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`DEFECTS IN CONTINUOUS HOT DIP GALVANIZED STEEL SHEET AND THEIR REMEDIES’

By Dr. N. C. Jain

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GALVANIZING PROCESS REALTED DEFECTS

Hot dip galvanizing is a metallurgical process which involves both physical (wetting) and chemical {Fe-Zn(Al)} interactions at the solid-liquid interface. Therefore, any change in the physical and chemical parameters will affect the coating. Important parameters which affect wetting are the strip inlet temperature, fluidity and surface tension of the molten zinc/alloy. The factors which affect chemical interaction are bath temperature, immersion time and chemical composition of the bath. Wiping and cooling operations also affect the quality of the coating quality.

Dross pimples

It is not good for body panel applications. In zinc bath, concentration gradient of aluminium can exist if there is no bath agitation. Further, at about 0.15% Al, the Al starts to react with Fe, which converts the bottom dross (Fe-Zn compounds) into top dross (Fe-Al-Zn compounds), In this way, the iron is floated to the top of the bath where it is removed by skimming at regular interval. Any temperature variation in zinc bath increases the level of iron saturation (via an “iron pump” effect). Thus, there is dross formation with some Al-Fe-Zn crystals growing to a large size due to iron super saturation. The resulting dross (Al-Fe-Zn inclusions) is a problem in the production of free zinc coating for exposed automotive panels.

Dross formation is particularly more in the Sendzimir type process. Many efforts are being made to suppress dross sticking to the coatings. Several continuous galvanizing line adopt a specific method to minimize top dross formation on the galvanizing bath surface by covering the bath with a seal box so as to reduce oxygen concentration in the box. Some uses nitrogen wiping also. However, the dross pick-up does not appear to be a mojor source of surface defects, provided skimming operations are performed frequently during galvanizing.

Edge Buildup

Other important defect is edge buildup which finally results into edge waviness. The main reason of this is the dross pick-up which mostly takes place from the top dross just after galvanizing. The dross pick-up is more near the coated strip edge where dross pick-up is generally not wiped off efficiently. When such dross particles are picked up by the coating, they can not be seen visually and will not be noticeable after skin rolling.

Practice has proved that a slightly heavier wiping action is required towards the edge of the strip. A slot profile known as the “bow-tie” which has a minimum opening in the center of the nozzle and a maximum at either end, yields the required impact pressure and resultant wiping action.

There are many methods to prevent iron build up in the zinc bath. Some of the important methods are, strict control of bath temperature, cleaning of non-adherent iron and dust from the strip, continuous filtering of zinc bath, molten zinc circulation/filtering using a bath/strip cooler and using a ceramic pot which eliminates the source of iron and sufficiently stirs zinc for a drossless galvanizing pot.

Flow Line Marks

Many product applications require high coating weight i.e. Z275 instead of the normal thin coating i.e. Z90. At these higher coating weights, new types of coating surface defects tend to occur looks like Ocean Waves, Curtain, Ripple and Air Angle marks. These defects normally become more prominent at lower line speeds i.e. < 30 meters per minute and at higher coating weights i.e.> 150 g/m2). While spangles tend to mask these defects, they become more prominent on non-spangled, minimum spangle or zero spangle skinpass coating surface.

The variation in the surface tension and viscosity (fluidity) of the molten zinc during the withdrawal of the sheet may change the flow characteristics of the coating. Lowering the lead content of the zinc increases the surface tension of the molten zinc. The major cause of these defects, however, appears to be the reaction between the outer surface of the molten zinc and the oxygen in the air. This interaction probably makes the top oxide layer of the coating more viscous than the bulk coating, creating ripples.

These defects can be avoided by increasing the line speed and using gas wiping. However, when thicker coatings are required (lower line speed), use of nitrogen gas gives the best results. As nitrogen prevents the formation of sticky oxides before the coating is wiped in the absence of oxygen, the coatings formed is smooth and bright. Nitrogen-wiped galvanized sheet is virtually free from defects.

Nozzle Line Marks

Wiping lines or header lines or jet line defects are longitudinal defects in the coating that appear as superficially raised and depressed bands respectively. Header lines are caused by the blockage of the wiping gas (steam, air, nitrogen) knife slot with zinc particles or other foreign debris. As a result the regions where the gas pressure is lower has a slightly heavier coating than the surrounding areas.

The obvious solution for these problems is to clean the knife slot and the lips with a relatively soft tool such as brush or scraper.

Black Stain

Al added in amounts of 0.15-0.25% forms a very thin, dense and adherent layer of Fe2Al5, which inhibits the growth of zinc ferrites at the steel surface. However, when the Al content in the bath exceeds 0.30%, the rate of attack by Al on steel also increases proportionately. Prior oxidation and reduction of the strip also results in finely divided, unattached particles of iron on the surface of the sheet as it enters the bath, which are quickly converted to FeAl3 particles by Al in concentration over 0.25%. They form films which may accumulate in the bath as clumps to be brought out irregularly in the coating or rise to the surface as floating dross and brought out as a black stain on the surface of the work, perhaps partly embedded in the coating.

