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 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

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 •

•

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 Cont'd • •

•

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 •

•

• •

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 • 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 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 •

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 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 • Straight or tapered threads are formed on round rods by passing the pipe though dies • Typical products include – Screws – Bolts

Thread Rolling Cont'd • 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 •

•

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

Continuous 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