Top 10 Common Defects in Hot Rolled Coils

Hot-rolled coils are widely used metal materials that play a crucial role in industries such as construction, machinery, automotive, shipbuilding, and petrochemicals. However, inevitable issues arise during the practical operation processes.

01 Roll Mark Defect

Defect Characteristics: A group of concave-convex defects with periodicity, similar size, and irregular appearance.

Causes:

  1. Fatigue or insufficient hardness of the rolls, leading to partial loss of material and concave edges.
  2. Presence of foreign substances on the roll surface, causing protrusions on the surface.
  3. Formation of concave-convex defects during steel rolling or finishing processing.
Roll Mark Defect

02 Surface Inclusions

Defect Characteristics: Irregular point, block, or stripe-shaped non-metallic inclusions on the steel plate surface, generally appearing in red-brown, yellow-brown, gray-white, or gray-black colors.

Causes:

  1. Subsurface inclusions in the slab exposed after rolling or pre-existing surface inclusions on the slab remaining on the steel plate surface.
  2. Non-metallic materials such as refractory materials and mud falling onto the slab surface during heating, getting pressed into the surface during rolling.
Surface Inclusions

03 Oxide Scale

Defect Characteristics: Oxide scale adheres to the steel plate surface, distributed locally or entirely, appearing in black or red-brown; some scales are loose and fall off easily, while others are pressed into the surface to varying depths, presenting different shapes like red, block, stripe, line, wood grain, shooting star, spindle, trailing, and scattered scales.

Causes:

  1. Generation of oxide scale depends on the heating conditions of the slab; prolonged heating time, higher temperature, and a stronger oxidizing atmosphere result in more oxide scale that is difficult to remove during rolling, getting pressed into the steel plate surface.
  2. Improper setting of large work rolls, insufficient squeezing of scales, making it challenging to remove them.
  3. Insufficient water pressure in the high-pressure descaling water pipe, water nozzle blockage, incorrect water nozzle angle, or improper usage, causing incomplete removal of scales on the steel plate surface, which then get pressed in during rolling.
  4. Oxide scales occur more in boiling steel and are more likely to form red scales in steels with higher silicon content.
Oxide Scale

04 Thickness Variation

Defect Characteristics: Inconsistent thickness of different parts of the steel plate, termed as thickness variation. Uneven thickness often indicates deviations exceeding the allowable tolerance in certain areas.

Causes:

  1. Improper adjustment of roll gaps and roll configurations.
  2. Uneven wear of roll bearings on both sides of the rolls.
  3. Non-uniform heating of the slab.
Pits

05 Pits

Defect Characteristics: Local or continuous pits on the steel plate surface, varying in size and depth.

Causes:

  1. Severe oxidation of the slab during heating, with pressed iron scales forming small pits after falling off.

Hot-rolled coils face more issues in practical operations than those mentioned above, including bubbles, tower shape (edge shifting), folding (imprint, wrinkles, edges, corners), loose coils, and flat coils.

06 Bubbles

Defect Characteristics: Circular protrusions on the steel plate surface, sometimes resembling straight-line patterns like earthworms; the outer edge is relatively smooth, and gas is present inside. After bubble rupture, irregular fine cracks appear; some bubbles do not protrude, and after leveling, the surface is bright, showing a layered cross-section.

Causes:

  1. Presence of gas bubble-type defects on the slab, not healed after multiple rolling processes, remaining on the steel plate.
  2. Prolonged exposure of the slab in the furnace results in bubble exposure.

07 Tower Shape (Edge Shifting)

Defect Characteristics: The upper and lower ends of the steel coil are uneven, forming a tower shape. Edge shifting between upper and lower coils is termed edge shifting.

Tower Shape (Edge Shifting)

Causes:

  1. Improper adjustment of the coiler roll gaps.
  2. The wedge-shaped gap between the feeding rolls.
  3. Poor centering when the strip enters the coiler.
  4. Inappropriate coiling tension setting.
  5. Improper adjustment of the gap in the forming guide plate.
  6. Unsynchronized action timing of the side guide plate before the coiler.
  7. Gap between the coiler drum and the recoiler.
  8. Severe wear at the driving end of the coiler drum, causing a significant centrifugal difference during rotation.
  9. Large knife bending or poor plate shape in the coiled state.

08 Folding (Imprint, Wrinkles, Edges, Corners)

Defect Characteristics: Local double-layer metal folds on the steel plate surface, resembling cracks with varying depths and generally forming sharp angles on the cross-section. Longitudinal folds are called straight folds, and transverse folds are called cross folds. Folding on the edge is called edge folding.

Causes:

  1. Scratching of the rolled piece, leading to folding during rolling, often appearing on the lower surface of the steel plate.
  2. Excessive extrusion by the vertical roll, gnawing the lower surface of the rolled piece.
  3. Excessive removal of defects during slab cleaning.
  4. Non-uniform slab temperature or improper roll configuration and unreasonable distribution of rolling load during rolling, causing large waves in the middle of the strip, which are then pressed together.
  5. Severe squeezing of the vertical roll or severe scratching of the rolled piece, combined with improper coiler tension, can lead to secondary squeezing during rolling.
  6. Severe wear of the side guide plate in front of the coiler, resulting in grooves, small openings, wedge-shaped feeding roll gaps, and easy deviation of the strip, causing it to be pressed by the feeding roll into the groove.
  7. Failure to coil in time, causing the coiling temperature to be too low or the coiling speed to be improperly set.
  8. Edge shifting of the steel coil or loosening of the coil, especially during lifting and lowering with a crane, can easily cause edge folding and corner folding, often occurring in thinner steel coils.
Loose Coil

09 Loose Coil

Defect Characteristics: The steel coil is not tightly wound, and there are gaps between layers.

Causes:

  1. Improper setting of coiling tension.
  2. Severe waves in the strip or deformation of the strip on the roller path due to coiling faults.
  3. High yield strength of the steel, combined with excessively low coiling temperature.
  4. After coiling, the coiler drum reverses due to a malfunction.
  5. Insufficient tightening of the strapping or breakage of the strapping.

10 Flat Coil

Defect Characteristics: The end of the steel coil is elliptical, termed as a flat coil. It often occurs in softer or thinner steel coils.

Causes:

  1. The steel coil experiences a significant impact during lifting.
  2. Over-tightening of the coil, high temperature, or placing the coil flat on the ground, with additional stacking of steel coils on top.
Flat Coil

These defects in hot-rolled coils may vary in severity and occurrence but collectively contribute to challenges in the production and application of these metal materials. Addressing these issues requires careful process control, maintenance, and adherence to quality standards throughout the manufacturing process. Contact us for more information.

Quick Contact

Mail Me

sales@shangangsteelsupply.com

WhatsApp

+86 153 1871 5360

If any steel product meet your demand, please share with me material grade or other requirement.