Galvalume Defect- Causes & Remedie

Defect in the 55% Al-Zn Alloy Coated Steel strip have been classified as Process Defect and Material Defect. Any defect is indicated by a four-letter code. The first letter signifies the stage i.e. Material M or Process P. The next three letters indicate the type of defect as per standard predefined codes.

Example – Material Defect

For a 55% Al-Zn Alloy Coated Steel coil diverted for material defect of scratch line, the defect is indicated as MSCR.

Example – Process Defect

For a 55% Al-Zn Alloy Coated Steel coil diverted for Process defect of dent mark, the defect is indicated as PDTM.

3.1 Material Defect

3.1.1 Emulsion Patch – MELP

Description

Emulsion Patch can vary in colour from black, gray black through to brown black. It can occur as matted stripes of varying width intermittently or continuously throughout a coil or as oval shaped/tear drops in a background of dark mottled (black and brown black) Emulsion Patch.               

It can vary from light to heavy.                                                                                       

Cause

Emulsion Patch is caused by rolling coolant (containing iron particles) which is not completely removed from the strip by the cold mill coolant

blow-off system. The finely divided iron will attract moisture leading to the characteristic pattern. Emulsion Patchis a result of incorrectly adjusted

blowers or too much coolant on the strip.                   

Remedies

Improve wiping system at rolling mill. Maintain wipe presser of Min 0.6 Kg/cm2.

3.1.2 Roll Mark  –          MRLM

Description

Roll Marks are easily distinguishable because they always appear as repetitive narrow elongated areas of various width, shapes, patterns and severity anywhere on the strip and they always run at various angles to the rolling direction throughout a coil.

Cause                                                                           

Damage to the cold mill work-roll surface as a result of the strip buckling and folding on itself which allows more than one thickness of the strip to pass through the work rolls as a result of:

a. Unequal screw down position

b. Poor strip shape

c. Strip track off

d. Power failure

Remedies

          Work Roll change.

3.1.3 Cold Mill Scratches Defect –     MSCR

Description

Cold-Mill Scratchescan vary in severity and appear as shiny intermittent or continuous longitudinal lines (in the rolling direction) on one or both surfaces and can be continuous throughout a coil.                                                                     

Cause                                                                           

Cold Mill Scratchesare caused by any contact of the cold-rolled strip with any non-moveable surface such as seized rolls.

Remedies

Checked all rolls.

3.1.4 Rolled in Scale Defect – MSLR

Description

Rolled-in-Scale can occur on one or both surfaces but is not as “deep rooted” as Primary Scale and occurs as random dark elongated areas or continuous bands after hot rolling. Rolled-in-Scale may not always be removed by pickling depending  on the severity.

            Cause

Rolled-in-Scale is rapidly and continuously formed on the hot steel surface between the roughing mill and the entry end of the Hot Strip Mill. Ineffective removal of this scale by high pressure water sprays in the descaling box at the entry end of the hot strip finishing              

 mill will result in scale called Secondary Scale, being rolled into the strip surface.

3.1.5 Edge Burst Defect–          MEBT

Description

Edge Burst is characterised by ragged, or otherwise disfigured strip edge increasing in severity as the degree of cold reduction increases. It can be continuous, in patches or throughout a coil on one or both edges of the strip.

                                                                                                                       

Cause

Poor side trimming practice during HR slitting. For example worn blades, poor initial set up or if a chip develops in the side- trimming blade producing a notch effect, forming potential site for edge crack formation

and propagation.

3.1.6 Edge Cut–   MECT

Description

Edge Cutself explanatory. It normally occurs on either one or both edges and mainly on light thickness material. It can vary in size and severity and can occur at intermittent intervals, in patches, or throughout a coil.

                                                                                                  

Cause

Poor side trimming at the HR Slitting line or severely serrated edges from the hot strip mill not completely side trimmed off, can cause Edge Cut as the degree of cold reduction increases.

3.1.7 Sliver-              MSIR

Description

Sliver can occur on one or both surfaces and generally appear as a thin relatively short and narrow tapered longitudinal (in the rolling direction) layer or flap of metal rolled into the surface with one end of the defect fused or welded to the base metal. The severity,                                  

frequency, size and distribution varies widely and shows no repetitive pattern.

