FIELD OF INVENTION
The present invention relates to a process for galvannealed coatings in
steel sheets/strips without galvannealing operation.
BACKGROUND OF THE INVENTION
The Zinc-base coated steel sheets have been extensively used in the
automotive industry to enhance the corrosion protection. Several
technologies have been developed to produce zinc coatings. Among them,
galvannealing process has received more attention because galvannealed
coatings exhibit corrosion resistance, better paintability and good
weldability.
The variables involved in producing the desired galvanneal
micro structure and properties are complex. A Good process control
requires that the effects of heating rate, holding temperature and time
and cooling rate on the Fe-Zn reaction kinetics be well understood so
that the optimum coating for the desired properties can be obtained.
Besides that, processing variables, the variations in bath chemistry and
the substrate composition all contribute to the final microstructure.

The incoming steel is most often "full-hard" sheet coming directly from a
cold reduction mill. The cold reduction mill is used to decrease the
thickness of the hot-rolled pickled strip to the desired thickness. The
cold rolling process makes the steel very hard with limited formability.
For heavier sheet thicknesses, the product may be entered into the
coating line directly after hot rolling and pickling. In either case, the
sheet is uncoiled and welded to the tail end of the coil ahead of it in the
processing line. It is thereafter cleaned in a process unit that typically
uses an alkaline liquid combined with brushing, rinsing and drying.
From the cleaning section, the strip passes into the heating (annealing)
furnace to soften the full-hard strip and impart the desired strength and
formability to the steel.
In the annealing furnace, the strip is maintained under a reducing gas
atmosphere to remove any vestiges of oxide on the steel furnace. The gas
atmosphere is composed of hydrogen and nitrogen. This oxide reduction
step (iron oxide converted to iron by reacting with hydrogen), is very
important to obtain complete wetting of the steel surface during the short
time, that it is immersed into the coating bath.
At the exit end of the furnace steel strip is cooled to 460°C by nitrogen.
The furnace is connected directly to the molten coating bath by a snout

to prevent any air from re-oxidizing the heated steel strip prior to it
reacting and alloying with the molten coating metal. As mentioned above
strip is cooled to 460°C before entering in the zinc bath and therefore
460°C is referred as strip entry temperature in commercial production
line. In the Zinc bath strip passes around a submerged roll and then
exits the bath in a vertical direction. At the exit point, a set of gas knives
(usually high pressure air), wipe off excess molten metal, leaving behind
a closely controlled thickness of molten metal. The steel strip remains in
the molten Zinc bath for 3-4 seconds.
Galvannealed coating is produced by in-line heat treatment of the zinc
coating, which is carried out by heating the coated steel immediately
after zinc bath to approximately 550°C and holding there for few
seconds. The Zinc bath of Zn-0.13% A1 is maintained at 460°C for
commercial GA production. Galvannealing leads to alloying of zinc with
the iron by diffusion. After this heat treatment, the average iron content
of galvannealed coating is about 10 wt% which varies from 7 wt% (at the
top of the coating) to 23 wt% (near steel interface) and coating becomes
layered structure of Fe-Zn intermetallic phases. The optimum alloyed
galvannealed microstructure consists of thin iron rich gamma (I') layer of
about 1µm thickness and an overlay containing delta (8) phase,
interspersed with a small amount of zeta (ζ) phase, at the top. The range
of iron content of the ζ, δ and I' phases are 5-6, 7-11 and 23-28 wt%

respectively. The schematic of production line is shown in Figure 1.
Typical micrograph of galvannealed coating showing stacking of different
Fe-Zn intermetallic layers has been shown in Figure 3.
OBJECTS OF THE INVENTION
It is therefore, an object of the present invention to propose a process for
galvannealed coating in steel sheets/strips without galvannealing
operation which eliminates the disadvantages of existing state of art.
Another object of the present invention is to propose a process for
galvannealed coating in steel sheets/strips without galvannealing
operation which reduces the process step for production.
A further object of the present invention is to propose a process for
galvannealed coating in steel sheets/strips without galvannealing
operation which save associated cost as compared to cost of conventional
galvannealing process.
A still further object of the present invention is to propose a process for
galvannealed coating in steel sheets/strips without galvannealing
operation which makes more attractive for the auto customer for less
powdering effect.

