FIELD OF THE INVENTION

The present invention relates to a method for effective performance of ceramic coat on galvanizing pots adapted for the reduction of the formation of dross at the galvanizing pot and molten zinc interface. Particularly the present invention relates to a method for the reduction in the galvanizing pot wear. More particularly the present invention relates to a method for pot wear measurement.

BACKGROUND AND PRIOR ART OF THE INVENTION

In a modern continuous hot galvanizing line, the galvanizing pot is lined with ceramic tiles internally and is equipped with induction heaters projected inside the pot. However, in many hot dip galvanizing lines, installed before 1990, galvanizing pots made of low carbon steel plates, are used. These pots contain molten zinc at around 460oC aided with external electrical heaters. During the process of galvanizing, iron zinc inter-metallic, called dross, is generated both at the moving strip surface and at the pot interface. It is a waste product and reduces availability of pure zinc in the bath.

The prior art was only to remove the dross formed in the bath so as to facilitate the galvanizing process. There was no control measure adopted. These pots are changed periodically to prevent excessive pot wear and leakage of molten zinc. However, a few traditional galvanizing lines have started applying ceramic coat before inserting a new pot. But, this method as such leads to minimal operational advantage.

US 4,056,657 discloses a ferrous metal strip having a continuous hot-dip coating consisting essentially of the zinc-5% aluminum eutectic alloy which contains as the only metallic additive a metal which reduces the surface tension of the coated bath, such as lead, antimony or tin, with the total amount of the metallic additives being a maximum of about 0.1 wt. %, and the coating being formable and exhibiting good corrosion resistance when exposed to marine-type environments and having a bright, smooth surface free of ripples and spangles which interfere with paintability and weldability.

US 5,399,376 disclose method and apparatus for meniscus coating one or two sides of steel strip with a metal or metal alloy. The apparatus includes a horizontally disposed coating tray for containing molten coating metal, means for maintaining the temperature of the coating metal above the melting point of the coating metal, means for moving steel strip transversely past a departure lip positioned on one side of the coating tray and means for maintaining the level of the coating metal in the coating tray relative to the upper elevation of the departure lip so that an uninterrupted flow of the coating metal can be delivered over the departure lip to a surface of the strip. The coating tray may be rotatably mounted for adjusting the level of molten metal in the coating tray. The coating tray also may include means for lateral displacement for positioning the departure lip a predetermined distance away from the strip. The terminal end of the departure lip preferably includes a planar upper surface having an acute angle of at least 15.degree. relative to the horizontal plane of the coating tray. Non-oxidizing gas may be passed through a jet nozzle to control the coating thickness on the strip.

US 2008/0274006 disclose a submerged pot roll and other articles for use in galvanizing baths including a metallurgically bonded super alloy cladding layer on a steel core layer. The cladding layer improves the corrosion resistance and dross buildup of the article and improves service life while reducing costs.

US 2004/0121083 disclose a method and apparatus for rapid and efficient changing-over the coating molten metal in a continuous steel strip coating line from a first aluminum-based coating composition to a second zinc-based coating comprising a first ceramic-lined tank provided with heating means for controlling the temperature of the molten metal in the first tank and a second smaller removable tank without heating means for containing the second coating molten metal adapted to be placed within the first tank, wherein the wall of the second removable tank is effectively heat conductive and preferably has a wall downwardly converging so that its positioning within the first tank is facilitated and damage to its ceramic lining is avoided. The weight of the second tank is mostly supported by the floating forces of the molten bath in the first tank. During use of the second tank, the composition of the first bath is adjusted away from its coating composition by removing a large portion of the aluminum-zinc content and adding zinc to the remaining bath; so that its melting point is lowered to prevent solidification at the zinc coating temperatures, preferably to within 400˚C and 480˚C.and its density is preferably in the range of 5.5 to 6.0 tons/m3. The volume of the second tank is designed so as to simplify the adjustment of composition of the first bath by withdrawing a predetermined volume of the aluminum-based coating metal (sufficient to accommodate the placement and immersion of the second tank in the first tank) and adding molten zinc thus obtaining the desired melting temperature and density and filling volume in the first tank.