Cause

In Continuously Cast Slabs,Slivers can originate from mechanical damage (gouges) to                              

the slab surface where material build up proud of the surface, is rolled flat onto the surface during hot rolling.

                                                                               

3.1.8 Ridging-      MRDG

Description

Ridging is similar in from to a very localised quarter buckle and is characterised by very short wave lengths of less than strip (less than 120mm) and, a high wave 150mm, a narrow buckle width across the height relative to its wave length and width running continuously                 

and parallel to the rolling direction. Ridging can occur anywhere across the strip surface, can vary in severity and can be continuous throughout a coil. There can be one or more ridges in a coil, more commonly one.

Cause

Where a rolling mill rolls one part of the strip more than another, the extra length produced must go somewhere. Ridging is a result of:

a. Uneven Cold mill recoil tension across the strip width

b. Worn coil mill work rolls

c. Localised distortion of the strip surface such as a small crease on the leading or front end of the strip as it enters the recoil mandrel on the Cold mill.

d. Localised heavy metal anywhere on the surface of hot dipped metallic coated strip.

3.1.9 Coil Break-  MCBK

Description

Coil Break is characterised by creases in the strip surface and appear as parallel irregular lines which are transverse to the rolling direction. They can vary in the length but more often extend the full width of the strip, the distance between each crease can vary.                                                                 

Cause

Coil Break is caused by localised or bending (fluting phenomenon) across the width of the strip, which can occur during uncoiling or as the strip passes over any small radius deflector or tension roll.

3.2 Surface Defects

3.2.1Knife Air Lines–     PKAL

Description                                                          High Coating

Air Linesare long lines up to 10mm Wide of different spangle pattern to that of the surrounding Zinc coating. These lines often appear at 45 degrees to the rolling direction towards the strip edge.                     

Cause                                                                           

  Low Coating

The cooling air above the jet stripping Controls the rate of spangle formation. This cooling air is controlled to chill the molten Zinc coating to a normal pattern. At times, and at certain cooling rates some spangles grow along a thermal gradient in a characteristic pattern in a                

line at 45 degrees to the rolling direction.

Remedies

Bring Knife Closer Maintain Knife Pressure.

3.2.2 Beaver Tail Mark –         PBTM

Description

Beaver Tail Markis a surface scrape mark characterised by narrow shinysmooth continuous band of low coating mass approximately 3-10 mm wide and up to15 mm in from one or both edges top or bottom surface flanked by heavy coating. The heavy metal can vary in severity and in the                  

worst case, edge build-up will occur.

Cause                                                                                                          

Beaver Tail Markis caused by the “beaver” guide plate. Poor strip shape (heavy edge wave) can cause the strip edge to come out of the guide as it leaves the molten metal coating pot. The guide rubs on the edge of the strip surface scraping off the molten Zinc.

Remedies

Clean jet lip .To avoid jet blockage.

3.2.3 Chatter –     PSCM

Description

Chatter appears as a series of parallel Shallow lines or bands up to 12mm wide running across the full width of the strip transverse to the rolling direction and is generally continuous throughout a coil. In its least severe form Chatter is virtually impossible to detect under normal processing condition.                                                  

Cause

Chattercan be caused by :

a. Poor skin pass mill work profile due to roll grinding practices and wear

and tear.                                                                                                 

b. Skin-pass mill vibration; wear in the mechanical components of the mill

can result in a resonance being set up.

c. Transfer from poor profile of skin-pass mill back-up rolls due to poor roll grinding practices and wear and tear.

d. Vibration in the line levellers causing wear and tear in the mechanical components of the levellers and which can result in resonance being set up.

e. Loose hold-down belts in leveller banks.

f. Poor leveller roll grinding practice.

Remedies

3.2.4 Dent –         PDTM

Description

A Dentwill show up as either a raised or recessed section in the strip surface, and generally occurs when the strip is distorted while passing at tension over a roll on which an object has been picked up. This results in a depression on one surface and a raised section on the other.                   

Cause

A Dent is a result of pieces of metal (or other foreign matter) adhering  to or embedding on processing rolls. The foreign particle then imprints into

 the strip surface once per revolution

Remedies

Frequent sink roll and Bridle Rolls cleaning.

