheet and plate stock

*The RollsRotating rolls perform two main functions:

Pull the work into the gap between them by friction between work part and rolls.

Simultaneously squeeze the work to reduce its cross section.

Two High Rolling Mill*Roll configurations in rolling mills Two High Rolling Mill.

Two High Rolling Mill

*Roll configurations in rolling mills Three High Rolling Mill.

Two High Rolling Mill*Roll configurations in rolling mills Four High Rolling Mill.

Two High Rolling Mill

Two High Rolling Mill*Roll configurations in rolling mills Multiple backing rolls allow even smaller roll diameters Cluster Rolling Mill.

Two High Rolling Mill

Two High Rolling Mill*Roll configurations in rolling mills A series of rolling stands in sequence Tandem Rolling Mill.

Two High Rolling Mill

Two High Rolling Mill*Production steps in rolling

Two High Rolling Mill

Two High Rolling Mill*Flat Rolling StagesHot rolling of ingot or a continuously cast slab converts it to a wrought structure called a bloom (square) or slab (rectangular)Bloom may next go to shape rollingSlabs may be rolled into plates and sheet

Two High Rolling Mill

*Rolling analysisIn flat rolling, the work is squeezed between two rolls so that its thickness is reduced by an amount called the draft: d = to - tfwhered: draftto: starting thicknesstf : final thickness

As a fraction of the starting thickness:

% reduction = % r = (d/ to) * 100%

*Rolling analysisRolling increases work width. This is called spreading.

Spreading is expected because of the volume constancy in plastic deformation. Since the material is compressed in the thickness direction, both the length and width will increase provided that the material is not constrained in the width direction.

Spreading is more pronounced with low width-to-thickness ratios and low coefficients of friction, since there is small resistance to flow in the width direction.

*Rolling analysisThe width-to-thickness ratio can be calculated as follows:

w/t Ratio = initial width/ initial thickness

After rolling, percentage spread can be calculated as follows:

% Spread = (Final width-initial width)/ (initial width) *100%

*Rolling analysisTo calculate the roll force required to maintain separation between the two rolls:

F = 1.15 * Yavg, i * Li * wi

where:F : roll forceYavg, i : the average flow stress in the ith passLi : the approximate contact length in the ith passwi : the width of the sheet in the ith pass

*Rolling analysisThe torque in rolling can be estimated by:T = 0.5 * F * LWhere:T: Torque (lb.in or N.m)F: Roll ForceL: Contact length

The Power required to drive the two rolls is calculated as follows:P = 2*N*F*LWhere:P: Power (in J/s =Watt or in-lb/min)N: Rolls rotational speed (RPM)F: Roll ForceL: Contact length

*Rolling DefectsDefects in rolling may be either surface or structural defects:Surface defects include scale and roll marks.Structural defects (see next figure) include:Wavy edges: bending of the rolls causes the sheet to be thinner at the edges, which tend to elongate more. Since the edges are restricted by the material at the center, they tend to wrinkle and form wavy edges.2. Center and edge cracks: caused by low material ductility and barreling of the edges.3. Alligatoring: results from inhomogeneous deformation or defects in the original cast ingots.Other defects may includes residual stresses (in some cases residual stresses are desirable).

*Defects in RollingSurface defects scale, rust, scratches, gouges, pits, and cracksWavy edges due to roll bendingAlligatoring complex phenomenon that may be due to non-uniform deformation or defects in the billetFigure 13.8 Schematic illustration of typical defects in flat rolling: (a) wavy edges; (b) zipper cracks in the center of the strip; (c) edge cracks; and (d) alligatoring.

*Rolled Metal CharacteristicsResidual stressesDimensional tolerances for cold-rolled sheet thicknesses +/- 0.1 mm to 0.35 mm (0.004 to 0.014 inch)Flatness tolerances to within +/- 15 mm/m (3/16 inch/foot) for cold rolling, +/- 55 mm/m (5/8 inch/foot) for hot rollingHot rolling and sand casting produce similar ranges for surface finishCold rolling produces a very fine surface finishGage number identifies standard thicknesses of sheet (the smaller the number, the thicker the sheet)

*A rolling mill for hot flat rolling. The steel plate is seen as the glowing strip in lower left corner (photo courtesy of Bethlehem Steel).

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