HISTORY

The term “GALVANIZING” is derived from the name of an Italian scientist, Galvani. He discovered in 18th century that there is a flow of electrical current between dissimilar metals which are in contact with each other in a conducting liquid.

In 1837, the French engineer Stanislaus Sorel submitted the first patent application in Paris for hot-dip galvanizing in accordance with the same principles which is in practice today.

Sorel called the method “Galvanizing” in reference to the galvanic cell that is created if the zinc coating is damaged and gives the steel cathodic protection. In order to avoid confusion, applying of a zinc coating on iron and steel by dipping in a bath of molten zinc is called hot dip galvanizing because electroplaters refer to their process as electro-galvanizing and zinc-dust painting is described as cold galvanizing.

Today, hot dip galvanizing is done by various means like semi-automatic or hand dipping of fabricated items or the mechanical dipping of sheet, wire and pipe or continuous dipping of sheet or wire in molten zinc. In this process, basically steel components, cleaned of rust, oxide scale and other impurities, fluxed and dried and then dipped in a bath of molten zinc (at 4500C). The galvanizing reaction takes place and different layers of iron-zinc alloys form. After the galvanizing reaction is over, when the jobs are withdrawn, they are covered with a layer of pure zinc on top. It is this combined coating, alloy and pure zinc that gives galvanized coating a superior resistance in a wide variety of environments. After withdrawal from the galvanizing bath, the jobs are quenched in a water tank to “stabilize” the coating and dipped in a chromating bath for passivation.

Technically, hot dip galvanizing is a metallurgical reaction between molten metal and steel surface. The iron should be in direct contact of molten zinc, therefore, the surface preparation of the steel object to be galvanized is very important

The alloy layers that form in galvanizing process are:

Gamma layer containing 21 to 28 percent iron
Delta layer containing 7 to 12 percent iron
Zeta layer containing 5.8 to 6.2 percent iron
The top layer which is pure zinc is known as Eta layer

The delta and zeta zinc-iron alloy layers are harder than the base steel. Abrasive or heavy loading conditions in service may remove the relatively soft eta layer of zinc from a galvanized surface but the very hard zeta alloy layer is then exposed to resist further abrasion and heavy loading.

DEVELOPMENTS IN CONTINUOUS HOT DIP SHEET GALVANIZING

Sheet galvanizing as a separate industry appeared to start in 1846. The process was distinguished by the control of the thickness of zinc by means of a pair of rolls at the exit of the galvanizing bath. Rapid development of the industry followed and a vital technical development was established by 1911; the addition of aluminium to the zinc bath in sufficient quantities (about 0.1 -0.3%) to suppress the formation of almost all the alloy layer. Galvanized sheet can be formed without the coating cracking or flaking.

By the outbreak of the 1914 -18 war the world output was over 1,500,000 tons of which 800,000 tons was made in the UK and 700,000 tons in the USA. After the reduced output during the war years U.K. production did not recover to the previous level until 1925. It reached a peak in 1928 at about 890,000 tons, a level that has not been approached in any subsequent year. The USA production increased fairly steadily, to reach a peak of 1.4 million tons in 1929. In the same year France reached 135,000 tons. In Japan, production started about 1925 and was only 22,000 tons in 1929: production in India started about this time. With estimates for production elsewhere, world output was well over 2.5 million tons in the boom year of 1929. In 1932 world output, severely affected by the slump, was reduced to less than half that for 1929 but by 1935 output had recovered to within 100,000 tons of the 1929 peak, mainly owing to the revival in the USA., expansion in Japan and Belgium, and a substantial increase in India. In UK, output was slow to recover from the slump, with the UK share of estimated world production dropping from about 33% in 1929 to 16% in 1935.

The introduction of cold-reduced coil in the United States in 1926 led to demands for continous galvanizing facilities, the product either being cut at the end of the line or recoiled for shipment. Unfortunately, no fluxes were available that would permit the use of the zinc bath containing aluminium which had become standard practice for galvanizing cut sheet. Research took two directions (i) alternative additions to the zinc bath (without success), and (ii) alternative cleaning methods. Sendzimir was successful with a gaseous oxidation-reduction cleaning technique that required to flux and by 1937 continuous galvanizing lines were in operation in Poland, France, USA and UK.

