Classification and characteristics of common defects in hot-rolled strip steel

1. Scarring

Defect characteristics:

M-shaped tongue-shaped defects similar to interlayers mostly appear on the edge of the strip and may appear on both the upper and lower surfaces.

CauseScarring of strip steel

Transverse cracks at the corners of the slab.

Precaution

Control the vibration parameters of the mould to avoid too deep vibration marks; the mold and the bending section are well-centred; the secondary cooling section is uniformly cooled; the billet straightening temperature avoids the brittle zone of the steel grade, and the general steel grade is ≥900°C.

2. Layering

Defect characteristics

There are unwelded gaps on the section of the steel strip, and sometimes there are inclusions visible to the naked eye in the gaps of the separation layer. Severe delamination causes partial splitting of the steel plate, and the parts of the delamination are irregular.

 

Layering of strip steel

Cause

There are internal cracks in the slab, the error of the opening of the nip rolls in the solidification terminal area of the secondary cooling zone is too large, or the rolls are bent and broken; the casting temperature is too high; the cooling at the solidification end is uneven.

Precaution

Strengthen the inspection and maintenance of the pinch rolls and spraying system at the end of the solidification of the slab to avoid excessive opening of the pinch rolls and roll bending and broken rolls; the secondary cooling should meet the requirements of the steel type and continuous casting process; avoid high temperature and slow drawing.

 

3. Longitudinal fracture

Longitudinal fracture of strip steel

Defect characteristics

Continuous or discontinuous cracks on the surface along the direction of rolling, which vary in length and depth.

Cause

Uneven cooling of the mold, excessive copper plate pits or scratches; inconsistent physical and chemical properties of mold slag, high superheat, low Mn/S; excessive fluctuations in mold steel liquid level and casting speed.

Precaution

The crystallizer structure is reasonable and the heat flux density is appropriate; the water joints of the copper plate of the mold have no scale, no rust, and no foreign matter; the depth of pits and scratches on the surface of the copper plate is less than 1mm; the casting speed is stable, and the fluctuation of the liquid steel level in the mold is ≤ ± 5mm; The performance of mold slag meets the process requirements, and the slag addition conforms to the principle of “less, frequent, and uniform”. The thickness of the mold liquid slag layer is 10-15mm; the taper of the mold matches the steel type; 160-190mm.

4. Descaling is not clean

Descaling is not clean, strip steel

Defect characteristics

Appears on one side or on the upper and lower surfaces of the strip along the entire width in the form of irregular blocks or spear points. This area is usually reddish after hot rolling, sometimes grainy and noticeably rougher than adjacent areas.

Cause

Does not descale well.

Precaution

Strengthen the control of descaling ability and adopt a reasonable heating process system.

5. Surface inclusions

Surface inclusions

Defect characteristics

Irregular, obviously punctate, blocky, and long strips, sometimes gradually disappearing in punctate or tongue-like form.

Cause

The inclusions under the slab skin are caused by violent extension and rupture during rolling.

Precaution

Strengthen the inspection and acceptance of slabs, do not use slabs with inclusions or slag on the surface; improve the purity of steel; stabilize continuous casting operations to avoid slag entrainment.

6. Cracked edge

Cracked edge

Defect characteristics

Different degrees of small notches in the edge of the strip.

Cause

Improper heating conditions such as too high a heating temperature, too long a heating time, partial burning phenomenon, etc. will cause such defects, and sometimes it may be related to too high a sulfur content.

Precaution

Adopt a reasonable heating process system to avoid excessive heating time and high temperature; ensure Mn/S≥15.

 

7. Edge cracks

 

Edge cracks

Defect characteristics

Most of the defects are on the edge, longitudinally distributed, and intermittent cracks.

Cause

There are small transverse cracks at the edge of the slab, and the edge of the strip after rolling is a small cross-peeling defect; it is caused by scratches or rollovers during the rolling process.

Precaution

Avoid overcooling of the edge to prevent transverse cracks on the edge; check the rolling line frequently to avoid scratches; ensure the surface quality of the vertical roll.

 

8. Pitting

Pitting

Defect characteristics

There are partial or continuous rough surfaces on the surface of the steel strip, which are orange peel-like in severe cases. It may appear on both the upper and lower surfaces, and the density is not uniform throughout the length of the strip.

Cause

When the temperature of the roll is relatively high, the oxide scale adheres to the roll, and pitting occurs on the surface of the pressed plate during rolling; if the material of the roll is poor or the temperature is high and the wear is serious, pitting may also appear on the surface of the plate during rolling.

Precaution

Keep the cooling water of the roll uniform, strengthen the inspection and control of the roll surface quality, and avoid the peeling off of the oxide film on the roll surface.

 

9. Scratch

Scratch

Defect characteristics

Along the strip direction, dark or light.

Cause

It is caused by friction between the rolled piece and the relative movement of the mechanical equipment components.

Precaution

Strengthen equipment inspection and maintenance to ensure the normal operation of the rolling line roller table and avoid dead rolls.

 

10. Tower shape

Tower shape

Defect characteristics

There are many inner rings at both ends of the steel coil, which are pagoda-shaped and bread-shaped.

Cause

Improper adjustment of the rolling mill causes sickle bends in the rolled strip or improper adjustment of the cooler.

Precaution

Reasonably configure the roll shape, strengthen the control and adjustment of the shape of the strip, and ensure that the strip enters the coiler straightly; strengthen the inspection and maintenance of the coiling equipment to ensure the stability of the coiling equipment.

 

11. Waves

 

Waves

Defect characteristics

The steel plate is not straight but presents undulating wavy bends along the rolling direction. According to the different distribution parts, it can be divided into unilateral waves, double-sided waves, middle waves and double-ribbed waves.

Cause

It is mainly caused by uneven deformation, and the part with a large extension appears wave-shaped. It is related to the roll shape of the roll and the distribution of the reduction.

Precaution

Reasonably match the roll shape, optimize the rolling process, and strengthen online adjustment.

12. Lost meat, sticky meat

 

Lost meat, sticky meat

Defect characteristics

The surface of the steel plate is dotted, and the periodic protrusions and pits in the form of flakes or strips are distributed over the entire length of the steel strip or a section of it.

Cause

Periodic bumps are caused by damage to work rolls or tension rolls; periodic pits are caused by bumps formed by work rolls or conveying rolls stuck to foreign matter.

Precaution

To improve rolling stability, adopt a reasonable roll shape configuration to avoid damage to the roll caused by flicking and the impact of the irregular shape of the head on the roll and the coiling tension roll, causing foreign matter to stick to the roll surface.

 

13. Loose roll

Loose roll

Defect characteristics

The strip coil is not tight, and there is a large gap or looseness between the coils.

Cause

The coiling tension is small; the coiling temperature is low; the strap is broken or the steel hardness is high.

Precaution

According to the characteristics of the steel grade, adjust the coiling tension; ensure the coiling temperature; and strengthen the strap for high-strength steel grades.

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