defects in hot rolled products1

1. Defects in Hot Rolled Products

Rolled in Scale (RIS)

Sliver / Seams

Blister

Edge Cracks

Spongy Edge

Rolled in Extraneous Material

Roll Marks

Indentation Mark

Scratch Mark

Folds

Lamination

Camber

Flatness

Ovality

BCL

TCC

Zig Zag Wrapping

Bulging

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2. Rolled in Scale (RIS)

Different types of RIS

Primary Scales due to Poor Descaling

Primary Scales due to High Temperature

Secondary Scales due to Poor Descaling

Secondary Scales (Drag / Comet type) from Coil Box.

Tertiary Scales due to Roll Roughness ( Salt & Pepper / Banded type).

Silicon / Red Scales (Tiger Scale)

Steps to control RIS

Cleaning off loose scales from slab surface Avoiding overheating of slab surface Drop out temp. Edging Descaling impression test Use of scale master nozzle & use of optimum height between

the strip and the nozzle tip. Coil box by-pass Oxidizing atmosphere Descaling pressure Roll cooling and roll bite lubrication Proper scheduling of surface critical grades. Damage of surface from apron plate. Use of interstand descaler

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Primary Scales due to Poor Descaling

Scale (Iron Oxides) generated at various Hot rolling Stages get rolled into the Strip Surface due to different reasons.

Prone Grades: Any grade

Location: Scattered

Appearance: Black-Grey

Origin / Root Cause

• Improper descaling / failure of Primary descaler.• Nozzle clogging of Primary descaler.• Sticky Scale (low O2, High Temperature & High Retention

Time).

Remedy / Process Control

• Maintain high Descaler Pressure ( >180 Bar).• Take Impression / Correction of Nozzle Impression.• Maintain Oxidizing atmosphere (>2%), low temperature

(1200°C) & low retention time 2 ½ to 3 hrs.).

Primary Scales due to High Temperature

Prone grades: Grades processed with high RMET-IFHS

Location: Scattered, in portions where high temp was experienced by slab

Appearance: ‘M’ shaped

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Origin / Root Cause

• High Furnace / Roughing Mill Exit Temperature


• Low Roughing Mill Exit Temperature (<1080°C)

Secondary Scales due to Poor Descaling

Origin / Root Cause

• Improper descaling / failure of Secondary descaler.• Nozzle clogging.• Low pressure of descalling water.


• Maintain high Descaler Pressure ( >180 Bar).• Take Impression / Correction of Nozzle Impression.

Secondary Scales (Drag / Comet type) from Coil Box

Prone grades : IF / SPH 440

Location: TE (1/4th) of HR coil, more on bottom (HR)

Origin / Root Cause

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• Ovality of Coil in Coil Box.• Un-flatness of Tr. Bar.• Worn out rolls in Coil Box.


• Maintain Roughing Mill Exit Temperature.• Coil (Tr. Bar) Tightness.• Use of leveler.• Frequent check / Replacement of Worn out Rolls.

Tertiary Scales due to Roll Roughness( Salt & Pepper / Banded type).

Prone grades: Thinner sections/high strength

Location: Banded, throughout width

Appearance: Small, spotted type

Origin / Root Cause

• High Temperature due to high Mill pacing.• Roll Cooling Water Quantity. • Amount of RBL.• Rolling Loads due to Adverse P Q S.• Scheduling Rules (Wear KM).


• Optimization of Roll Cooling Water.• Optimization of RBL.• Right product mix.• Schedule Length w.r.t. Wear KM

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Silicon / Red Scales (Tiger Scale)

Prone grades: Grades having Silicon (>=0.20%), Cu , Cr (>=0.40%)

Location: Scattered, both surfaces

Appearance: Elongated, patchy nature

Origin / Root Cause

• Presence of Si, Cu, Cr type Elements leading to Stickiness. • Soaking Temperature. • Retention Time.


• Increase Descaling Pressure (>180 Bar).• All Pass Descaling in Roughing Mill.• Use of ISD in Finishing Mill.

3. Sliver / Seams

Defect Description:

Irregular flaky overlapping material with non-metallic inclusions or scales under it. It is found along the rolling direction and can appear on both surfaces. Sometime also called seams, shell.

Possible causes:

Slag or nonmetallic material entrapment during casting or rolling.

Transverse crack in slab.

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Types of Sliver

Body Sliver / Seams

Edge Line Slivers

Body Line Slivers

Body Sliver / Seams

Sub Surface Defects in Slabs, Like Non-metallic Inclusions, Mould Powder entrapment, Gaseous inclusions etc. which open up in subsequent Hot Rolling / Cold Rolling Stage.