CONTINUOUS HOT DIP SHEET GALVANIZING AFTER 1937

In the next twenty years the industry has expanded steadily, first in the USA and subsequently in Japan and W.Europe. By 1957, almost all the USA production was from continuous lines and in the UK and most other industrialized countries the percentage of sheets cut and then galvanized has become very small. There were over 120 continuous galvanizing lines capable of producing each year about 10 million tons of hot dip galvanized steel in sheets up to 72 inches wide.

The established continuous hot dip process for sheet galvanizing can be classified into several main types. The principal differences between these are (i) the methods used for cleaning the strip; (ii) the absence or presence of in-line annealing. The Cook-Norteman principle does not include in-line annealing; for a soft product the coils must be box-annealed prior to entering the line. As employed by the originators, the Wheeling Steel Corporation, it comprises liquid cleaning (by acid pickling and an alkali dip) followed by passage through a flux solution. The flux is dried on to the sheet when it passes through an oven before entering the molten zinc. Many variations are possible in the preliminary cleaning procedure, while the Japanese “Economy” lines coat cold-reduced strip by direct immersion into molten zinc through a flux layer without any prior cleaning.

In-line annealing is an essential part of the Sendzimir system which is still the most widely used continuous galvanizing technique, although it is slower than the Cook-Norteman type. In a Sendzimir line, strip cleaning comprises flame oxidation of the oil on the surface with subsequent reduction of the oxidized surface and in-line annealing while still in the furnace. No flux is then needed either on the strip before dipping or on top of the molten zinc. Alternative cleaning procedures-such as electrolytic alkali cleaning are employed by some large companies such as the United States Steel Corporation and the Weirton Steel Company prior to annealing in-line.

Lines now being installed are usually required to be able to produce both hard and soft galvanized sheet and to operate at higher speed to obtain a greater output. This can be achieved by using radiant furnaces for heating the strip. Radiant heating removes all rolling oils by volatilization and there should be no solid carbon particles present either before or after passage through the radiant heating section of the furnace. Radiant heating can be used in conjunction with preliminary chemical cleaning and/or with subsequent annealing of the strip. By avoiding the oxidizing treatment of the Sendzimir lines no reducing treatments are needed.

CONTINUOUS HOT DIP SHEET GALVANIZING TODAY

Sheet metal is hot-dip galvanized in continuous production lines where all of the processes are linked together in a closed furnace system. The basic material is cold-rolled sheet in coils. One coil is welded to the next in an “endless” strip.

After degreasing, the strip is pickled or oxidized. Oxides are then removed by reduction at 9500C. At the same time, the strip is soft-annealed. The surface of the strip is then metallurgically clean and goes in a protective gas atmosphere directly down into the zinc bath.

The strip runs vertically up out of the Zinc bath and passes through jet knives. Fine jets of air or steam are blown through these knives wiping the zinc coating to the desired thickness. The thickness of the zinc coating is checked and the jet knives are controlled with the aid of a thickness meter and a computer.

After cooling, leveling and treatment (often chromating) against wet storage staining, popularly known as white rust, the strip is either cut into sized sheets or coiled for delivery or subsequent plastic coating, painting and/or profiling.

CONTINUOUS HOT DIP GALVANIZING PROCESS AND TECHNOLOGY

All the available empirical data demonstrate unequivocally that the special features which characterize the reaction between molten zinc and steels containing silicon can be attributed to the silicon in solution in the ferrite. Internal oxidation of the silicon in the ferrite to form oxide inclusions has the immediate effect that the reaction follows the same course as with steel containing no silicon.

Taking now a closer look at the individual reactions, it is necessary to consider separately five different ranges, i.e. silicon contents below 0.03%, from 0.03 to 0.12%, from 0.12 to about 0.25%, from 0.25 to about 2.5% and the range of silicon contents above 2.9%.

