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FIELD OF THE INVENTION:
The present invention relates to a method of measuring the dull and bright spangle on hot-dip galvanized steel sheets. More particularly, the present invention relates to a method of quantitative measurement of the area of fractions of dull and bright spangles in galvanized steel sheets.
BACKGROUND OF THE INVENTION
Galvanized steel sheets are manufactured commercially by hot dipping steel strips in a zinc bath. Alloying elements are added in the zinc bath to improve coating properties and surface appearance. The zinc bath typically contains 0.15 to 0.25 wt% aluminium, and up to 0.2 wt % lead/antimony. Aluminum is added to suppress the brittle Fe-Zn alloy layer at the interface between the coating and steel by forming a layer of Fe2AI5 intermetallic compound. Lead and/or antimony is known to produce a structure consisting of large grains of zinc, termed as spangles, at concentration a greater than 0.04 wt%.
In a hot-dip galvanized steel sheet, the regular spangle (Figure 1) produced during galvanizing process are of two types classified by a parameter called reflectivity; one is dull (or frosty) and the other is bright (or mirror) type. The contrasting spangles and their ratio have

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major effect on corrosion properties of the coating and their lasting value.
In the construction sector, spangle morphology is an important factor that determines the acceptability of hot-dip galvanized steel sheets. It has been observed that all galvanized steel sheets with spangles do not have the same surface appearance. In some cases the coating apparently has an overall brilliance but a close examination shows presence of large number of the spangles which are smooth and very reflective. In other cases, the coating has a dull appearance as most of the spangles are frosty with lower reflectivity. Such a feature not only degrades the coating film appearance but also apparently affects its surface reactivity. Experimentation results have confirmed that corrosion attack is more pronounced in the dull spangle than the bright spangle. The galvanized steel sheet with large number of dull spangle is also prone to blackening, particularly when stacked as cut sheets or in coil form. In summary, bright spangle have better surface reflectivity and corrosion resistance property as compared to the dull spangle.
The formulation and distribution of dull and bright spangle in hot dip galvanized sheet is generally influenced by the solidification condition prevailing immediately after the sheet exits the zinc bath. The solubility of the lead present in zinc reduces correspondingly with the progress of solidification, and is almost completely expelled out as lead particles at room temperature. Higher solidification time (i.e.

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lower cooling rate), facilitates the lead particles to form precipitates on the zinc surface. It has been observed that the dull spangles are highly populated with such lead globules. To control the blackening problem in sheets with spangle, it is necessary to reduce the area fraction of dull spangle on the galvanized surface. To achieve such reduction, it is necessary to evaluate the factors that influence formation of different types of spangle and their proportion.
In the prior art method, only qualitative measurements of the contrasting spangles are resorted to. The result of such qualitative measurement fails to provide an accurate and effective data about the ratio of dull to bright spangles developed during hot dip galvanizing. In the absence of such data, the processing parameters cannot be optimized leading to production of inferior quality of hot-dip galvanized steel sheets.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a method for quantitative measurement of the area fractions of dull and bright spangles in galvanized sheets which eliminates the disadvantages of the prior art.
Another object of the invention is to propose a method of quantitative measurement of the area fractions of dull and bright spangles in galvanized sheets which provides an accurate

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measurement of the spangles to optimize the processing parameters during galvanizing.
A further object of the invention is to propose a method of quantitative measurement of the area fractions of dull and bright spangles in galvanized sheets which provides accurate parametric values useful for carrying out a corrosion probability analysis.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Figure 1- Shows a galvanized sheet having regular spangle.
Figure 2- A square grid used in the method of quantitative measurement of the spangles according to the present invention.
Figure 3- Shows marked grids indicating areas covering dull spangles used according to the method of the invention.
Figure 4- Marked grids for a first sample showing dull area and bright area measured according to the present invention.
Figure 5- Marked grids for a second sample showing dull and bright area measured according to the invention.

