FIELD OF INVENTION:
This invention relates to a method of developing an intermediate metallic
layer by a displacement process on high strength steel (HHS) and advanced
high strength steel (AHSS).
BACKGROUND OF INVENTION:
For engineering applications a material is often chosen to satisfy the strength
requirement. That material may not possess satisfactory surface properties
such as wear resistance, corrosion resistance, abrasion resistance etc. To
impart specific properties, coatings are used. For long durability of the
product, the top coating plays a vital role. There are many coating
technologies available to enhance the life of steel substrate which includes
the hot-dip coating, electrodeposition, spraying etc.
Hot dip galvanizing coating has many advantages over other metallic
coating, in terms of mode of application, longer protection and cost. Despite
vast research that has been carried out on this old technology, still there is
tremendous scope for further improvement in this field. Current
developments in this field include the influence of presence of alloying
elements as buffer layer on the corrosion resistance, formability, coatability.
Hot-dip galvanised steel is product by coating steel sheet with molten zinc
containing very small amounts of elements like Al, Mg, Ni, Si. The extra
elements are used to control alloying reaction, retard formation of suspended
dross particles and improve bath fluidity.
Aluminum in the level of around 0.2wt% suppresses the growth of a brittle
Fe-Zn alloy layer at the interface between the coating and steel surface by
forming a layer of Fe2Al5 intermetallic compound.
In present world where industrialization is occurring at a very steady rate the
demand of automobile has increased rapidly. Moreover, the depletion of fuel
resources an environmental pollution has posed a real challenge to the
automobile industry to reduce vehicle body weight without compromising
with safety, cost competitiveness or pollution. This led to demand of
advanced high strength (AHSS) steel such as transformation induced
plasticity (TRIP), dual phase (DP) and high strength steel (HSS) like
interstitial free high strength (IFHS) etc. These steels have great strength
enhanced ductility. Hence, thinner gauge strips of HSS and AHSS can be
used in automobile sector that improve the fuel efficiency and reduction in
pollution. These thin sheets are to be made corrosion resistant using the most
economical zinc coating technique. A significant technical hurdle for
implementations of AHSS is to achieve good quality galvanized coatings
while retaining the target mechanical properties due to formation of surface
oxides as a result of segregation of constitute elements during high
temperature annealing. The thin layer of surface oxides affects the
wettability of the substrate with liquid zinc.
Different types of intermediate layer or buffer layer on the steel substrate
may improve the coatability and remove wettability problem in hot dip
galvanization of AHSS and HSS. The present work was intended, with
previous background knowledge of published literature, to thoroughly
understand the effect of buffer layer on hot dip galvanized coating to
improve coatability and corrosion resistance, and to obtain optimized
process parameters to make most efficient hot dip zinc coatings with
following advantages.
(1) Automobile fuel consumption will be less.
(2) Less consumption of natural minerals.
(3) Less pollution.
(4) Defect free coating.
OBJECTS OF THE INVENTION;
An object of this invention is to propose a method of developing an
intermediate metallic layer by a displacement process on high strength steel
and advanced high strength steel;
Another object of this invention is to propose an excellent Zn coating for
HSS and AHSS;
Still another object of this invention is to propose a corrosion resistance and
abrasion resistant coating;
Further object of this invention is to propose a noble metallic layer on
different steel substrates to improve zinc coatibility and adhesion.
BRIEF DESCRIPTION OF THE INVENTION;
According to this invention there is provided a noble intermediate metallic
layer. The noble metallic layer on different steel was introduced before hot
dip galvanization to improve zinc coatability and adhesion.
In accordance with this invention there is also provided a method for
developing an intermediate metallic layer coating. The noble metallic layer
on the steel substrate is the pure Cu, Sn, or alloys of two. The noble metallic
layer on steel substrate is deposited by displacement process. This process
consists of oxidation of Fe from the steel surface as Fe and donate
electrons being used for reduction of noble metallic ion and is deposited on
the steel substrate.
DETAILED DESCRIPTION OF THE INVENTION:
In the present invention suitable displacement process has been developed
before hot dip galvanization on different grades of steel substrate. The
process will be useful for galvanization on HSS and AHSS steel substrate
without any defect. The process described in the accompanying figure 1.