The drawbacks of the above mentioned prior art is that the proper application technique for effective dross reduction is not known. The measurement technique for the pot wear was not known effectively.

Thus there is a need to provide a method for electrically heated pots while restricting iron diffusion from pot to molten zinc. For enhanced coating life and effectiveness, the present inventors invented the present method.

OBJECTS OF THE INVENTION

One object of the present invention is to overcome the disadvantages / drawbacks of the prior art.

A basic object of the present invention is to provide a method for the reduction of the formation of dross at the galvanizing pot and molten zinc interface

Another object of the present invention is to provide a method for the reduction in the galvanizing pot wear.

Yet another object of the present invention is to provide an effective method for the reduction of the formation of dross at the galvanizing pot and molten zinc interface with ceramic coat.

Yet another object of the present invention is to provide a method for the pot wear measurement.

These and other advantages of the present invention will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings.

SUMMARY OF THE INVENTION
According to one of the aspect of the present invention the system is provided a method for dross and wear reduction in steel galvanizing pots, said method comprising steps of

cleaning of said pot surface adapted to get roughness and scale free surface;
wiping of said pot surface;
mixing of a coat in a trough with a bowl for homogenization of settling compound in the solvent;
coating of said coat in said pot with a brush to have the first coating layer ;
drying /curing of said coat in the pot for the first time;
mixing of a coat in a trough with a bowl for the second time;
coating of said coat in said pot with a brush to have the second coating layer adapted to cover non-adhesive spots;
drying /curing of said coat in the pot for the second time;
maintaining stipulated bath/molten zinc temperature;
maintaining stipulated aluminum content in zinc melt/bath.

According to another the aspect of the present invention there is provided a method for pot wear measurement , said method steps comprising steps of :

marking of three measurement spots on all four walls of the used pot in a vertical orientation;
selection of two sets of vertical spots on the walls having welds such that
one set on weld zone and the other on pot body;
fine hand grinding of external surface at the marked spots;
application of rust preventive grease on ground spots;
measurement the Ultrasonic thickness at these spots ;
finding the difference of measured thickness from original unused pot thickness in mm.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Other features as well as the advantages of the invention will be clear from the following description.
In the appended drawing:

Figure 1 illustrates a graph showing the comparison of dross reduction with the prior art and modified practice.
Figure 2 illustrates layout of measurement spots on four walls of pot external surface.
Figure 3 illustrates a graph showing the comparison of reduction of pot wear with the prior art and modified practice.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method for effective performance of ceramic coat on galvanizing pots for the reduction of the formation of dross at the galvanizing pot and molten zinc interface. The method is for the reduction in the galvanizing pot wear and pot wear measurement.
The ceramic coat, added with graphite coat has the effect of providing a barrier between the pot and the molten zinc in a combined form.

The present method discloses not only usage of ceramic and graphite based coats on internal surface of steel pots but also associated measures for its effectiveness.

The material and coating thickness should be such as to permit electrical heating while restricting iron diffusion from pot to molten zinc. For enhanced coating life and effectiveness, a proper methodology is suggested. It includes pot surface preparation with wire brush to achieve a scale free surface and average roughness of 2~3 microns. The ceramic coat should be mixed well for homogenization of settling compound in the solvent. The coating thickness should be around 1.2 kg per square metre per layer. Minimum two coats are needed to cover the non adherent spots. The curing time per coat should be 18 hours in ambient temperature. The bath temperature was reduced by around 6oC and aluminium level was increased by 0.1% in the zinc bath. Dross reduction of 0.8 kg/ton could be achieved above normal coat application technique. With improved coating technique, the pot wear could reduce by 1.1 x10-3 mm per ton.

The main constituent of the coat is a ceramic compound. However, another constituent is graphite.

The coating material comes with all its constituents in proper mix from the supplier.