Where the silicon content is very low, under 0.03%, its effect is so slight that the reaction takes place as between molten zinc and pure iron. Up to about 4900C, compact and cohesive layers of alloy are formed which build up by the diffusion of the iron and zinc through the layers in accordance with the laws governing diffusion. The inhibited formation of the phase in the temperature range from 4950C to 5300C has the result that no cohesive layer is formed. This leads to non-equilibrium conditions which, in turn, result in zinc-permeated, structurally uncohesive layers whose thickness increases as a linear function of time since the very thin compact layer next to the steel is unable to build up over a period of time, so that diffusion through this layer is always constant and is not retarded by increasing thickness. Over 5200C compact layers are formed once more, and the speed of the reaction is determined by the rate of diffusion through these layers.

With silicon contents between 0.03 and 0.12% at temperatures from about 4300C to 4650C structurally unsound deposits are formed which are made up of numerous small zeta crystals which do not bond together to make a cohesive layers. In view of this structure of the alloy layer it is to be supposed that, in these conditions, nucleation of new zeta crystals occurs with such rapidity that they hinder each other’s growth and force one another apart, thus constantly allowing molten zinc to penetrate into the interstices between the crystals. It is possible that this process is aided by silicon enrichment of the zinc melt in the vicinity of the steel surface. Since alloy layers having this type of structure are found only in the temperature range mentioned. One must assume that at these temperatures a critical point for nucleation is passed which triggers off the phenomenon. At lower and higher temperatures, this thermal trigger is not actuated, so that compact, layers of alloy are formed as in the reaction between zinc and unalloyed steel. At temperatures above about 4850C the nucleation of the zeta phase is inhibited to such an extent that no cohesive zeta layer can be formed and structurally unsound deposits are created. Over about 5200C the layers made up of the gamma and delta phases are then compact once more.

Above about 0.12% Si, the nucleation of the Zeta phase, which is given such extraordinary impetus by lower levels of silicon, is once more reduced to such an extent that the individual crystals no longer interfere with each other’s growth. At ordinary galvanizing temperatures, compact deposits are therefore obtained. In this range the reaction is similar to that between zinc and steel containing no silicon. The transition to the formation of structurally uncohesive layers made up of fragments of delta and angular zeta crystals begins at about 4700C. This temperature range then extends up to around 5200C above which compact layers are formed once more.

As the silicon content is increased, the temperature range within which the formation of the zeta phase is inhibited and structurally uncohesive layers therefore occur drops to increasingly low levels. Consequently, with a silicon level of above 0.25%, alloy layers of this kind are already formed in the upper range of galvanizing temperatures around 4650C, while with silicon contents in excess of about 0.45% this occurs at very low temperatures around 4300C. Above about 5200C compact layers are obtained once again.

With very high levels of silicon in excess of 2.9% the speed of reaction between steels of this kind and molten zinc has dropped off to such an extent that, even at temperatures around 5000C, a compact zeta layer can form which isolates delta layer underneath from the zinc melt. This prevents any dissociation of the layers of alloy. At all temperatures, the reaction then follows a course which declines parabolic ally as a function of time.

CHEMICAL CLEANING LINES

The galvanizing lines based on chemical cleaning of CR strips consist of the following sections:

Degreasing: Degreasing is done in alkali solution such as sodium hydroxide/orthosilicate based chemicals. The solution temperature is maintained at nearly 75-800C to remove rolling oils, lubricants and other foreign materials like grease etc.

Degreasing is done either by immersion of strip in solution tank or by continuous spray of degreasing solution over the entire width of strip on top and bottom surface.

Brush rolls with nylon bristles are used, rotating at high speed, to remove the oils and lubricants mechanically as well as chemically.

Rinsing : Cold rinsing, followed by hot rinsing, is done after degreasing to remove alkali solution from the strip surface.

Pickling: The pickling is done in 10-15% concentrated hydrochloric acid at a temperature of 45-500C. The purpose of pickling is to remove rust and other surface impurities from the CR strips. Brush rolls with nylon bristles are also used in pickling tanks to achieve fast reaction of Hcl acid and fast cleaning of strips.

Rinsing : Cold and hot rinsing are again done to remove any kind of acid contents or ferrous chlorides from the surface of the CR sheets/strips so that the material becomes free from all kinds of surface contaminants.