3.2.5 Indentation –           PIDM

            Description

An Indentationis a repetitive crater or depression on the strip surface throughout part or the whole coil, the distance between depressions determined by the processing roll circumference.                                                                                               

Cause                                                                             

An Indentation is a result of pieces of metal (or other foreign matter) adhering or embedding on the mill work rolls, as the strip passes through the work rolls under roll force a crater or depression is imprinted in the strip surface. If heavy enough can affect the thickness of the strip in that area.

            Remedies

Frequent sink roll and Bridle Rolls cleaning.

3.2.6 Dross –        PDSP

Description

Dross are pieces of raised roughened  areas, lumps or streaks resulting in increased thickness in the metal coating that can vary in size, shape and severity and have a slightly different colour and texture (duller)                     

 compared to the surrounding spangled Al-Zinc alloy coating.

Cause

                        Dross can be caused by:

a. Dross on the surface of the bath being taken up into the coating by emerging strip, especially at the edges during gravity stripping on the heavier coating mass products.

b. Drosscarried down into the bath by the metal flowing down the

launder into the bath.

c. Changes in the pot composition can affect any drossbuild-up on the pot floor causing it to disintegrate (large spots) or a change in the bath equilibrium that could result in precipitation (small spots) of dross.

d. Drossbuild-up on the Zinc/Aluminium bath surface and not cleaned off at regular intervals.

                        Remedies

Dissolve less Fe. Keep pot stirred. Use of ferrite free pot equipments. Frequent sink Roll cleaning. Watch dissolution rate of pot equipments by estimating Ni content in bath. Manage pot temperature with max fluctuation of ±1ºC.

3.2.7 Edge Build-up –    PEBU

Description

Where there is a distinct roughness of thicker than normal metal coating along       the edge of the strip compared to the remainder of the surface. Can be up to 50mm wide and of frosty appearance.                                              

Cause

Edge Build-up can be caused by :

a. Uneven coating distribution caused by uneven jet control.

b. The Beaver Tail Guide blocking the airflow resulting in a band of

higher coating mass.

c. Bad strip shape eg. Edge wave moves sections away from the air jets resulting in uneven metal coating along the strip edge.

Remedies

Use of well designed edge baffle, Jet-stripping line must be placed on start of curvature of the baffle, good surface condition of cotton reel touching the edge of the strip. Frequent oiling (once per shift) of reel surface. Saw tooth edges of base metal damages chrome plating, Frozen metal stick to strip called Beavertail pickup.

3.2.8 Edge Stretcher Strain  – PESS

Description

Edge Stretcher Strain(Luders bands) marks appear as short, narrow, shallow tapered stretch depressions in the metal surface at the strip edge, extending up to 20mm or occasionally more on one or both edges. Generally                   

occurs on skin-passed products but can be seen on non-skin passed leveled products as well.

                   Cause

Edge Stretcher Strainis caused when the base steel yield point has not been suppressed or eliminated during skin-passing and material is stretched passed the yield point due to insufficient rolling of the edge (eg. Incorrect roll bending). Roll bend should be such to ensure the full width of strip is evenly skin-passed.

If non skin-passed and leveled strip is stretched unevenly at the edge and yielding will occur in the form of stretcher strain. This is actually coil break confined to the edge of the strip due to one tight edge and one loose edge or excessive full center with very tight edge.

Remedies

        Adjust backup roll pressure. Ensure backup roll rotation.

                        3.2.9 Heat Creases –     PHCR

                        Description

Heat Creases are generally creases in the centre of the strip that appear as the strip exits the pot or after the Turn Around Roll above the pot. Can be single or multiple creases, continuous or intermittent and predominantly          

affects soft wide light thickness product, but has been recorded on light thickness wide G550.

Thought to be generated by the bowing under tension and when feeding onto a roll the edge print is narrower than the strip width resulting in excess strip left at the middle which forms into creases. Wide material bows more but light thickness soft products will crease rather than force the strip to move across the roll surface to accommodate the excess width.

Cause

Heat Creases are caused by:

a. Furnace temperature too high. This could lead to a softer product, which is more prone to creasing. If the material is under-annealed, the strip would generally be harder and move across the rolls without the likelihood of creasing.

b. Tension control eg. Tension is too high in the process section, which       may be affected by the tensions in the sections either side of the process section tension regulators.

c. Sink roll build-up results in the initiation of a crease.

d. Upleg Blower pressure causing the strip to bow outwards from the Turn Around Roll over the pot.