An yet further object of the present invention is to propose a process for
galvannealed coating in steel sheets/strips without galvannealing
operation which ensures good spot weldability on the new coatings.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 - shows a schematic line diagram used for commercial production
of galvannealed coatings
Fig. 2 - shows a schematic line diagram used for new invention
Fig. 3 - Typical micrograph of galvannealing coating showing stacking of
different Fe-Zn intermetallic layer
Fig. 4 - Comparison of coating phases for plant and lab samples
Fig. 5 - Comparison of powdering behaviour for plant and lab samples
Fig. 6 - Comparison of spot weldability for plant and lab samples
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
The present invention relates to a process which would develop
Galvannealed coating phases without galvannealing operation. Thereby
present innovation eliminates a very critical step which determines

quality of the product and at the same time saves associated cost of
conventional galvannealing operation.
As shown in Fig.2, a process for galvannealed coating in steel
sheets/strips without galvannelaing operation comprising the steps of:
(1) Cleaning the steel sheets/strips by passing the uncoiled
sheets/strips (1) through an electrolytic cleaning chamber (2)
wherein brushing the steel sheets/strips with alkaline liquid,
rinsing and drying take place.
(2) Annealing the cleaned steel sheets/strips by passing the uncoiled
hard sheets/strips through a heating chamber (3) at 600-650°C
under a hydrogen or nitrogen controlled furnace atmosphere to
impart strength and formability property.
(3) The heated steel sheets/strips are quenched in a coating bath (4)
containing a molten mixture of Zn-(0.14-0.16%A1) for full wetting
within 3-4 seconds.
(4) Zinc coated sheets/strips are skin passed (5) for uniform
lamination of coated material.
(5) The zinc coated sheets/ strips are coated with organic compound
in a organic coating chamber (6).
(6) The steel sheets/strips are recoiled in a recoiler (7).

These coated steel sheets/strips are when examined it reveals the same
micro structure as that of conventional galvannealing coating.
The coating were obtained on the steel strip as per new process. The
coating properties were evaluated and compared with the commercially
available Galvannealed strips. The commercially available Galvannealed
and coatings of per new process have been named as "Plant" and "Lab"
respectively in the following figures. Quantification of the coating phases
and their comparison for plant and lab samples has been shown in the
Figure 4. Similarly powdering and spot weldability of the plant and lab
samples have been evaluated and compared in the Figure 5 and 6
respectively.
The new coating process does not require any galvannealing treatment
however the coating micro structure of the new coating is similar to the
commercial Galvannealed coatings. The powdering shown by new coating
is very less as compared to commercial product. The good powdering
resistance (less powdering) makes new coating even more attractive for
the auto customers. The spot weldability of the new coating is at par with
the commercial product.

WE CLAIM
1. A process for galvannealed coating in steel sheets/strips without
galvannealing operation comprising the steps of:
- cleaning the steel sheets/strips (1) by passing the uncoiled
sheets/strips through an Electrolytic cleaning chamber wherein
brushing the steel sheets with alkaline liquid, rinsing and drying
take place;
- annealing the cleaned steel sheets/strips by passing the uncoiled
hard sheets/strips through a heating chamber (3) at 600-650°C
under a hydrogen or nitrogen controlled atmosphere to impart
strength and formability property;
- dipping the heated sheets/ strips in a coating bath (4) containing a
molten mixture of Zn-(0.14-0.16% Al) for full wetting for 3-4
seconds;
- skin passing (5) the coated strips for uniform lamination of coated
material;
- coating the Zn-coated strips with organic compound in an organic
coating chamber (6); and
- finally recoiling the sheets/strips in a recoiler (7),
characterised in that the said steel sheets/strips coated with Zn
(0.13-0.14% Al) yield better property in respect of spot weldability and

powering resistance and in that the coating is provided without any
galvannealing furnace.
2. The process as claimed in claim 1, wherein the microstructure of
steel coated sheets/ strips are same as that of sheets/strips of
conventional galvannealing process.
3. The process as claimed in claim 1, wherein the powering resistance
of the coated steel sheets/strips is increased by 70-80%.
4. The process as claimed in claim 1, wherein the spot weldability of
new coated steel sheets/strips is same as that in conventional
galvannealing coating.

The present invention relates to a process for galvannealed coatings in
steel sheets/strips without galvannealing operation comprises the steps
of cleaning the steel sheets/strips by passing the uncoiled sheets/strips
through an Electrolytic cleaning chamber wherein brushing the steel
sheets with alkaline liquid, rinsing and drying takes place and annealing
the cleaned steel sheets/strips by passing the uncoiled hard
sheets/strips through a heating chamber at 500-650°C under a
hydrogen or nitrogen controlled atmosphere to impart strength and
formability property and dipping the heated sheets/strips in a coating
bath containing a molten mixture of Zn-(0.14-0.16% Al) for full wetting
for 3-4 seconds and skin passing the coated strips for uniform
lamination of coated material and coating the Zn-coated strips with
organic compound in an organic coating chamber and finally recoiling
the sheets/strips in a recoiler.