Fluxing: The cleaned CR strips are then passed through a flux solution. The flux solution is a combination of various salts such as zinc chloride, ammonium chloride, barium chloride etc. The purpose of providing a flux layer on the cleaned and reactive surface of the strip is as follows:

The flux removes all harmful and offending impurities from the degreased and pickled strips. Flux also protects the degreased and pickled strips from further contamination and oxidation. The flux temperature is maintained at between 70 and 800C and the viscosity is maintained at nearly 250Be. Sometimes, a wetting agent like tallow is added in the flux solution to improve the adherence properties of the flux. Treatment of free iron percentage is flux tank is done which helps in reducing the dross formation in zinc bath.

Drying: The strip is then passed through a drying chamber where preheating of the strip takes place prior to immersing the strip in the molten zinc bath. Usually, the waste heat coming from flue gases of the zinc tank furnace is utilized in the dryer. The dryer temperature is maintained at up to 1200C.

Immersion in Zinc Bath (molten zinc) : In a continuous process, the strip is immersed in molten zinc and zinc iron alloy layer formation takes place over the strip in the form of zinc coating.

The rate of withdrawal of the strip from the zinc bath varies from plant to plant. At the time of withdrawal, the sheet passes through an air jet / air-knife, where excess zinc is removed from the surface in order to obtain the desired coating thickness. For better quality and uniform zinc coating, the zinc bath composition analysis is done at regular intervals and the bath chemistry is strictly maintained.

Generally, aluminium percentage in zinc bath is maintained at between 0.12 and 0.18% for better zinc coating adherence and the lead percentage is maintained at between 0.22 and 0.32% for better and uniform spangles.

Other alloying elements such as antimony, tin, bismuth are added depending on requirements and this differs from plant to plant.

The zinc bath temperature plays a very vital role in zinc coating thickness and adherence. The ideal temperature for zinc is 455-4600C for such lines because the strip entering the pot does not have adequate heat.

Cooling: After the zinc pot, the galvanized strip is passed through a series of air-cooling units to bring down the temperature of the strip from 4500C to ambient. The capacity of cooling units is designed as per the desired throughput. Water cooled heat exchangers are used to bring down the temperature of ambient air so that rapid cooling can be achieved.

Chromating: The purpose of chromating or chromating treatment is to provide stable and uniform coating of chromic acid over the entire surface of the galvanized sheet to prevent white rust formation on the zinc coating. White formation may take place on untreated zinc coating even under very mild conditions of corrosion, either in transportation or in warehouse due to condensation. Any kind of water or moisture can lead to white rust formation on the zinc coating and the product can deteriorate.

The solution is prepared from chromic acid flakes and concentration is maintained at nearly 1.00 to 1.10% at a temperature of 500C.

The chromating does not harm the product physically or metallurgically, except giving a light yellow luster finish.

NON-OX FURNACE CONTNUOUS GALVANIZING LINES

The non-ox process has been found to be highly effective and has various added advantages over other processes. This process is adopted worldwide for mass production lines. It provides simultaneously the degreasing, complete pickling ad annealing of the strip. A galvanizing line based on non-oxidizing furnace has the following sections:

Non-ox zones: The strip entering into the furnace is directly exposed to a set of burners. The burners are preset on a fixed air:as ratio to enable an oxidizing atmosphere in the first zone and slightly reducing in the second and third zones.

The burners are preset and the composition of flue gases are maintained very precisely and remains stable in all conditions, excluding any traces of oxygen and maintains a substantial quantity of unburnt combustibles in the form of hydrogen + Co in the range of 4 to 8% by volume. The combustion products provide a reducing atmosphere in the second half zone and prepares the strip fully cleaned.

The strip is degreased as soon as it passes through the first zone of non-ox furnace. The ambient temperature in the first zone of the non-ox furnace is maintained at between 1150 and 12500C.. The residues of rolling and rolling oil are vapourised in the zone and a light film of oxides is formed over the surface. These oxides are reduced in the last part of the non-ox furnace where the ambient temperature is maintained at between 1200 and 1200 0C, depending upon strip thickness and line speed.

The strip temperature at the exit of non-ox furnace is maintained at between 550 and 7500C, depending on the end properties required and whether soft product or full hard sheets are required.

