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 1
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
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Appearance: ‘M’ shaped Origin / Root Cause • High Furnace / Roughing Mill Exit Temperature Remedy / Process Control • 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. Remedy / Process Control • 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) 4
Origin / Root Cause • Ovality of Coil in Coil Box. • Un-flatness of Tr. Bar. • Worn out rolls in Coil Box. Remedy / Process Control • • • •
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).
Remedy / Process Control • Optimization of Roll Cooling Water. • Optimization of RBL.
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• Right product mix. • Schedule Length w.r.t. Wear KM
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. Remedy / Process Control • 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:
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Slag or nonmetallic material entrapment during casting or rolling. • Transverse crack in slab. •
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. 7
• Speed Variation. • Mould level fluctuation. Remedy / Process Control • 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 Remedy / Process Control • 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). 8
• Mould powder quality (viscosity). • High casting speed. Remedy / Process Control • 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. Remedy / Process Control • Control above parameters.
5. Edge Crack There are cracks observed on the edges of HR coil due to excess quenching effect. Origin / Root Cause
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• Excess water in FM / ROT / DC cooling the strip edges / body • Un-flatness leading to water carryover. Remedy / Process Control • 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. Remedy / Process Control • 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. 10
Remedy / Process Control • • • •
Proper maintenance practices. 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. Remedy / Process Control • • • •
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
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Remedy / Process Control • • • •
Avoid Tail Chewing. Proper slab Quality w.r.t. Burrs. 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. Remedy / Process Control • Sufficient cropping. • Maintain high temperature at head end.
Fire-Crack 13
These are the transfer marks from the cracks generated on the finishing roll surface due to thermal shocks. Appears on the regular intervals. Origin / Root Cause • Thermal Shock on the rolls due to the strip remaining in contact with the roll bite for sometime particularly during cobbles. Remedy / Process Control • 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.
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Indentation Marks 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).
Remedy / Process Control • 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. Remedy / Process Control • 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 Remedy / Process Control • Control wedge / camber at RM. • Optimize / maintain side guide pressure. •
Body Fold Origin / Root Cause • Loop in strip before coiling Remedy / Process Control • 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). Remedy / Process Control 17
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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. Remedy / Process Control • Control Wedge in Tr. Bar <0.15 mm. • Control Camber <10 mm. • Servo valve nil offset <12%.
13. Flatness Types of flatness defects Edge Waviness Centre Buckle Origin / Root Cause • Improper Swiveling / Bending. • Improper CVC setup. 19
• Servo value Response. • Faulty Pressure transducer. Remedy / Process Control • 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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Remedy / Process Control • 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. 21
Remedy / Process Control • • • •
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. Remedy / Process Control • Control Camber. • Control DC guide opening. • Avoid abrasion / control while pulling out coil from mandrel. 22
17. Zig Zag Wraping Origin / Root Cause • Shifting of wraps on either side (O/S & D/S) while coiling. Remedy / Process Control • Use of correct. Hot Y.P valves. • Correct strip tension values. • Higher CT. 23
• Proper Product Section / grade.
18. Bulging Origin / Root Cause •
Belly shaped coiling due to Camber from RM, low strip tension in down coiler.
Remedy / Process Control • Control Camber in RM. • Maintains proper tension. • No dia fluctuation of mandrel. 24
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 900 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) 31
CENTRE LINE SEGREGATION
The defect was in the form of deep continuous line on the right side of fig above. Multiple line defects were also observed on the left side of fig 1.
Line
Close up of surface defect as shown on the right side of Fig 1.
Close up of surface defect as shown on the left side of Fig 1.
HIGHERSEGREGATIO
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