metal rolling

Chapter 13

Rolling of Metals

This chapter describes Flat rolling Shape rollingProduction of seamless tubing & pipe

Rolling – process of reducing the thickness of a long work piece

Plates – having thickness greater than 6mmSheets – generally less than 6mm thick

Introduction

Flat Rolling ProcessFlat Rolling

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 = µ2RRoll Force• F= W0.L.Yavg

• L=" sqrt{R(ho-hf)}

Flat-Rolling PracticeFlat-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 & SheetsDefects 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 CharacteristicsOther 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 arrangementsSchematic 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 OperationsShape-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 RollingRing 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 ROLLINGRING 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 RollingThread 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'dThread 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 & MinimillsContinuous 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'dContinuous 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 ENDTHE END