FIELD OF APPLICATION
The present invention relates to a method for etching of cold rolled and annealed
interstitial free (IF) steel sheet. In particular the invention relates to an improved
Marshall's reagent used for etching cola rolled and annealed interstitial free steel sheet.
BACKGROUND OF THE INVENTION
One of the most popular and standard method of revealing the microstructure of steel is
through application of specific chemical reagent to the steel surface which attacks /
corrodes preferentially some features in the microstructure to reveal it while viewing
through optical microscope or any other tools.
The cold rolled and annealed sheets of interstitial free (IF) steels are abundantly used for
the fabrication of automobile component. It is important to characterize this commercially
important material for various properties like strength, ductility, fatigue behavior etc. in
correlation to its microstructure (consisting of ferrite).
There are specific etchants for various types of steels depending upon which feature is to
be revealed. However, it is necessary to optimize or modify a particular etching process for
application to a specific material to get optimum results, to avoid any associated damage
or deterioration of the surface and to eliminate any undue error in measurement or
interpretation.

The standard and most popular method of revealing microstructure in single phase ferritic
steel is done through the use of chemical etching by Nital. Nital is basically a solution of
concentrated nitric acid (2-5 % by volume) in methyl / ethyl alcohol. When Nital solution is
applied over polished steel surfaces, it attacks / selectively corrodes the ferrite grain
boundaries and thus reveals them. The electrochemical etching using Nital is the standard
and most popular method of revealing ferrite grain boundaries in low and medium carbon
steels. But it has been found that ferme grain boundaries in cold rolled and cold rolled and
annealed interstitial free steel sheets (both surface and the cross section) is not completely
revealed by etching with Nital. Althoudh ASTM Handbook (Volume 9, Page 599) prescribes
Marshall's Reagent, it was found that for specific application to cold rolled and annealed IF
steel sheets this Reagent cannot be used directly as prescribed in the handbook and should
be modified to obtain optimum results and to avoid any undue damage to the etched
surface.
Some problems are faced during etching samples of cold rolled and annealed IF steel sheet
with conventional Marshall's Reagent.
The ASTM Handbook prescribes Marshall's reagent for IF steel. The composition of the
chemical solution is given. But some of the vital details are missing as shown in Table 1.

There was therefore, a need for an optimized method of revealing microstructure in cold
rolled and annealed IF steel sheet.
It was found that there are problems n etching with Nital as it is not capable of revealing
all the grain boundaries in cold rolleo and annealed IF steel. Hence the samples of cold
rolled and annealed IF steel sheets etched with nital exhibits islands where no grains are
revealed. Only a few packets of disjointed ferrite grains are seen. Such incomplete
revelation of microstructure may result into serious error and mis-interpretation in
measurement of grain size of the surface and cross section using automatic image analysis
and other studies where the grain boundaries are to be delineated completely.
SUMMARY OF THE INVENTION
The main object of the present invention therefore, is to provide an improved Marshall's
reagent for controlling the rate of etching reaction.
Another object of the present invention is to avoid etching pits introduced by the echants.
Yet another object of the invention is to avoid over etching for preventing the darkening of
the surface.
These and other objects of the invention are achieved as summarized in Table 2.

Etching for extended period may cause darkening of the surface. A moderate amount of
graying can be cleaned through ultrasonic cleaning using ethylene-diamine tetra-acetic acid
(EDTA), it is a method of cleaning the surfaces of a metallographic sample using certain
solution and an ultrasonic vibrator. The specimen is generally kept in a suitable container
(breaker etc.) full of solution and then the entire container along with specimen in it is
placed in the ultrasonic vibrator. The liquid solution in contact with the specimen cleans
the surface from oil, dirt and other impurities very effectively when the ultrasonic vibration
is applied 3 % ethylene-diamine tetra-acetic acid (EDTA) solution is a specific solution that
can be used for ultrasonic cleaning of metallographic sample of steel.
The present invention provides a method for etching of cold rolled and annealed interstitial
free (IF) steel sheet, comprising the steps of: preparing a chemical etchant in two parts, a
first part comprising oxalic acid, sulphuric acid and distilled water at room temperature,
and a second part comprising 30% solution of H2O2 at a temperature of 15-20° C for
controlling the rate of etching; mixing freshly made solutions of said first and second part
in equal proportion; etching samples of said steel sheets with said chemical etchant for 30-
40 seconds; and ultrasonic cleaning of the etched surface for about 15-20 minutes.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention can now be explained with the help of the accompanying drawings where
Figure 1 shows the microstructure of a cold rolled and annealed IF steel sheet
revealed by etching with Nital. The resulting microstructure exhibits
areas where no grain boundaries are revealed shown with arrows.
Figure 2 shows the darkened surface of a cold rolled and annealed IF steel surface
darkened by using Marshall's reagent with HF acid. Etching time 2-3 seconds.
Figure 3 shows the surface of IF steel sheet etched with Marshall's reagent
showing pitting with arrows. The pits developed during etching
as the sample was not absolutely stationary during etching.
Figure 4 shows the microstructure of cold rolled and annealed IF steel sheet revealed
using modified Marshall's eagent after etching for 30 seconds.
Figure 5 shows the microstructure of surface of IF steel sheet as seen in a SEM 5(a)
is etched with Marshall's reagent showing complete network of grains as all
the grain boundaries are revealed and 5(b) is etched with Nital showing
areas where no grains are revealed.