The different noble metals were deposited on high strength steel by
displacement process. The noble metal coated steel substrate then dipped in
molten zinc. Subsequently, the zinc coatability and zinc adhesion of the steel
surface with noble intermediate layer was compared to the steel without
intermediate metallic layer. The high strength steel substrate with noble

intermediate metallic is showed better zinc coatability and zinc adhesion as
shown in Figure 2.
The displacement process is conducted in solution containing one or more
than one noble metal ions under varied concentration depends on
requirement. The pure Cu coating on the steel surface is conducted in
solution containing only Cu ions under varied pH conditions at room
temperature. The pH range is in between 1 to 4 and the room temperature
varies from 5 to 45 degree C. The alloy coating deposited on the steel
surface includes different Cu-Sn alloys.The different Cu-Sn alloys includes
the Sn content in the Cu-Sn alloy varied in the range of 5 to 35wt%. Hot dip
galvanization is done on the noble metal coated steel surface includes the
pure zinc bath as well as different Zn-Al alloy baths. The different Zn-Al
alloy baths includes the Al content in the range of 0.08 to 5 wt%. The hot
dip galvanization was conducted under variable bath temperature. The
variable galvanization bath temperature is in the range of415 to 470oC.the
preferable bath temperature is 450o C for pure zinc bath and 415o C for Zn-
5wt% alloy bath. Galvannealing operation is done under variable process
condition on galvanized steel surface to obtain Zn-Fe alloy coating with
better spot weld ability paint ability. The variable process condition for
galvannealing operation includes variable temperature and time. The
variable temperature is from 470 to 560° C. the preferable galvannealing
temperature is 510° C. The variable time required is from 6 to 60 seconds.
Preferable galvannealing time is 20 seconds. After galvannealing coated
steel surface cool in different medium. The different cooling medium
includes normal air cooling as well as water quenching.
The improvement in zinc coatability can be defined as practically no bare
spot appears after galvanization for the steel substrate with intermediate
noble metallic layer. The improvement in zinc adhesion can be defined as no
flaking occurs after bending of the galvanized coating for the steel substrate
with intermediate noble metallic layer. The different steel substrate includes
interstitial free (IF), interstitial free high strength (IFHS), CQ, extra deep
drawing (EDD), micro alloyed steel, dual phase, transform induced
plasticity, martensitic and complex phase.
From the present invention, various grades (IF, IFHS, CQ, EDD, DP, TRIP
martensitic and CP) of steel can be galvanized through hot dip process
without any defect. This galvanizing process is not only beneficial to
improve coatability and zinc adhesion but also galvanizing process will be
much more flexible. The stringent reducing atmosphere requirement for
annealing of cold rolled steel sheet before galvanizing is expected to be no
more stringent. Excellent galvanizing operation can be done on the noble
metal plated steel substrate even in the atmosphere free from hydrogen.
WE CLAIM:
1. A noble intermediate metallic layer comprising of metals, pure Cu, Sn
or alloys of the two on the still substrate.
2. A noble intermediate metallic layer as claimed in claim 1 wherein the
metal coated steel substrate is dipped in molten zinc for zinc coating.
3. A method for developing a novel intermediate metallic layer
comprising depositing a metal layer on high strength steel surface by
displacement process, subjecting the metal coated steel to the step of
dipping in molten zinc for zinc coating.
4. The method as claimed in claim 1 wherein the metallic layer is the
pure Cu, Sn or alloys of two metals.
5. The method as claimed in claim 1 wherein the said displacement
process comprises of oxidation.
6. The method as claimed in claim 1 wherein the pH range of the copper
solution is 1 to 4.
7. The method as claimed in claim 1 wherein the temperature in plating
solution is in between 5 to 45o C.
8. The method as claimed in claim 1 wherein the hot dip is done in the
pure zinc bath as well as in different zinc alloy baths.
9. The method as claimed in claim 1 wherein the bath temperature is in
between 415 to 470oC.
10.The method as claimed in claim 1 wherein the preferred temperature
of the bath is 450o C.

The present invention describes the displacement noble metallic layer
coating on high strength steel (HSS) and advanced high strength steel
(AHSS) steel surfaces followed by hot dip galvanization and subsequently
comparison of zinc coatability compared that the high strength steel
substrate without any metallic layer. The study confirmed that the noble
metallic has good adhesion with steel substrate as well as with zinc. The so-
called un-coated bare spot on HSS and AHSS steel substrate has been
resolved significantly by applying noble metallic layer before galvanization.