The non-ox zone is followed by a radiant tubes furnace where the strip annealing takes place in the p0resence of protective atmosphere of nitrogen and hydrogen, The strip temperature in the radiant tube furnace is maintained at between 550 and 7500C,depending upon the end use of the strip.

A precisely controlled electrically heated soaking zone, either vertical or horizontal, is installed in line to achieve the desired properties in the material so as to be suitable for bending, forming and drawing operations.

The soaking zone is followed by controlled jet cooling section to bring down the temperature of the strip very near to the zinc bath temperature prior to entering the zinc bath. The balance processes are the same as stated above for Sendzimir and conventional chemical cleaning lines.

DAGAL PROCESS

DAGAL stands for Degreasing, Annealing and Galvanizing. In this process, the cleaning of strip (degreasing) is obtained by the reaction of furnace atmosphere with strip under a controlled temperature. The degreasing atmosphere is created by injecting a specific mixture of air + fuel ratio under a protective atmosphere of nitrogen + hydrogen in the furnace. The hydrogen percentage in these lines is limited to a maximum of 15% by volume. Except degreasing burners, generally two in number, the whole heating section is indirectly heated through radiant tubes.

Degreasing is done when strip temperature reaches 600 to 6500C, which is why degreasing burners are installed in between zones 1 and 2 of radiant tube furnace. There are a total of three zones in RTF. The length of furnaces and number of radiant tubes in each zone depends upon the capacity of the line.

The radiant tube furnace is followed by electrically heated soaking zone and controlled jet cooling zone as stated in Sendzimir and Non-ox lines. Thereafter, the strip is immersed in molten zinc; the strip enters the bath at a temperature of 475-4800C. The zinc bath temperature is maintained at between 455 and 4600C.

All other processes after the zinc bath remain the same as stated for Sendzimir and Non-ox lines.

ADVANTAGES OF CONTINUOUS SHEET GALVANIZING

Continuous sheet galvanizing offers a number of advantages to both the producer and user. To the producer it makes possible higher production speeds in which cleaning, annealing and zinc coating are combined in one operation. Cleaning is an integral part of the annealing process ad no chemical fluxing is required prior to galvanizing. This has the advantage that control of the amount of aluminium in the zinc bath which in turn controls brittle alloy formation is simple and zinc coatings with only thin intermetallic layers can easily be produced. Coatings produced on strip lines are therefore adherent and flexible and such sheets can be pressed, drawn, lock seamed and bent without flaking of the coating, Strip line operation also simplified coating control and more uniform coatings are possible.

Some important advantages are as under:

1. Competitive costs
2. Low life time cost<>br 3. Long life to first maintenance
4. Reliability
5. Speed of application
6. Complete coating coverage
7. Dual protection
8. Ease of inspection

NEW PRODUCTS AND PROCESSES

The applications of zinc coated products are increasing day by day. The cold rolled steel sheet users are switching over to zinc coated steel sheets for the purpose of aesthetic look as well as overall life of the product particularly in roofing, automobile, and appliances industries. The quality requirements particularly surface finish in all these sectors is similar to CR and electro galvanized sheets. In view of this, the coating industry is also developing new products and processes to meet the specific needs of the end users. The most significant features in the present market demand are the mechanical properties of the base material, formability and drawability of the coated product and the surface finish. The surface finish plays an important role for the characteristics like paintability, weldability, etc.

Coating properties depends on the surface preparation and heat treatment and the parameters like bath temperature, alloying elements (aluminium, antimony, and lead) in the zinc bath, use of spangle minimizer, and skinpass parameters if the material is skinpass. Considerable changes have been made in the area of strip surface preparation and heat treatment. In surface cleaning, the important change is the introduction of a chemical cleaning section before the furnace. The main purpose of this chemical cleaning section is to eliminate iron fines and excess oil from the strip in order to avoid surface defects on coated strip.

NEW ALLOY COATINGS

GALVALUME

It is Zinc – 55% Aluminium – 1.5% Silicon alloy coating on steel. It gives excellent corrosion resistance in many environments and suitable for heat resistant applications. Its use in roofing and cladding is increasing day by day in the country due to its unique appearance and enhanced corrosion free life.