The process steps at application site is as follows:
a. Surface preparation (wire brush cleaning and cotton cloth wiping) of new galvanizing pot,
b. Pouring and manual mixing of coat in a trough, (trough is a separate mixing apparatus). First coat application with brush (1.2 kg per sq. meter for one coating layer),
d. Drying/curing for 18 hours,
e. Pouring and manual mixing of coat in a trough,
f. Second coat application and
g. Final Drying/curing for 18 hours.
Further process steps are related to subsequent usage time and involve process control:
h. maintain stipulated bath/ molten zinc temperature and
i. maintain stipulated aluminium content in zinc melt.

The effect is adequate reduction in dross generation is as shown in figure 1.

The important steps are:
a. Pot surface preparation with a suitable roughness,
b. Quantification of coating material to be adopted and time of drying,
c. Repeat coat to cover non adhesive spots,
d. complementary effect of process parameters like temperature and aluminium content on coating effectiveness and e. method of pot wear assessment.

The coating material contains some sediments and colloidal particles. After pouring it from the container to the trough, the sediments settle at the bottom and the thinner type solvent remains on top. So, it needs a proper mixing with a bowl. The non adherent spots are found on the surface of the pot to be coated. Generally, metallic oxides, sulphides or silicates present at the surface do not bond with the coat easily. The Metallic oxides (scale), sulphides or silicates are isolated undesired impurities found on the steel plate used for pot fabrication. Wire brushing of pot is primarily to remove them from pot interior surface. In case, still some impurities remain, they will resist bonding with ceramic coat. Therefore, there is a need for double layer coating. Pot wear measurement steps explained below explains it.

The figures 2 illustrates pot wear measurement spots after usage and comparative test results are in figure 3.

Here, the curing time is 18 hours. In prior art, either there is no coat application or curing time of around 6-8 hours. The advantages are reduced dross generation and pot wear.

Here also, with reduced bath temperature and increased aluminium, the advantages are reduced dross generation and pot wear.

A method for pot wear measurement was also developed. Three spots along the pot height were marked on all sides of the pot. Additional three spots were marked along the pot weld interface. With fine grinding and lubricating, the pot wall thickness was determined with an ultrasonic thickness gauge. The reduced pot wear also indicates the effectiveness of the ceramic coat.

The selected spots on external surface are subjected to fine grinding, application of grease for rust protection and measurement with ultrasonic probe at these spots for the wall thickness. The selected wear measurement spots on four walls of the pot are shown with numbers in Fig. 2. The maximum wear data is considered in mm. When divided by production in tons it gives the specific pot wear.

According to the first embodiment of the present invention there is provided a method for dross and wear reduction in steel galvanizing pots. The method comprising steps of cleaning of the pot surface adapted to get roughness and scale free surface; wiping of the pot surface; mixing of a coat in a trough with a bowl for homogenization of settling compound in the solvent; coating of the coat in the pot with a brush to have the first coating layer ; drying /curing of the coat in the pot for the first time; mixing of a coat in a trough with a bowl for the second time; coating of the coat in the pot with a brush to have the second coating layer adapted to cover non-adhesive spots; drying /curing of the coat in the pot for the second time; maintaining stipulated bath/molten zinc temperature and maintaining stipulated aluminum content in zinc melt/bath.

The coat is ceramic and graphite, mainly ceramic. The cleaning is done with wire brush and wiping is done with cotton cloth.
The first coating layer thickness is 1.2 kg/sq.m and second coating layer is 1.2 kg/sq.m.
The drying/curing duration is for 18 hours in ambient temperature and the roughness of the surface of the pot is 2~ 3 microns. Further the bath/molten zinc temperature is reduced by around 6oC and the aluminum content is increased by 0.1% in the zinc bath. The dross reduction is of 0.8 kg/ton. The pot wear reduction is by 1.1 x10-3 mm per ton.

According to the second embodiment of the present invention there is provided a method for pot wear measurement.