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BRIEF DESCRIPTION OF THE INVENTION
Accordingly, there is provided a method of quantitative measurement of the area of fractions of the dull and bright spangles in galvanized steel sheets for optimizing the processing parameters of the galvanized steel sheets. The method comprises the steps of cutting a full width galvanized sheet from a coil. Firstly, at least two A4-size sample-pieces are cut from two locations of the full width sheet. One piece is cut from 100mm of the edge, and the other one from the center of the sheet. A square section of size 80mm x 80mm is then made on both the A4 size sample sheets. A square grid (~1mm) is produced through a scanner on a transparent A4 size graph paper. The transparent graph paper is laid over the marked area of ~ 80mm x 80mm (80grids x 80grids) in both the sample sheets. The dull spangles are coloured by marking all the small grids covering the dull area. The number of small square grids covering the dull area is counted. The counted number is assigned a value 'X', which represents the dull area. The number of grids covering the bright area 'Y' is then determined by subtracting the dull area 'X' from the total number of grids (6400). The percentage dull area is calculated by dividing the dull area 'X' by the total number of grids (6400) and multiplying by 100. The percentage bright area [Y/(6400X100)] the ratio of dull to bright area (X/Y) is then calculated.

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DETAILED DESCRIPTION OF THE INVENTION
Figure 1 shows a galvanized sheet having regular spangle. According to the invention the method of quantitative measurement of dull and bright spangles in a sheet sample consists of the following steps:-
- cutting a full width galvanized sheet from a coil;
- cutting atleast two A4- size sample-pieces from two locations of
the full width sheet, one piece being cut from 100mm of the
edge, and the other one from the center of the sheet;
- making a square section of size 80mm x 80mm on both the A4
size sample sheets;
- producing a square grid (~1mm) through a scanner on a
transparent A4 size graph paper;
- laying the transparent graph paper over the marked area of ~
80mm x 80mm (80grids X 80grids) in both the sample sheets;
- colouring the dull spangles by marking all the small grids
covering the dull area;
- counting the numbers of small square grids covering the dull
area, the counted number being assigned a value 'X' which
represents the dull area,

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- obtaining the number of grids covering the bright
area 'Y' by subtracting the dull area 'X' from the total number of
grids (6400);
- calculating the percentage dull area by dividing the dull area 'X'
by the total number of grids (6400) and multiplying by 100;
- calculating the percentage bright area [Y/(6400 x 100)];
- calculating the ratio of dull to bright area (X/Y).
Example:
Two sets of samples (A and B) were collected from coils with different contrast levels. The quantitative measurement of the dull and bright spangles was carried out using the described method. As shown in Figure 4, 65% of the area is bright and 35% dull in case of sample A. Figure 5 shows that 32% bright and 68% dull in case of sample B.
A precise quantitative evaluation technique for spangle type distribution could be a useful tool for measuring the ratio of bright to dull spangle during the manufacture of galvanized sheet. Such a method can optimize the processing parameters of the galvanized steel sheets. The values would also be useful for carrying out detailed corrosion studies.

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WE CLAIM;
1. A method of quantitative measurement of the area of fractions of the dull and bright spangles in galvanized steel sheets for optimizing the processing parameters of the galvanized steel sheets. The method comprises of:
- cutting a full width galvanized sheet from a coil;
- cutting atleast two A4- size sample-pieces from two locations of
the full width sheet, one piece being cut from 100mm of the
edge, and the other one from the center of the sheet;
- making a square section of size 80mm x 80mm on both the A4
size sample sheets;
- producing a square grid (~1mm) through a scanner on a
transparent A4 size graph paper;
- laying the transparent graph paper over the marked area of ~
80mm x 80mm (80grids X 80grids) in both the sample sheets;
- colouring the dull spangles by marking all the small grids
covering the dull area;
- counting the numbers of small square grids covering the dull
area, the counted number being assigned a value 'X' which
represents the dull area.
- obtaining the number of grids covering the bright area 'Y' by
subtracting the dull area 'X' from the total number of grids
(6400);

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- calculating the percentage dull area by dividing the
dull area 'X' by the total number of grids (6400) and multiplying
by 100;
- calculating the percentage bright area [Y/(6400 x 100)]; and
- calculating the ratio of dull to bright area (X/Y).
2. A method of quantitative measurement of the area of fractions of the dull and bright spangles in galvanized steel sheets for optimizing the processing parameters of the galvanized steel sheets as substantially described and illustrated herein with reference to the accompanying drawings.
Dated This 12th day of April, 2006