Remedies

Control the following parameter within narrow range especially for wider and thinner material. NOF temperature not very high, Avoid too high process tension, check Sink Roll buildup. Balance up leg blower pressure for top and bottom surfaces

3.2.10 Jet Cleaning Line –      PJCL

Description

Jet Cleaning Lineis a continuous “snakey” band of heavy metal

approximately 5-10mm wide which can vary in severity. It can occur on one or both surfaces of the strip and vary in length from 30-40 metres to 100-200 metres in the rolling direction.                                 

Cause

Jet Cleaning Linesare a result of cleaning a blocked air jet with a special jet-cleaning tool. This causes the characteristic “snakey” continuous defect as the tool moves across the jet lips cutting off the stripping air pressure.

Remedies

Preventive measures are important to avoid jet blockage.

3.2.11 Jet Line – PJLN

Description

Jet Linesare continuous narrow longitudinal bands (in the rolling direction) of heavy metal coating generally 5-10 mm wide. The thicker coating may cause a ridge or edge build-up as the coil wrapping continues              

due to the increase in thickness. One or more Jet Lines can occur on one or both surfaces in varying severity.

Cause

Jet Line or heavier coating is caused by localized reduction in the stripping air pressure from blockage inside the air jet lips between the jets and the molten metal covered strip. The thicker coating may cause a ridge or edge build-up as the coil wrapping continues due to the increase in the thickness and this may result in shape distortion on uncoiling.

Remedies

Frequent cleaning of Jet.

3.2.12 Leveler Track Mark –    PLTM

Description

Leveller Track Marksare continuous parallel superficial fine shiny polish lines along one or both surfaces in the rolling direction.

Cause                                                        

Leveller Track Markscan be caused by :

a. Excessive backup leveller roll pressure.

b. Seized leveller back -up rolls marking the leveller works  rolls thereby marking the Zinc/Iron coated steel.

Remedies

Adjust leveler backup roll pressure. Ensure backup roll rotation.

3.2.13 Metal Spots –      PMSP

Description

Metal Spotsare pieces of coating or base metal on or in the coating and raised above the background coating level. Can vary in severity from isolated spots, scattered pattern/patches to bands running in the rolling direction.                                                                                                                               

Cause

Metal Spots can be caused by excessive metal spray at the strip edge (misaligned jets, incorrect beaver tail setup). Pre-galvanized scratching can cause a build-up of base metal that results in extra thickness of coating metal as Metal Spots. Scratching or scuffing of the metallic coating can result in the build-up of pieces of metal that can be then rolled into or onto the surface as metal spots.

Remedies

  • Control Strip entry temp Pot temp. 600+/-1
  • Check bottom dross condition for mushy dross
  • Dissolve less Fe.
  •  Keep pot stirred.
  •  Use of ferrite free pot equipments.
  • Frequent sink Roll cleaning.

3.2.14 Micro Fluting –    PMFL

Description

Micro-flutingis not unlike coil break except that it is on a much finer scale. Micro-fluting is characterised by very fine creases in the strip surface which are very close together and transverse to the rolling direction.                 

It can vary in severity and generally extends across the full width of the strip.

Cause

Micro-flutingis caused by localised yielding (fluting phenomenon) across the width of the strip which can occur after metal coating during tension leveling or by passing over small diameter rolls under tension. Non-skin-passed leveled product is more prone to micro fluting and is more noticeable on material with lower coating masses.

Remedies

Check diameter of leveler work roll the optimum diameter is to be maintained according to strip thickness and temper.

3.2.15  Phosphate Pattern-               PPPN

Description                                                         

Parallel grey lines in the rolling direction with a 17mm gap between lines.Can appear similar to sink-roll lines on a Zinc/Iron alloy coated steel surface although these lines have a different gap (25mm). Zinc/Iron alloy coated steel             

 is the only product, which is sometimes

phosphated. (Metal Coating Line 6).

Cause

It is thought to be caused by excessive phosphate strip loading, which allows the dunk roll engraved pattern to be seen. Either the phosphate tank concentration is too high or the squeegee rolls are

not removing excess solution due to a damaged roll or insufficient pressure.