Galvalume, is developed in the mid-60 from a research programme at Bethlehem Steel Corporation, USA. The programme consisted in applying and evaluating coatings containing from 1% to 70% Al, the remainder being zinc. After extensive and long-term testing, the 55% Al-Zn, or Galvalume composition proved to have optimum properties of appearance, corrosion resistance and resistance to heat oxidation. Patents were applied for and granted worldwide to Bethlehem, covering the range of Al-Zn alloy compositions and related processes.

Bethlehem started production in 1972 and the product was accepted and grew in the USA market. Today a number of the world’s most prominent steel companies are licensed to produce and sell 55% Al-Zn coated sheet in the USA, Canada, Europe, Australia, Japan and Korea. The product is sold in these area as under the trade names of Galvalume, Zincalume Aluzink,Zincalit, Aluzinc, Alugalv and Zalutite.

Galvalume’s superior performance in certain environments, as compared to that of conventionally coated sheet products, has been reported. Producers may realize specific operating cost benefits by producing Galvalume rather than galvanized. These benefits result from the lower specific gravity of 55% Al-Zn coatings compared to galvanized. This lower specific gravity makes it possible to apply lighter coatings by weight for an equivalent coating thickness compared to galvanized. As a consequence, coating metal cost savings may be realized with Galvalume. The extent of these savings depends on the relative prices of aluminium zinc.

Galvalume coating because of two phase structure (aluminium rich phase and zinc rich phase) gives the combination of the properties of zinc and aluminium both to some extent and is a suitable materials for some specific environments. The two-phase structure shows heterogeneous behaviour, owing to the presence of the high content of aluminium, which is the most important phase in volume of coating and determinant for its characteristics. The coating is, therefore, primarily of an inactive type. It has better resistance in acid environments but presents less or no sacrificial protection.

Current worldwide applications for Galvalume sheet include those in the construction, appliance, automotive and agriculture markets. Galvalume building panels, both base and painted, comprise the largest tonnage. Next in size are appliance and automotive applications that require good corrosion and heat resistance. Finally, farm applications include those for animal feeders, shelters and storage buildings.

GALFAN

It is Zinc – 5% Aluminium – Mischmetal alloy coating on steel. It has excellent formability and suitable for auto components where the draw is very high. It corrosion resistance is good with sacrificial properties. This product could not get much response in the country. Ispat Ind. Ltd. recently developed a product ZINKALU which is similar in properties containing 5 to 10 % Aluminium and gives good corrosion as well as formability.

Experiments on laboratory scale were conducted to develop other coatings to overcome the deficiencies associated with Galvalume. Several agencies around the world investigated in depth the Zn 5% Al (eutectic) alloy with various minor additions of a number of alloying elements singly and in combination. This alloy system was of particular interest because of its eutectic composition which exhibits high fluidity and lower melting point than that of conventional galvanizing and other Zn-Al alloys.

The centre de Recherches Metallurgiques (CRM) in Liege, Belgium, initiated research for ILZRO in April 1979, to study the effect of addition of a number of elements in improving the coating performance and corrosion resistance of the Zn 5% Al alloy. The new alloy with optimized composition was named “Galfan”. It consists of the classical 95% Zinc-5% Aluminium eutectic alloy with small but critical additions of Mischmetal (a rare-earth alloy made up primarily of cerium and lanthanum). Purity of zinc is crucial, so 99.99% pure EC-grade zinc is used. The Mischmetal used is of commercial grade.

Tests carried out have shown Galfan’s performance in terms of corrosion resistance, formability, paintability and weldability to be far superior to that of conventional galvanized steel and competitive with Galvalume in a number of environments.

Conversion of a standard Sendzimir or Heurtey type continuous galvanizing line to operate with the Galfan alloy requires the molten zinc metal holding pot to be constructed of ceramic or cast iron.

The operation of a Galfan coating line has definite advantages in regard to energy requirements. A Galfan bath due to its eutectic composition (melting point about 380 deg.C (716 deg.F), operates at a temperature slightly lower than a conventional galvanizing bath and much lower than Galvalume, thus lower energy requirement. Today Galfan is licensed to many companies all over the world.