Origin / Root Cause

• Severe mould oscillations.• Flushing Operation.• Speed Variation.• Mould level fluctuation.

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• Control over above Parameters

Edge Line Slivers

Transverse cracks on the slab edges lead to these defect during Hot Rolling.

Origin / Root Cause

• Quenching effect of the slab edges in sec. zone.• Poor ductility.• Deep oscillation marks at the edges


• Reduce water in secondary cooling zone.• Control mould level fluctuation.

Body Line Slivers

Star / longitudinal cracks on slabs (body) leading to Sliver on Hot Roll Strips.

Origin / Root Cause

• Varying mould flux (Heat extraction).• Mould powder quality (viscosity).• High casting speed.


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• Control of mould water.• Use of high viscosity mould powder.• Slower casting speeds.

4. Blister

Sub Surface blow holes of varying size and shape located closely, mainly due to argon entrapment during casting, leading to Blisters in HR / CR after rolling.

Origin / Root Cause

• Excess argon purging. • Mould level fluctuation• Rapid solidification near the mold wall causes gas entrapment.


• Control above parameters.

5. Edge Crack

There are cracks observed on the edges of HR coil due to excess quenching effect.

Origin / Root Cause

• Excess water in FM / ROT / DC cooling the strip edges / body• Un-flatness leading to water carryover.


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• Avoid cooling water in FM, ROT (cross spray) and Pinch roll water in DC.

• Roll flat strip / remove water on ROT

6. Spongy Edge

There are cracks / pin holes on the edges due to improper ductility

Origin / Root Cause

• Pin holes on slab edges.• Deep oscillation marks.• Poor Mn / S ratio.


• Avoid pin holes / deep oscillation marks.• Maintain Mn / S >12

7. Rolled in Extraneous material

Bolt, Mould powder, etc., get rolled in the strip.

Origin / Root Cause

• Bolts falling on the slab / Tr. Bar.• Mould powder on the slab.• Roll material after spalling.• Burrs on the slabs.


• Proper maintenance practices.

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• Proper casting parameters / slab quality.• Better roll quality & Profile.• Maintain operational practices.

8. Roll Marks

Type of roll marks

Lump

Sticker

V-Shape

Fire-Crack

Zebra Pattern Mark

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Lump

Roll Spalling or portion of roll surface chipping out giving rise to Lump marks at regular interval.This interval is depending on roll diameter.

Origin / Root Cause

• Material of roll clipping off due to tail chewing.• Extraneous material denting the rolls.


• Avoid Tail Chewing.• Proper slab Quality w.r.t. Burrs.• Sufficient cropping of Tr. Bar.• Follow up remedies as suggest for Rolled in extraneous

material.

Sticker

This defect is observed when small pieces of scrap sticks on the roll surface generating dent marks (replica of the scrap) at regular interval depending on the roll diameter.

Origin / Root Cause

• Sticking of extraneous material (Burrs) on rolls


• Avoid Tail Chewing.• Proper slab Quality w.r.t. Burrs.

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• Sufficient cropping of Tr. Bar.• Follow up remedies as suggested for Rolled in extraneous

material.

V-Shape

Cold head mark of the Tr. Bar on the roll surface.

Origin / Root Cause

• Cold head end of Tr. Bar.


• Sufficient cropping.• Maintain high temperature at head end.

Fire-Crack

These are the transfer marks from the cracks generated on the finishing roll surface due to thermal shocks. Appears on the regular intervals.

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Origin / Root Cause

• Thermal Shock on the rolls due to the strip remaining in contact with the roll bite for sometime particularly during cobbles.


• Avoid cobbles in FM.• Proper dressing of rolls.

Zebra Pattern Mark

This is a mark which is generated on the strip surface due to undulation of roll surface.

Origin / Root Cause

• Improper grinding of the rolls.

Remedy / Process Control• Skill of the grinder.• Proper finish of the roll surface.

Indentation Marks

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This is a Dent mark which is observed at the Head End of the strip approx. 20-30 meters.

Origin / Root Cause

• Tongue shape of the strip head end.• Low temperature of the tongue.• Tension of the strip while coiling.• High temperature of the strip (excluding tongue).


• Head end un-cooled to keep tongue hot.• Reduce tension (maintaining coil tightness) to avoid excess

pressure.• Reduce CT to avoid sticking of wraps.

9. Scratches / Grooves

Line marks on the strip surface which is generated from stationary parts.

Origin / Root Cause

• Mechanical abrasion of strip with stationery part.


• Strip should not touch stationery parts.• All (table / ROT etc.) to be in rotation.

