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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.

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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
slide3

Flat Rolling

Flat Rolling Process

flat rolling process
Flat Rolling Process
  • Metal strip enters the roll gap
  • The strip is reduced in size by the metal rolls
  • The velocity of the strip is increased the metal strip is reduced in size
  • Factors affecting Rolling Process
    • Frictional Forces
    • Roll Force and Power Requirement
frictional forces
Frictional Forces

Friction Forces acting on strip forces

  • Max Draft
  • h0-hf = µ2R

Roll Force

  • F= W0.L.Yavg
  • L=̃ sqrt{R(ho-hf)}
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
  • 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 con t
Flat-Rolling Practice Con’t
  • Billets – smaller than blooms and rolled into bars and rods
  • Cold rolling
    • carried out at room temperature
    • Produces sheet and strip metal
    • Better surface finish – less scale
  • Pack rolling – when two or more layers of metal are rolled together

Changes in grain structure during hot-rolling

defects in rolled plates sheets
Defects in Rolled Plates & Sheets
  • Undesirable
    • Degrade surface appearance
    • Adversely affect the strength
  • Sheet metal defects include:
    • Scale, Rust, Scratches, Gouges, Pits, & Cracks
    • May be caused by impurities and inclusions
  • Wavy edges – result of roll bending
  • Alligatoring – complex phenomenon
other characteristics
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
slide11
Schematic Illustration of various roll arrangements : (a) two-high; (b) three-high; (c) four-high; (d) cluster mill
shape rolling operations
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
slide13

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
  • 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 rolling1
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 ring-rolling

thread rolling
Thread Rolling
  • Cold-forming process
  • Straight or tapered threads are formed on round rods by passing the pipe though dies
  • Typical products include
    • Screws
    • Bolts
thread rolling con t
Thread Rolling Con’t
  • Threads are rolled in the soft condition
  • Threads may then be heat treated, and subjected to final machining or grinding
  • Uncommon or special-purpose threads are machined
production of seamless pipe tubing
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
  • Tensile stresses develop at the center of the bar when it is subjected to compressive forces
continuous casting integrated mills minimills
Continuous Casting & Integrated Mills & Minimills
  • Continuous casting
    • Advantages
      • Highly automated
      • Reduces product cost
      • Companies are converting over to this type of casting
continuous casting integrated mills minimills con t
Continuous Casting & Integrated Mills & Minimills Con’t

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
slide21
Spray Casting : In spray casting the molten metal is sprayed over a rotating mandrel to produce seamless tubing and pipe
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