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Rolling

 

Process description

Continuous forming of metal between a set of rotating rolls whose shape or height is adjusted incrementally to produce desired section through imposing high pressures for plastic deformation. It is the process of reducing thickness, increasing length without increasing the width markedly. Can be performed with the material at a high temperature (hot) or initially at ambient temperature (cold).

Materials

  • Most ductile metals such as low carbon, alloy and stainless steels, aluminum, copper and magnesium alloys.
  • Metal ingots called blooms, slabs or billets, used to load the mill. Blooms are used to produce structural sections (beams, channels, rail sections), slabs are used to produce flat products such as sheets and plate, and billets are rolled into rods and bars using shaped rolls.
  • Continuous casting also used for higher efficiency and lower cost.
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Process variations

  • Variety of roll combinations exist (called mills):

    • Two high: commonly used for hot rolling of plate and flat product, either reversing or non-reversing type.
    • Two high with vertical rolls: commonly used for hot rolling of structural sections. Vertical rolls maintain uniform deformation of section and prevent cracking.
    • Three high: for reversing one length above the other simultaneously.
    • Four high (tandem): backing rolls give more support to the rolls in contact with product for initial reduction of ingots.
  • Cluster mills: very low roll deflection obtained due to many supporting rolls above the driven rolls that are in contact with product. For cold rolling thin sheets and foil to close dimensional tolerances.
  • Leveling rolls: used to improve flatness of strip product after main rolling operations.
  • Flat rolling: for long continuous lengths (long discontinuous lengths in reality) of flat product. The height between the rolls is adjusted lower on each reversing cycle, or the product is passed through a series of tandem rollers with decreasing roller gap and increasing speed, to reduce the product to its final thickness. Tandem roll system has higher production rates.
  • Shape rolling: billet is passed through a series of shaped grooves on same roll or a set of rolls in order to gradually form the final shape. Typically used for structural sections.
  • Transverse or cross rolling: wedge shaped forms in a pair of rolls create the final shape on shortcropped bars in one revolution. For parts with axial symmetry such as spanners.
  • Ring rolling: an internal roller (idler) and external roller (driven) impart pressure on to the thickness of a doughnut-shaped metal preform. As the thickness decreases, the diameter increases. For creating seamless rings used for pressure vessels, jet engine parts and bearing races. Rectangular cross sections and contours are also possible. Can be readily automated.
  • Pack rolling: operation where two or more layers of metal are rolled together.
  • Thread rolling: wire or rod is passed between two flat plates, one moving and the other stationary, with a thread form engraved on surfaces. Used to produce threaded fasteners with excellent strength and surface integrity at high production rates and no waste.
  • Roll forming: forming of long lengths of sheet metal into complex profiles using a series of rolls (see 3.9). . Calandering: thermoplastic raw material is passed between a series of heated rollers in order to produce sheet product.

Economic considerations

  • Production rates high. Continuous process with speeds ranging 20–500 m/min.
  • Production rates for related processes: transverse rolling up to 100/h and thread rolling up to 30 000/h.
  • Lead times typically months due to number of mills required and complexity of profile.
  • Long set-up times for shaped rolls.
  • Hot rolling requires less energy than cold rolling.
  • Material utilization very good (rolling is a constant volume process). Less than 1 per cent scrap generated, commonly through line stoppages or when cutting to lengths. Can be recycled.
  • High degree of automation possible.
  • Plane rolls flexible in the range of flat products they can produce. Shaped rolls dedicated and therefore not flexible . Economical for very high production runs. Minimum quantity 50 000m of rolled product (equivalent to 100 000+).
  • Tooling costs high.
  • Equipment costs high.
  • Direct labor costs low to moderate.
  • Finishing costs very low.

Typical applications

  • Rolling is an important process for producing the stock material for many other processes, e.g. machining, cold forming and sheet metal work. Around 90 per cent of all stock product used is produced by rolling for many industries:

    • Flat, square, rectangular and polygon sections
    • Structural sections, e.g. I-beams, H-beams, T-sections, channels, rails, angles and plate
    • Strip, foil and sheet
  • Sheet for shipbuilding
  • Structural fabrication
  • Sheet metal for shearing and forming operations
  • Tube forming
  • Automotive trim

Design aspects

  • Simple shapes using flat rolling, fairly complex 2-dimensional profiles using shape rolling and 3-dimensional shapes for transverse rolling.
  • Re-entrant angles possible on profile.
  • No draft angles required, except in transverse rolling.
  • Hot rolling: Minimum section =1.6 mm. Maximum section =1m.
  • Cold rolling: Minimum section =0.0025 mm. Maximum section =200 mm.
  • Maximum width =5m.

Quality issues

  • Coarse grain structure and porosity of hot ingot or continuous casting is gradually improved and finer grain structure produced with little or no voids.
  • Hot rolling takes place above recrystallization temperature, and therefore sections are free from residual stresses. No working hardening of material.
  • Anisotropy in cold-rolled sections are due to directionality of grains during rolling and work hardening. Can be used to advantage, but does mean high compressive residual stresses that exist in surface are balanced by high tensile residual stresses in section bulk. Can lead to surface delamination.
  • High sulfur contents in steels can cause cracking and flaring of rolled section ends. Possibility of jamming when introduced to a subsequent set of rolls. High scrap rates and downtime can be experienced if this occurs.
  • Hot-rolled material is more difficult to handle than cold rolled. Cold-rolled strip product can be coiled for subsequent processing, hot rolled cannot.
  • Rough surface finish of rolls is used in hot rolling to aid traction of metal through the rolls. Cold rolling rolls have a high surface finish.
  • Lubrication can be used for ferrous alloys (graphite) and non-ferrous alloys (oil emulsion) to minimize friction during rolling.
  • Cold rolling can be performed with low viscosity lubricants such as paraffin or oil emulsion.
  • Hot rolling requires the preparation of stock material to remove surface oxides before processing.
  • Maintenance of rolling temperature dictates quality. Too low and becomes difficult to deform. Too high and surface quality is reduced.
  • Roll material must be highly wear resistant. Made to withstand 5 000 000m of rolled section production. Can be re-coated and ground back to size.
  • Surface defects may result from inclusions and impurities in the material (scale, rust, dirt, roll marks, and other causes related to prior treatment of ingots).
  • Surface detail is poor in hot-rolled product (oxide layer called mill scale is always present). Oxide layer can be removed by pickling in acid.
  • Surface detail is excellent for cold rolling.
  • Surface roughness values ranging 6.3–50 µm Ra for hot rolling, 0.2–6.3µm Ra for cold rolling.
  • Process capability charts showing the achievable dimensional tolerances for cold rolling various materials are provided .
  • Achievable tolerances ranging ±1–±2.5 per cent of the dimension for hot rolling. Dimensional variations are greater than cold rolling due to non-uniformities in material properties such as hardness, roll deflection and surface conditions.

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