Remedies

3.2.16 Pick Off – PPOF

Description

Pick-off are small irregular shaped areas up to 2mm in size where the metal coating has been completely removed from the strip surface exposing the steel base and or alloy. They only occur on the bottom surface and mainly in the center which is the hottest part of the strip and can                          

be continuous throughout a coil varying in severity and frequency.

Cause

Pick-offalways occurs on the bottom surface where the coated strip comes into contact with the turn around roll at the top of the cooling tower located above. When insufficient cooling of the coating takes place, the incompletely solidified coating will tend to stick or weld to the turn around roll thus exposing the steel base or alloy layer.

Remedies

  • Keep strip cooler at less than 350 deg. Celsius aim 320 max.
  • Interlock to start the line with top cooler must run.
  • Use Roll scraper to remove pickoff from turn roll.          

3.2.17 Pick Up –   PPUP

Description

Pick-upis a repetitive defect that comes in m any different shapes and severity and appears as a crater or depression resulting in reduction of thickness in the steel strip. Pick-upcan occur on one or both strip surfaces.                                                              

Cause

Pick-upis a result of pieces of coating metal or other foreign matter building up or embedding in the various rolls (ie. Skin-pass mill) on the metal coating line and being forced into the strip surface due to the resistance of the work roll on the reverse surface once per revolution of whatever roll is involved.

Remedies

Frequent sink roll and Bridle Rolls cleaning.

3.2.18 Pin Hole/Fine Uncoated – PPHL

Description

Fine Uncoated are small holes in the metallic coating down to the steel base approximately 0.5 mm in diameter, devoid of the Metal Coating. The exposed steel base if visible is oxidized to a                             

blue/black discolouration. They can be isolated, in clumps or groups scattered across the strip or either one or both surfaces. Fine Uncoated may be associated with poor coating adherence.

The uncoated areas will result in premature corrosion failure, as the coating adjacent to the exposed steel will be sacrificed to protect these areas. When Fine Uncoated is confined to the edge, (approx. 10-15 mm in from the edges), the defect is known as Pinhole Uncoated.

Cause

Fine Uncoated can be caused by:

  • The cold-rolled feed can be contaminated by a substance that  prevents wetability of the surface during the hot dipping process.
  • The strip surface becoming oxidised prior to entering the molten metal coating pot preventing the coating metal adhering to the strip surface.

Remedies

  • Removal of scale at pickling.
  • Strong wiping system to avoid coolant carry over, Avoid drips from coolant headers and condensation from fume hoods.
  •  Normal operating range: Maintain Dew point –30 to –50 ºC and temp 500- 750 ºC. In case of FH coating increase H2 %.

3.2.19 Pock Marks – PPMK

Description

Pock Marks are squarish to slightly rectangular shaped shiny areas 1 to 5 mm long in the rolling direction and 1 to 10mm across in the rolling direction of localized reduction in the coating thickness, often to almost zero. They can occur anywhere across the strip and on one or both surfaces and is highlighted by skin passing and more prevalent on lighter coating classes.                                                                       

Cause

Pock Marks usually occur under conditions of high air knife (jet) pressure or low coating metal fluidity. It is thought that under high jet pressure, molten metal flow disturbance from the jets produces small rectangular areas of low coating. This defect is more common on low coating mass products.

Remedies

Bring Jet in (closer). Reduce jet pressure to compensate. Reduce speed in combination with other parameter.

3.2.20 Post Metal Coating Line Scratches –   PSCL

Description

Post Metal Coating Line Scratches can vary in severity and appear as shiny intermittent or continuous longitudinal lines in the rolling direction on one or both surfaces and can be continuous throughout a coil.                                                                                                                                       

Cause

Post Metal Coating Line Scratches are caused by any contact by the metal coated strip surface with non-moveable surface such as seized rolls, tables and rolls that move at a different speed to the strip.

Remedies

Check rolls movement and gap between tables and strip.

3.2.21 Pre Metal Coating Line Scratches –   PSCR

Description

Pre Metal Coating Scratches are narrow Raised intermittent or continuous lines of metal coating in the rolling direction. They can occur on one or both surfaces  and can vary in number and severity and can be intermittent or             

continuous throughout a coil.