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10. Folds

Different Types of folds are:

Edge Fold

Origin / Root Cause

• Camber in strip (mainly HE / TE) while coiling.• DC side guide pressure


• Control wedge / camber at RM.• Optimize / maintain side guide pressure.•

Body Fold

Origin / Root Cause

• Loop in strip before coiling


• Tension between F6 / Pinch roll / down coiler to be maintained.

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11. Lamination

Defect in sheets and plates, which appear in a local area in layers and are generally caused due to segregation

Origin / Root Cause

• Segregation in slab (mainly centre portion).


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• Low ‘S’ content (<0.005). • Low super heat (<30°C). • Proper machine health. • Soft reduction after casting.

12. Camber

C & S Type

This occurs due to difference in heat concentration between the different parts of the profile or faulty setting of rolls. It is measured by the minimum deviation of the middle region from the straight line joining the corresponding points on the two boundary sides or edges.

Origin / Root Cause

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• Transfer Bar Wedge.• Transfer Bar Camber.• Servo valve response.


• Control Wedge in Tr. Bar <0.15 mm.• Control Camber <10 mm.• Servo valve nil offset <12%.

13. Flatness

Types of flatness defects

Edge WavinessCentre Buckle

Origin / Root Cause

• Improper Swiveling / Bending.• Improper CVC setup.

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• Servo value Response.• Faulty Pressure transducer.


• Control Swiveling / Bending.• Provide un-flatness correction.• Maintain basic conditions of equipments.

14. Ovality

The Inner diameter of coil is not same in all direction and it was of a oval shape and having a minor & major axis. Such a defect is known as ovality. Origin / Root Cause

• Sagging of coils after taking out of Down Coilers. It happens due to heat dissipations after coiling and on conveyor.

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• Control of flatness.• Control of CT to complete transformation on ROT itself.• Tension in DC.

15. BCL

Type:a) Loose Coiling.

a) Wrap Shifting.Origin / Root Cause

• Loose wraps after coiling.• Protruding outer wraps (like TCC) after coiling.

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• Control of tail end tension during coiling.• Proper coiling gear selection.• Control Camber.• Control Cradle roll speed / alignment.

16. TCC

Origin / Root Cause

• Protruding inner wraps after coiling.


• Control Camber.• Control DC guide opening.• Avoid abrasion / control while pulling out coil from mandrel.

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17. Zig Zag Wraping

Origin / Root Cause

• Shifting of wraps on either side (O/S & D/S) while coiling.


• Use of correct. Hot Y.P valves.• Correct strip tension values.• Higher CT.

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• Proper Product Section / grade.

18. Bulging

Origin / Root Cause

• Belly shaped coiling due to Camber from RM, low strip tension in down coiler.


• Control Camber in RM.• Maintains proper tension.• No dia fluctuation of mandrel.

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19. Some Practical Examples Of HR Products Defect

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Longitudinal line defect.(BSK- 46)

Presence of bainite in the microstructure and subsequent higher hardness resulting from faster rate of cooling at the edge of coil, caused edge cracking.

Surface rupture (E-38)

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Above observation indicates that the "rupture marks" were caused by some mechanical abrasion in cold condition as revealed by grain deformation along the defect.

Cracking through weld zone (K3)

Prominent longitudinal crack was observed just outside the weld joint of the tube.Above observations indicate that the crack outside weld joint was caused by MnS inclusions in steel.

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Multiple cracking at the edge of Disc (FE-330)

Multiple cracks occurred at the edge of the Wheel Disc after forming.

Cracking during 90 0 bending (Gr.B)

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Feedback: IS 2062 Grade B, 12mm thick HR coil supplied to M/S International Auto Ltd., was cracking at 900 bend with a radius of 18, while making Air Bracket for TELCO.

Gas cut sample from Air Bracket shows multiple cracks at the 900

bend. The sheared face shows cracks extending up to mid thickness zone of the strip. While one large crack occurred exactly on the bend, other cracks appeared slightly away from the bend. Cavity like defect was also observed on the sheared face, close to mid thickness zone.

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HOLE

HOLE DUE TO SLIVER (TMBP-2)

EDGE CRACK (RIM)

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DEPRESSION MARK (LPG)

DEPRESSION MARK (LPG)

PITTING MARK / SCRATCH (LPG)

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Line Defect (Gr A)

The defect was in theform of deep continuousline on the right side of figabove. Multiple linedefects were alsoobserved on the left sideof fig 1.

Close up of surfacedefect as shown on theright side of Fig 1.

Close up of surfacedefect as shown on theleft side of Fig 1.

Fig.1:

Fig.2:

Fig.3:

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defects in hot rolled products1

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