The method steps for Pot wear measurement are:
• Marking of three measurement spots on all four walls of the used pot in a vertical orientation. On walls having welds, two sets of vertical spots are selected. One set on weld zone and the other on pot body.
• Fine hand grinding of external surface at the marked spots
• Application of rust preventive grease on ground spots
• Ultrasonic thickness measurement at these spots. The measuring probe is kept perpendicular to the pot wall and the wall thickness is displayed in the digital meter of the instrument. Such practice is not normally used for vertical wall thickness measurement of galvanizing pots.
• Find the difference of measured thickness from original unused pot thickness in mm. When this data is divided by the tonnage of material processed, specific wear in mm/ton is obtained. It is a numerical division of wear data in mm by the production data in tons.

Features:
• Procedure for ceramic coat application
• Usage of ceramic coat in galvanizing pots made of low carbon steel
• Optimized galvanizing bath parameters like temperature and aluminium level
• Measurement technique to assess pot wear

WE CLAIM:

1. A method for dross and wear reduction in steel galvanizing pots, said method comprising steps of :

cleaning of said pot surface adapted to get roughness and scale free surface;
wiping of said pot surface;
mixing of a coat in a trough with a bowl for homogenization of settling compound in the solvent;
coating of said coat in said pot with a brush to have the first coating layer ;
drying /curing of said coat in the pot for the first time;
mixing of a coat in a trough with a bowl for the second time;
coating of said coat in said pot with a brush to have the second coating layer adapted to cover non-adhesive spots;
drying /curing of said coat in the pot for the second time;
maintaining stipulated bath/molten zinc temperature;
maintaining stipulated aluminum content in zinc melt/bath.

2. Method as claimed in claim 1 wherein said coat is ceramic and graphite, mainly ceramic.
3. Method as claimed in claim 1 wherein said step of cleaning is with wire brush.
4. Method as claimed in claim 1 wherein said step of wiping is with cotton cloth.
5. Method as claimed in claim 1 wherein said step of first coating layer thickness is 1.2 kg/sq.m.
6. Method as claimed in claim 1 wherein said step of second coating layer thickness is 1.2 kg/sq.m.
7. Method as claimed in claim 1 wherein said drying/curing duration is for 18 hours in ambient temperature.
8. Method as claimed in claim 1 wherein said roughness of the surface of said pot is 2~ 3 microns.
9. Method as claimed in claim 1 wherein said bath/molten zinc temperature is reduced by around 6oC.
10. Method as claimed in claim 1 wherein said aluminum content is increased by 0.1% in the zinc bath.
11. Method as claimed in claim 1 wherein said dross reduction is of 0.8 kg/ton.
12. Method as claimed in claim 1 wherein said pot wear reduction is by 1.1 x10-3 mm per ton.
13. A method for pot wear measurement , said method steps comprising steps of :
marking of three measurement spots on all four walls of the used pot in a vertical orientation;
selection of two sets of vertical spots on the walls having welds such that
one set on weld zone and the other on pot body;
fine hand grinding of external surface at the marked spots;
application of rust preventive grease on ground spots;
measurement the Ultrasonic thickness at these spots ;
finding the difference of measured thickness from original unused pot thickness in mm; and
dividing data by the tonnage of material processed to obtain specific wear in mm/ton .

14. Method as claimed in claim 14 wherein said step for the measurement of Ultrasonic thickness at these spots comprises keeping the measuring probe perpendicular to the pot wall and the wall thickness is displayed in a digital meter of the instrument.

ABSTRACT

The present invention relates to a method for effective performance of ceramic coat on galvanizing pots for the reduction of the formation of dross at the galvanizing pot and molten zinc interface. A method for dross and wear reduction in steel galvanizing pots, the method comprises steps of cleaning of the pot surface adapted to get roughness and scale free surface; wiping of the pot surface; mixing of a coat for homogenization of settling compound in the solvent; coating of the coat in the pot with a brush to have the first coating layer; drying /curing of the coat in the pot for the first time; mixing of a coat for the second time; coating of the coat in the pot with a brush to have the second coating layer adapted to cover non-adhesive spots.