Cause

Pre Metal CoatingScratchesoccur as a result of:                             

a. Scratches in Cold-rolled (full hard) feed from the cold-rolling mill. 

b. Scratches in hot-rolled pickled galvanizing (Zinc coated) feed.                              

c. Continuous galvanizing line scratches prior to entry into the metal coating pot.

d. Metal coating pot sink roll scratches where the roll is seized or not rotating at strip speed.

                        Remedies

Check rolls movement and gap between tables and strip.

3.2.22 Sag Lines – PSLN

Description

Sag Lines are areas where the molten coating runs or flows down the vertical strip just above the stripping jets and before solidification occurs leaving shiny areas of low coating adjacent (preceding) to a raised area of high                  

coating. Due to the nature of their formation they appear as narrow “sagged” areas or bands of alternating heavy metal and shiny low coating mass areas. The sagged coating or heavy band can be up to 3-4mm wide leaving lower coating mass bands up to 10-40mm wide. Sag Linesare mainly confined to heavier steel base thickness and higher coating masses eg. Z600 and can vary in severity on one or both surfaces.

Cause

Sag Linescan be caused by:

  • Low jet stripping pressure from: Low process line speed, jets too high above the bath, heavy coating mass.
  •  Slow solidification rate from: Heavy coating mass, low processing line speed, low jet cooling air flow, strip entry temperature too high.
  • Oxide film formation from: Incomplete flame coverage at strip/bath exit interface during gravity stripping.

Remedies

  • Maintain proper distance and wiping pressure of jet.
  • Maintain strip entry temperature 580±5 ºC.

3.2.23 Sink Roll Lines – PSRL

Description

Sink-Roll Linesare a pattern of different surface textured parallel bands. Different appearance running in the rolling direction alternating in width at approximately 2 mm and 5mm wide across the entire width of strip occurs on top surface only because it is in contact with the sink roll. They can                           

 vary in severity and can be continuous throughout a coil and correspond to the threaded groove on the sink roll.

Cause

Sink-Roll Linesare caused by a seized sink-roll or the sink-roll not rotating at strip speed. The actual mechanism is not fully understood but it is believed to be related to a variation in the alloying rate between areas in contact with the sink-roll surface and those areas adjacent to the grooves, which do not contact the sink-roll.

Remedies

Frequent maintenance of sink roll bush to attain sink roll rotation in line with the strip speed.

3.2.24 Streaky Surface –         PSSF

Description

Streaky Surfaceis characterised by a silver streak appearance on a grayish matte surface. Streaky surface occurs on both surfaces to varying degrees.

Cause                                                                           

Streaky Surfaceis not fully understood but is believed to be linked to excess titanium  in the steel base and hot strip mill descaling  practice leading to alloying rate variances on the strip surfaces.

                        Remedies

  3.2.25 Unalloyed Area –         PUYA

                   Description

Unalloyed Areas are areas where the Zinc coating has not been fully converted to a Zinc/Iron alloy. They can occur anywhere on one or both surfaces of the strip but predominantly occurs on the strip edges. Unalloyed Areas can be shiny to dullish silver in                           

colour compared to the surrounding light grey of the Zinc/Iron and can vary in severity throughout a coil in form of patches or continuous irregular shape bands.

Cause

Unalloyed Areas are caused by:

a. Aluminium content of molten Zinc

bath too high which inhibits alloy growth.

b. Line speed too fast.

c. Incorrect furnace setting (temperature too low).

d. Variable coating mass, thicker than “normal” coating takes longer to form the Zinc/Iron alloy.

                        Remedies

3.2.26 Uncoated Areas Defect –        PUCT

Description

Uncoated Areas are areas devoid of metal coating. The exposed steel base is always oxidised to a blue/black discolouration varying in size, shape and frequency on one or both surfaces of the strip and can be continuous or patchy throughout a coil.            

Cause

Uncoated Areas can be caused by a variety of reasons:

  • Cold-mill carryover contamination.
  • Any surface contamination (eg. Grease, oil) that is not fully removed by the Non Ox furnace.
  • Furnace problems eg. Air/gas ratio, gas contamination, water leaks, low furnace pressure resulting in strip surface oxidation.
  • Incompletely removed hot strip mill surface scale on cold-rolled full hard metal coating feed.

Remedies

  • Removal of scale at pickling.
  • Strong wiping system to avoid coolant carry over, Avoid drips from coolant headers and condensation from fume hoods.
  •  Normal operating range: Maintain Dew point –30 to –50 ºC and temp 500- 750 ºC. In case of FH coating increase H2 %.