GALVANNEALED

Galvannealed is a product which is annealed after galvanizing. This may be inline or offline. In this the top zinc layer is converted into Zinc-Iron alloy layer by keeping it at a temperature for pre calculated time. The thickness and composition of alloy layer is controlled by the speed and temperature. The product is suitable where welding is critical and very important.

Galvannealed sheet is made along the lines of the conventional hot-dip galvanizing process, merely followed by a heat treatment at approx. 550 deg.c. subsequent to galvanizing. In this process, the not solidified zinc coat is converted into an iron-zinc alloy coat containing 7 to 12% iron. The surface is thus given the typical dull grey appearance.

To obtain favourable mechanical properties and prevent ageing, post-annealing is required here, followed by temper rolling.

The surface thus obtained presents advantages during processing over that of hot dip galvanized sheet, especially during welding and painting, and good corrosion properties. With its coating weight of approx. 150 g/m2, galvannealed sheet is used, for instance, for the exposed panels of doors in small vans and also for the difficult inner sides of the bonnets.

LAVEGAL

Lavegal-30% Al-0.2% Mg-0.2% Si-Zn is actually produced in Italy on the continous strip line at N.Italsider’s Frosinone works. The function of Magnesium addition is to slow down intergranular corrosion of the coating, thus permitting a marked reduction on the Al and Si contents while equaling – and in some case improving on – the superior corrosion resistance of Galvalume.

The Lavegal coating process went into production on the continuous strip line at Lave-metal’s Frosinone work in January 1985. The melting point of Lavegal is 495 0C.

Presence of magnesium ensures that, it provides very good galvanic protection, better than that of all other materials used in the comparison. From the hardness and scratchability point of view also, Lavegal is superior to other coatings investigated.

GRADES WITH DIFFERENT SURFACE FINISH AND PROPERTIES

MECHANICAL PROPERTIES

The mechanical properties are achieved by the designed steel chemistry and the controlled heat-treatment in in-line annealing furnace. A wide range of grades starting from the IF steel grade with super EDD properties to the very high strength grade up to 700 Mpa yield strength are available for various applications in appliances and auto industries. The thickness range 0.20mm to 3.20mm and width up to 1500mm is available in the country with zinc coating in the range of 60 gsm to 750 gsm.

SURFACE FINISH

The surface finish is controlled by the roughness during cold rolling and also after galvanizing as well as the bath composition. The roughness can be achieved in the range of 0.40 Ra to 2.0 Ra with very big spangle to zero spangle with or without skinpass. The skinpass material looks like silver with uniform roughness and is best for painting purpose i.e. in continuous painting line as well as powder coating. It gives excellent paint adherence.

CONCLUSIONS

Continuous hot dip sheet galvanizing lines in the country are equipped with modern facilities like online skinpass, galvannealing, tension leveling. The can meet the requirements of table top flatness, bright and matte surface, big and zero spangle as desired by the customer. The mechanical properties can be achieved by designing a suitable chemistry and heat treatment in online annealing furnace. The products like Galvalume, Galfan, and Galvannealed are being produced in the country for indigenous and export market.

REFERENCES

1. ”Some important factors controlling the surface of hot dip zinc coated steel sheet”, presented in National Conference on Zinc Coated Steel Sheets – Technology & Applications, organized by Indian Lead Zinc Development Association, 3-4 December 2003 in Mumbai.
2. Theme Paper, “Coated Steel Sheet : An Overview”, by V R Subramanian, Executive Director, ILZIC, National Conference on Coated Steel Technology, Applications & Markets, 28-30 April, 1994, organized by ILZIC, New Delhi.
3. “Continuous Galvanized Sheet : Technology & Applications”, by V R Sharma, Vice President, Bhushan Steel & Strips Limited, Sahibabad, National Conference on Coated Steel Technology, Applications & Markets, 28-30 April, 1994, organized by ILZIC, New Delhi.
4. “Coated Steel Sheet : Properties & Applications” N C Jain, Chief Technical Manager, ILZIC, National Conference on Coated Steel Technology, Applications & Markets, 28-30 April, 1994, organized by ILZIC, New Delhi.