DETAILED DESCRIPTION
Figure 1 shows the microstructure of IF steel sheet (surface and cross section) etched with
Nital. There are areas where grain boundaries are not revealed.
Figure 2 shows a micrograph showing darkened and over-etched microstructure revealed
by conventional Marshall's reagent containing HF acid. The above mentioned two
situations can be avoided by etching old rolled and annealed sheets of IF steel sheet using
the improved Marshall's reagent of the present invention.
The modified Marshall's reagent of he present invention can be prepared in two parts.
The first part comprising oxalic acid, sulphuric acid and distilled water; and the second part
consisting of a 30 % solution of H2O. The first part is taken at room temperature whereas
the second part can be under refrigeration at around 15 - 20° C.
Although the first part cannot be stored for more than two days, the second part of the
reagent comprising readymade 30% H2O2 solution available in dark bottles can be taken
out for instant use.

During etching the first part and the second part are to be freshly mixed in equal
proportion. Mounted samples can be immersed in the etching reagent vertical down and
should be kept absolutely stationary to avoid pitting. If the sample size is too large for
mounting, then the etchant can be swabbed on the sample surface with the help of a
cotton ball by keeping the sample stationary to avoid pitting. Ultrasound cleaning can be
done with 3% EDTA solution for removing graying of the steel sheet during etching.
Ultrasonic cleaning of the etched surface should be for not more than 20 minutes to avoid
pitting of the surface.
After required time the sample can be washed with water and alcohol followed by blow
drying.
The inventive method of the present invention is summarized in Table 3.


WE CLAIM
1. A method for etching of cold rolled and annealed interstitial free (IF) steel sheet,
comprising the steps of:
- preparing a chemical etchant in two parts, a first part comprising oxalic acid,
sulphuric acid and distilled water at room temperature, and a second part
comprising 30% solution of H2O2 at a temperature of 15-20° C for controlling
the rate of etching;
- mixing freshly made solutions of said first and second part in equal
proportion;
- etching samples of said steel sheets with said chemical etchant for 30-40
seconds; and
- ultrasonic cleaning of the etched surface for about 15-20 minutes.
2. The method as claimed in claim 1, wherein said samples of steel sheets arc
mounted and immersed in the etchant solution vertically down to keep said
samples stationary, so as to avoid pitting.

3. The method as claimed in claim 1, wherein the etchant solution can be swabbed on
the sample surface with the hep of a cotton ball, when the sample size is too large
for mounting.
4. The method as claimed in claim 1, wherein for ultrasonic cleaning comprises using
ethylene-diamine tetra-acetic acid (EDTA) solution and an ultrasonic vibrator for
removing graying of the steel sneet during etching.
5. The method as claimed in claim 1, wherein said ultrasonic cleaning of the etched
surface should be for not more than 20 minutes to avoid pitting of the surface.
6. The method as claimed in claim 1, wherein after etching the sample steel sheet can
be washed with water and alcohol followed by blow-drying.
7. A method for etching of cold oiled and annealed interstitial free (IF) steel sheet,
substantially as herein described and illustrated in the accompanying drawings.

The main object of the present invention therefore, is to provide an improved Marshall's
reagent for controlling the rate of etching reaction. Another object of the present invention is to avoid etching pits introduced by the echants. Yet another object of the invention is to avoid over etching for preventing the darkening of the surface.
These and other objects of the invention are achieved as summarized in Table 2.
Etching for extended period may cause darkening of the surface. A moderate amount of
graying can be cleaned through ultrasonic cleaning using ethylene-diamine tetra-acetic acid
(EDTA), it is a method of cleaning the surfaces of a metallographic sample using certain
solution and an ultrasonic vibrator. The specimen is generally kept in a suitable container (breaker etc.) full of solution and then the entire container along with specimen in it is placed in the ultrasonic vibrator. The liquid solution in contact with the specimen cleans the surface from oil, dirt and other impurities very effectively when the ultrasonic vibration is applied 3 % ethylene-diamine tetra-acetic acid (EDTA) solution is a specific solution that can be used for ultrasonic cleaning of metallographic sample of steel.
The present invention provides a method for etching of cold rolled and annealed interstitial
free (IF) steel sheet, comprising the steps of: preparing a chemical etchant in two parts, a
first part comprising oxalic acid, sulphuric acid and distilled water at room temperature,
and a second part comprising 30% solution of H2O2 at a temperature of 15-20° C for
controlling the rate of etching; mixing freshly made solutions of said first and second part
in equal proportion; etching samples of said steel sheets with said chemical etchant for 30-
40 seconds; and ultrasonic cleaning of the etched surface for about 15-20 minutes.