TOPICS Rolling Types of Rolling Flate Rolling Shapes rolling Ring rolling Rolling mill
Rolling (metalworking) Rolling…… process of reducing the thickness of a long work piece. OR The process of plastically deforming metal by passing it between the rolls is known as rolling.
• Rolling….. Is widely used to convert steel ingots into blooms, billets, and slabs, and subsequently into plates, sheets and strips. Rods
Advantages: Provides high throughput. Provides good control over the dimensions of the finished product.
Rolling Process Rolls transfer energy to the strip through friction. As the strip is dragged by the rolls into the gap between them, it decreases in thickness while passing from the entrance to the exit. Meanwhile its speed gradually increases from the entrance to the exit. V0 = input velocity Vf = final or output velocity R = roll radius hb = back height hf = output or final thickness
Classification of Rolling Process Hot rolling Cold rolling
Hot rolling Hot working ---- above the re-crystallization temperature. It is employed where large reduction in cross- sectional area is required. Used for to produce bars, rods, rails etc.
Advantages: Machanical strength is increase. Porosity is reduce.Dis-Advantages: Poor surface finish. Loss of precise dimension.
Cold rolling Cold working ---- below the re- crystallization temperature. Employed for finishing the metal to given specification of sizes and surface quality. Produces sheets, strips and foils.
Advantages: with good surface finish increased mechanical strength.
Re-crystallization Re-crystallization is a process by which deformed grains are replaced by a new set of undeformed grains. Re-crystallization temperature can be defined as the temperature at which destroyed grains of a crystal structure are replaced by the new strain grains
→ Solvent added (clear) to compound (orange) → Solventheated to give saturated compound solution (orange) →Saturated compound solution (orange) allowed to cool over timeto give crystals (orange) and a saturated solution (pale-orange
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
Flat Rolling 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 defectsProduct of the first hot-rolling operation - Bloom or slab Square cross section of 150mm (6in) on one side Processed further by shape rolling I-beams Railroad rails
Flat-Rolling 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. 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, Pits, & Cracks May be caused by impurities and inclusions Wavy edges – result of roll bending
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 finish 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 Rollarrangements
Shape-Rolling Structural shape rolling, also known as shape rolling and profile rolling. It is a metal forming process where structural shapes are passed through rollers to bend or deform the workpiece to a desired shape while maintaining a constant cross-section. Structural shapes that can be rolled include: 1) I-beams 2) H-beams 3) T-beams 4) U-beams 5) channels 6) railroad rails 7) pipes
Stages in Shape Rolling of an H-section part. Various otherstructural sections such as channels and I-beams, are rolled bythis kind of process.
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
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 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 ring-rolling
ROLLING MILLS: A rolling mill consists basically of rolls, bearings, a housing for containing these parts, and a drive for applying power to the rolls and controlling there speeds. Rolling mills can be conventionally classified with respect to the number and arrangement of the rolls.Classification of rolling mills: i) Two-high mills ii) Tow-high reversing mills iii) Three-high mills iv) Four-high mills v) Cluster mills vi) Continuous mills vii) Planetary mills
Schematic Illustration of various roll arrangements : (a) two-high; (b) three-high; (c) four-high; (d) cluster mill
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’tIntegrated Mills utilize everything from the production of hot metal to the casting and rolling of the finished productMinimills 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
ROLLED PRODUCTSi) Bloom: First breakdown product of ingots with a minimum cross-sectional area of 36 in2. Generally, square in cross-scetion.ii) Billet: Further reduction of bloom by rolling results in a billet. Cross-sectional area 1.5 in. X 1.5 in.iii) Slab: A hot rolled ingot with a width to thickness ratio of 2:1 at least. Cross-sectional area over 16 sq. in.**Blooms, billets and slabs are known as semi-finished products because they are subsequently formed into other mill products.
iv) Plate: It has thickness greater than ¼ in.v) Sheet and Strip: Generally have thickness lesser than ¼ in.vi) Foil: Thickness is very small just as 0.002 in.* Billets and other unfinished rollings are further rolled to I-sections, T-sections, Angles, Channels, Girders etc.