3.2.27 Wash Pattern –   PWSP

Description

Wash Pattern is a coating defect where localized variation in coating thickness produces a transverse, wave like appearance (trough and crest) across the strip. It can vary in severity on one or both surfaces throughout a coil.                                                                                                                   

Cause

Wash Patternis due to a rippling effect caused by the air knives (jets) stripping force on the molten metal surface in the stripping zone. Higher jet pressures tend to cause more rippling before solidification.

Remedies

Adjust jet angle and jet pressure.

3.2.28 Wrap Mark –       PWPM

Description

Wrap Mark is a single narrow repetitive transverse corrugation like mark or “bump” running across the full width of the strip. Generally seen on the heavier thickness product eg. greater than 1.20mm and due to the nature of this defect it will start in its                           

severest form at distance equal to the circumference of the gripper mandrel. This distance will gradually increase as the coil is rewound until such time as the defect diminishes. Depending on strip thickness, the amount of material affected will vary. Defect is always in the bore of coil.

Cause

When the front end of a coil to be rewound, is fed into the belt wrapper or mandrel bite, depending on the strip thickness, the point at which the leading end of the strip touches the next wrap, will be raised across the width of the strip. As tension is applied, the inside bore wraps and subsequent wraps will be damaged by the high point (pressure point).

Remedies

3.3 Shape Defect                 

3.3.1 Camber –    PCAM        

Description

Camber is the curvature in a sheet or length of strip 2000mm long. Camber can result from conditions during rolling or side trimming where one edge is worked or distorted more than the other.

How to Measure Defect

Measuring Camber with a string line. Use a length of string fixed in a similar manner to the method used for a straight edge with the ends of the string touching the edge of the strip at intervals of 2000 mm. The maximum distance between the centre of the string and strip edge is the Camber. Measuring Camber with a tape measure. Camber can also be measured by placing adjacent sheets or pieces of the same length of strip with concave edges together as shown right. Actual camber is one half of the maximum distance between the two edges. Camber is a measured value divided by length, expressed as a percentage.                     

3.3.2 Coil Set –    PCST         

Description

Coil Set is present in sheet when the ends curl up or down under its own weight. Coil Set can be induced from the incorrect roller leveller tilt set up or from the inherent stresses from recording strip around a small bore mandrel.                                                                         

How to Measure Defect

The Coil Set Jig method of assessing Coil Set is considered to give a more accurate reading than that specified in AS/NZS 1365. The method in AS/NZS 1365 requires the sheet or length of strip to be laid down with the convex surface                             

up, resting on its ends under its own weight, which overcomes part of the Coil Set. The amount of Coil Set is read directly from the measuring tape.

3.3.3 Bow –                    PBOW

Description

Crossbow occurs when the sides of sheet or strip curl up or down around the centre line in the rolling direction. Crossbow can be positive or negative depending on which way the strip edges curl relative to the top surface of the strip.            

How to Measure Defects                                                         

Measuring crossbow in a Crossbow jig. A 50 mm sample is taken from the full width of strip or sheet. The sample is placed in the Crossbow jig and a straight edge used to make contact with the edges of the sheet.                                                  

Measuring Crossbow with a straight edge and taper gauge or tape. A straight edge is placed on the concave surface perpendicular to the edges. The taper gauge is then used to measure the deviation from the centre of the strip to the face of the straight edge.                                                                                               

  • Centre Buckling   PCBG

Centre Buckle is defined as symmetrical or regular fullness along the centre of the strip. The fullness is not pronounced enough to result in any buckle or localised distortion and is characterised by a long wavelength, greater than 500 mm. The wave height of a Full Centre, quarter buckle and ridging can be measured in any direction but is normally measured in the rolling direction along the strip or sheet. A straight edge is rested on the crests and a taper gauge is used to measure the wave height.

3.4 Passivation Defects

3.4.1 Passivation Stains – PPST

3.4.1.1 Description

Passivation Stain can occur anywhere on one or both surfaces in any pattern of various sizes and severity, with color ranging from light yellow to brown patches throughout a coil.

