Metal Rolling
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Transcript of 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