Control of iron impurities and Al content in the bath is very important for avoiding these defect. The active concentration of Al should not exceed 0.30 and iron contamination of the bath should be less than 0.02%. Use of easily soluble Al-Zn master alloys and bath circulation helps in this regard.

COATING WEIGHT NON-UNIFORMITY

Coating weight uniformity on the sheet in width-wise direction and on top and bottom sides of the sheet is greatly affected by bending of the sheet in width-wise direction at gas wiping nozzle position. As the nozzle to strip distance increases, coating thickness increases. This indicates that non-uniformity of the zinc coating weight at continuous galvanizing line is attributed to cross-bow at the gas nozzle in the strip’s lateral direction and strip vibration in the longitudinal direction.

In general, the temperature of the molten zinc bath is controlled to about 450 - 4600c and that of the strip is controlled to a similar level. Consequently, the strip in the bath is soft and provides a low yield point and Young’s modulus.  Under this soft condition, the strip is subjected to bending by sink roll as well as tension bending and back bending due to the line tension.  In this process, the internal stress exceeds the yield stress at the strip surface layer, generating localized plastic deformation in the material.  This plastic deformation produces longitudinal and lateral stress distributions (residual stress) in and through thickness direction of the strip.  The strip subjected to lateral constraints by sink roll allows the lateral residual stress to develop after leaving SR and generates lateral cross-bow.

Therefore, to give a uniform coating weight, it is necessary to flatten the cross-bow and suppress strip vibrations at the wiping section.  To flatten the cross-bow, plastic back bending is carried out with front support roll located above the sink roll. The roll gap is controlled to straighten the sheet bending.  The rolls may be driven or be idlers.   It is possible now to control the coating weight with an accuracy of ± 5-10 %.

Non Uniform spangles

Most continuous galvanizing lines use air knife, 30-60 cm above the bath to control the coating thickness.  Just below this knife the strip temperature across the strip width is fairly constant.  However, immediately after air wiping, there is a temperature variation across the width, which depends upon the coating weight, strip shape, thermal history and ambient temperature.  Since the strip edges lose heat faster than the centre, areas at the edges of the strip solidify at a faster rate.

Moreover, if the strip has a centre buckle, this will be further so.  This gives rise to a non-uniform spangle growth rate areas at the edges of the strip solidify at a faster rate, providing smaller spangles.

Rough Surface

Hot dip continuous galvanizing baths containing about 0.25 % of Al and small amounts of lead (0.20 % Pb) produce a medium to large spangled surface with bright appearance due to the dendritic growth of zinc arms. A higher lead content yields a larger spangle with pronounced boundaries while lower lead content (< 0.03 %) yields smaller spangles and slightly duller appearance.  Thus lead plays an important role in the formation of spangles in a galvanizing line in addition to providing smoothness of the surface.  However, if the content of lead in the bath is more than 0.2 %, due to the wide separation of zinc nuclei, there may be unhindered and unfavourably oriented dendritic growth of zinc giving rise to large size spangles and a very rough coating.  This happens especially when the cooling rate is slow.

Roughness of the surface can be reduced by decreasing the size of the spangles.  The spangles can be controlled or eliminated by the following methods:

  1. Controlled addition to the bath: Galvanizing lines which are designed to produce exclusively the minimized spangled sheets adopt the coating bath with less than 0.03 % of lead so as to reduce spangle without minimized spangle processing.
  2. Skinpass after galvanizing.

CONCLUSIONS

Surface quality of the zinc coating is very important in many of the modern applications of galvanized sheets.  In addition to good adhesion and ductility which are critical for forming operations, uniformity of thickness, surface finish and paintability are also important for all such applications.  The surface quality of the coating depends not only on the quality of the steel used but also on many other important process parameters which influence the coating process.  Understanding of each stage of the coating process and the physical and chemical surface phenomena involved is, therefore, important to alter and control the coating quality.

REFERENCES

  1. “Defects in Continuous Hot Dip Galvanizing Coatings”. By E P Rajiv, Technical Manager, India Lead Zinc Information Centre, New Delhi.
  2. “Some Important Factors Controlling the Continuous Hot Dip Zinc Coated Steel Sheet Surface”, by N C Jain and Nilesh Mahakalkar from Ispat Industries Limited, Kalmeshwar, Nagpur, Conference organized by ILZDA, Mumbai on 3 & 4th Dec 2003.
  3. “Effective Operation of Furnaces used in Galvanizing Line to Save Fuel” paper presented by Mr B D Pawar, one day programme in Ispat Industries Limited, December 22, 2006.
 

 

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