Metal Rolling

Rolling of Metals

Chapter 13

Introduction  This chapter describes Flat rolling Shape rolling Production of seamless tubing & pipe

 Rolling – process of reducing the thickness of a

long work piece  Plates – having thickness greater than 6mm  Sheets – generally less than 6mm thick 

Flat Rolling

 Flat Rolling Process

 Flat Rolling Process •

Metal strip enters the roll gap

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The strip is reduced in size by the metal rolls

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The velocity of the strip is increased the metal strip is reduced in size

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Factors affecting Rolling Process  –  Frictional Forces  –  Roll Force and Power Requirement

Frictional Forces

Friction Forces acting on strip forces • Max Draft 

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h0-h f = µ2R

 Roll Force •  F= W 0.L.Y avg 

•  L=̃ sqrt{R(ho-h )}  f 

 Flat-Rolling Practice  Flat-Rolling Practice •

Hot rolling  –  The initial break down of an ingot  –  Continuously cast slab  –  Structure may be brittle  –  Converts the cast structure to a wrought structure • Finer grains • Enhanced ductility  –  Reduction in defects

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Continuous Casting   –  Is replacing traditional methods  –  Faster & better 

Product of the first hot-rolling operation - Bloom or slab  –  Square cross section of 150mm (6in) on one side  –  Processed father by shape rolling • I-beams • Railroad rails

Flat-Rolling Practice Cont'd •

Billets – smaller than blooms and rolled into bars and rods

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Cold rolling  –  carried out at room temperature  –  Produces sheet and strip metal  –  Better surface finish – less scale

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Pack rolling – when two or more layers of metal are rolled together  Changes in grain structure during hot-rolling

 Def  ects in Rolled Plates & Sheets  Defects •

Undesirable  –  Degrade surface appearance  –  Adversely affect the strength

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Sheet metal defects include:  –  Scale, Rust, Scratches, Gouges, Pits, & Cracks  –  May be caused by impurities and inclusions

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Wavy edges – result of roll bending

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Alligatoring – complex phenomenon

Other Characteristics • Residual stresses – produces:  –  Compressive residual stresses on the surfaces  –  Tensile stresses in the middle • Tolerances  –  Cold-rolled sheets: (+/- ) 0.1mm – 0.35mm  –  Tolerances much greater for hot-rolled plates • Surface roughness  –  Cold rolling can produce a very fine finish  –  Hot rolling & sand have the same range of surface finish • Gauge numbers – the thickness of a sheet is identified by a gauge number 

Schematic Illustration of Various of Various Roll arrangements

Schematic Illustration of various roll arrangements : (a) two-high; (b) three-high; (c) four-high; (d) cluster mill

Shape-Rolling Operations • Various shapes can be produced by shape rolling  –  Bars  –  Channels  –  I-beams  –  Railroad rails • Roll-pass design requires considerable experience in order  to avoid external and internal defects

Stages in Shape Rolling of an H-section part . Various other structural sections such as channels and I-beams, are rolled by this kind of process.

 Ring Rolling   Ring Rolling  •

A thick ring is expanded into a large diameter ring  –  The ring is placed between the two rolls  –  One of which is driven  –  The thickness is reduced by bringing the rolls together 

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The ring shaped blank my be produced by:\  –  Cutting from plate  –  Piercing  –  Cutting from a thick walled pipe

Various shapes can be produced by shaped rolls • Typical applications of ring rolling:  –  Large rings for rockets  –  Gearwheel rims  –  Ball-bearing and roller-bearing races • Can be carried out at room temperature • Has short production time • Close dimensional tolerances

 RING ROLLING

(a) Schematic illustration of  Ring-rolling operation. Thickness reduction results in an increase in the part diameter. (b) Examples of cross-sections that can be formed by ringrolling

Thread Rolling  •

Cold-forming process

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Straight or tapered threads are formed on round rods by passing the pipe though dies

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Typical products include  –  Screws  –  Bolts

Thread Rolling C  ont'd  Cont'd  •

Threads are rolled in the soft condition

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Threads may then be heat treated, and subjected to final machining or  grinding

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Uncommon or special-purpose threads are machined

 Production of Seamless Pipe & Tubing  •

Rotary tube piercing (Mannesmann process)  –  Hot-working process  –  Produces long thick-walled seamless pipe  –  Carried out by using an arrangement of rotating rolls

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Tensile stresses develop at the center of the bar when it is subjected to compressive forces

Continuous Casting & Integrated Mills & Minimills •

Continuous casting  –  Advantages • Highly automated • Reduces product cost • Companies are converting over to this type of casting

Continuous Casting & Casting & Integrated Mills & Minimills Cont'd   Integrated Mills utilize everything from the production of hot metal to the casting and rolling of the finished product  Minimills

 –  Scrap metal is melted  –  Cast continuously  –  Rolled directly into specific lines of products  –  Each minimill produces one kind of rolled product • Rod • Bar  • Structural steel

Spray Casting : In spray casting the molten metal is sprayed over a rotating mandrel to produce seamless tubing and pipe

 THE END