3.4.1.2 Cause

Passivation Stain are caused by:

  • Damage or worn passivation tank squeegee rolls.
  • Loss of passivation tank squeegee roll pressure. Carryover of passivation solution due to excessive roll gap. It can also occur when there is a change to a different width.
  • Leaks or condensation inside the passivation tank, misdirected sprays etc that causes drips of passivation solution to fall onto the strip after the squeegee rolls.

2.4.1.3 Inspection Norms

2.4.1.4 Remedies

Manage squeeze roll gap and pressure. Roll surface shall be uniformly ok, cut & worn out free. Ensure proper leveling & shape correction. Minimize coating abnormality  (Smooth surface free from dross, heavy Z line, Excessive coating band etc.) Check squeeze roll condition when changing from lower to higher width production, avoid dripping from Passivation unit duct, and spray header.

  • Off Line Defect
  • Storage, Transportation & Handling Defect

3.6.1 Rain Water Black Rust Defect – TWBR

Description

Rainwater Black Ruston zinc/alumi nium alloy coated strip or sheet is black in colour and occurs in patches. Unlike zinc-coated product, the corrosion product on a 55% Al-Zn Alloy Coated Steel® steel surface is only a superficial black corrosion stain. The degree and extent of the black corrosion stain depends on the amount of water retained between the strip or sheet surfaces and exposure time.

Cause

Rainwater Black Ruston zinc/aluminium alloy coated product is caused by:

  • Rainwater falling directly onto the zinc/aluminium alloy coated coil or Sheet where it has entered through the mill roofing eg leaky roof. Or Gutter overflow or during transportation.
  •  Rainwater flooding the floor of the mill storage areas allowing rainwater to penetrate the bottom portion of any zinc/aluminium  alloy coated coil or sheet packs.
  • Condensation with some plastic wrapped coils.

Remedies

  • Ensure proper roofing condition before monsoon.
  • Proper storage of the packed coils to avoid rain water spillage. Avoid storage of 55% Al-Zn Alloy Coated Steel coils near the shop floor entrance.
  • Proper packaging and sealing with umbrella packing on the truck during transportation.
  • Ensure material receipt in good condition with tarpaulin seal inact while unloading.

3.6.2 Rain Water Black Rust Defect- TWWR

          Description                   

Rainwater White Ruston Zinc-coated strip or sheet is normally in patches and can vary in colour, severity and extent depending on the amount of water retained between the strip or sheet surface and exposure time.

The corrosion product can be:

a. Very light white corrosion patches-generally superficial.

b. Heavier white corrosion patches with loose white corrosion product where the zinc coating has been partly consumed.

c. Very heavy white corrosion patches together with blackish patches where the zinc coating has been consumed to a fair degree.                                     

d. Extremely heavy corrosion where there is a combination of white and blackish corrosion product together with base metal red rust where the zinc coating has been totally removed.

Note: It is rare for (d) to occur at works.

Cause

Rainwater White Ruston zinc-coated product is caused by:

a. Rainwater falling directly onto zinc-coated coils or sheet where it has entered through the mill roofing eg. Leaky roof  or gutter overflow.

b. Rainwater flooding the floor of the mill storage area allowing rainwater to penetrate the bottom portions of any zinc-coated coil or sheet.

Remedies

3.6.3 Galling Defect  – PGAL

                   Description

Gallingcan occur on hot rolled, cold rolled, metallic coated and prepainted products. Galling, often referred to as “Road burn” in the transport industry is a roughened area on the strip or sheet                      

surface that can vary in size, severity and extent. These roughened areas can be individual spots, patches of spots or areas. In the worst case galling can result in total removal of metallic coating and paint film in the galled area.

Cause

Galling is the unintentional roughening or abrasion of metal or organic finished steel surfaces where rubbing (under pressure) results from relative movement of adjacent strip or sheet surface during road, rail or shipping transport. Galling is aggravated by

a. Insufficient coil tension

b. Poor packing practice

c. Incorrect loading and securing of coil and sheet packs in  

    all  modes of transport

d. Any surface pressure points emanating from any source  

such as damaged chocks (Not covered by rubber matting), sheet pack skids etc.

                        Remedies

Provide sufficient coiling tension. Establish good practices for packing, loading and securing the packs on any mode of transport to avoid wrap to wrap rubbing of strip surfaces in coils and sheet to sheet in packets. Nullify high pressure points with sufficient rubber packaging.

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