Tech Matters

TECH MATTERSbailey Metal Processing Limited - Products



Coil Breaks

by Esther Mar | Sep 28, 2017
What are Coil Breaks and why are they objectionable?
  • Coil Breaks are creases which appear as lines transverse to the rolling direction and generally extend across the width of the steel.
  • If a coil is sheeted and used in flat form the condition is considered unsightly.
  • Coil Breaks are purely cosmetic.They are non-injurious and will not affect formability.Coil Breaks will not affect the integrity of formed parts.The presence of Coil Breaks is a clear indication of very formable and ductile material.

What causes Coil Breaks?

  • Coil Breaks are caused by the phenomenon of “yield point elongation” or “elongation at the yield point” which is inherent in low carbon steel.
  • When a tension test specimen is subjected to load there is an initial range of loading in which no permanent deformation occurs, i.e. if the load is removed at any value within this range the specimen will return completely to its original dimensions (elastic range).
  • As the tensile load on the specimen is increased through the elastic range a stress will be reached at which the specimen will begin to deform in a plastic manner, i.e. it will undergo a permanent set which is not recoverable upon release of the load.
  • Some materials, with increasing stress, show a gradual departure from elastic behaviour.Many steels, however, exhibit an abrupt yielding and show an increase in strain without any appreciable increase of stress when yielding occurs.Such materials are said to have a yield point.The amount of extension which occurs between the initial yield point and the point at which the load begins to rise steadily again is called the “yield point elongation”.
  • When hot rolled coils are uncoiled for further processing, such as pickling, slitting, cutting to length or temper rolling there are strains associated with the uncoiling.Localized yielding occurs at the point of unbending when the strain exceeds the yield point of the material.This is what Coil Breaks are.The strain is a function of the ratio of the thickness of the material to the diameter of the coil and it is greatest at the wraps closest to the inside diameter and so Coil Breaks are most prevalent there.
  • Since the mechanism of formation of Coil Breaks is dependent on the yield point it is clear that materials with higher yield strength (e.g. HSLA) are far less susceptible to Coil Breaks.
  • Other factors that affect coil breaks are:
    • Tension
    • Shape
    • Length of time allowed for cooling after hot rolling (longer is better)
  • Coil Breaks will further occur at other points of bending, such as roller leveling.Roller leveler breaks are said to be controlled Coil Breaks in that they are caused by bending the sheet over regularly spaced rolls.

     

    Eliminating Coil Breaks

  • Modification to chemistry with Boron to change the characteristics of yielding behaviour can minimize Coil Breaks.

  • Skin rolling or temper rolling can be used to alleviate coil breaks in two ways.A small cold reduction is applied to create numerous nucleating points for Coil Breaks that are so small so as not to be obvious.In addition, pre-existing Coil Breaks are flattened and masked.
  • In order to guarantee freedom from Coil Breaks on hot rolled steel the product must be skin rolled or temper rolled.
  • Coil Breaks are not usually seen on fully processed cold rolled sheet because the processing includes temper rolling after annealing.Similarly coated products are either tension leveled or temper rolled in line.

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