FIELD OF THE INVENTION
The present invention relates to an improved process of galvanizing high
strength steel substrate to enhance zinc coatability and zinc adhesion.
BACKGROUND OF THE INVENTION
High-strength steels (HSS) and advanced high-strength steels (AHSS) allow
automobiles makers to substantially reduce vehicle weight without compromising
safety.High fuel prices in the recent years have certainly encouraged a broader
application of HSS and AHSS in the automotive industry.
This trend poses a challenge to the galvanizers because alloying elements in
AHSS, such as Mn and Si, are preferentially oxidized that on steel surfaces during
annealing prior to hot dipping. The surface oxide layer reduces the wettability of
the steel and result in bare spots.
For engineering applications a material is often selected to satisfy the strength
requirement, eventhough the selected 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 in the art to enhance the life of steel substrates which
includes hot-dip coating, eletrodeposition, spraying etc.

Hot dip galvanizing coating has many advantages over other metallic coating, in
terms of ease in application, longer protection and cost. Despite a vast research
that has been carried out on this old technology, still there is tremendous scope
for further improvement in this field. Current researches in this field include the
impact of presence of alloying elements as a buffer layer on the corrosion
resistance, formability, and coatability. Zinc coating by continuous hot dip-
galvanized steel is produced 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 cross particles and
improve bath fluidity. Aluminum in level of around 0.2 percent suppresses the
growth of a brittle fe-Zn alloy layer at the interface between the coating and the
steel substrate by forming Fe2 Al5 intermetallic compound layer.
The depletion of fuel resources and environmental pollution have posed a real
challenge to the automobile industry to reduce vehicle body weight without
compromising the safety, cost competitiveness or pollution. This led to a higher
demand of advanced high strength (AHSS) steel such as transformation induced
plasticity (TRIP), dual phase (DP) and high strength steel (HSS) for example,
interstitial free high strength (IFHS).The IFHS has higher strength and enhanced
ductility. Hence, thinner gauge strips of HSS and AHSS can be used in
automobile sector that improves the fuel efficiency and reduces pollution.
However, thin sheets of IFHS are required to be made corrosion- resistant using
a most economical zinc coating technique. A significant technical hurdle for
implementation of AHSS in automobile sector, constitutes achieving an effective
quality of galvanized coating while retaining the targeted mechanical properties.

Normally, the targeted mechanical properties of IFHS deteriorate due to
formation of surface oxides as a result of segregation of the constituent elements
of the coating during the high temperature annealing. The thin layer of surface
oxides affect the wettability of the substrate with liquid zinc.
The inventors recognized that different types of intermediate layer or buffer layer
on the steel substrate seem to improve the coatability and remove wettability
problem in hot dip galvanization of AHSS and HSS.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved process of
galvanizing high strength steel substrate in which a nickel or nickel-phosphorus
alloy coatings is first formed through electro or electroless plating to enhance
zinc coatability and zinc adhesion.
Another object of the invention is to propose an improved process of galvanizing
high strength steel substrate in which a nickel or nickel-phosphorus alloy
coatings is first formed through electro or electroless plating to enhance zinc
coatability and zinc adhesion, which allows a defect-free galvanized articles of
manufacture in particular auto-components.
A further object of the invention is to propose an improved process of
galvanizing high strength steel substrate in which a nickel or nickel-phosphorus
alloy coatings is first formed through electro or electroless plating to enhance

zinc coatability and zinc adhesion, which requires lesser quantity of natural
minerals and constitutes an environment -friendly process.
SUMMARY OF THE INVENTION
The present invention discloses an improved process of galvanizing high strength
steel substrate in which a nickel or nickel-phosphorus alloy coatings is first
formed through electro or electroless plating to enhance zinc coatability and zinc
adhesion, followed by hot-dip galvanizing of the substrate.
A comparison of zinc coatability upon implementation of the inventive process as
opposed to a directly galvanized high strength steel substrate without any
intermediate metallic layer, establishes that the metallic layer has good adhesion
with the 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 the
novel metallic layer before galvanization.
According to the present invention, a nickel or nickel-phosphorous alloy layer is
coated on HSS or AHSS substrate by electro or electroless plating process before
hot dip galvanization.
The present invention eliminates any bare spots on the substrate after
galvanization over intermediate metallic layer.

The improvement in zinc adhesion is established by observing non-occurrence of
flaking after bending of the galvanized steel substrate with intermediate metallic
layer.
The different steel substrates which are susceptible according to the invention
includes but not limited to, 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.
According to the present invention, various grades (IF, IFHS, CQ, EDD, DP, TRIP
martensitic and CP) of steel can be pre-coated and thereafter galvanized through
hot dip process without any defect. This intermetallic coating process followed by
the known galvanizing process is not only beneficial to improve coatability and
zinc adhesion, but is much more flexible. The stringent reducing atmosphere
requirement for annealing of cold rolled steel sheet before galvanizing remains
no more stringent. Excellent galvanizing result can be obtained on the nickel or
nickel-phosphorous alloy plated steel substrate even in an atmosphere free from
hydrogen.
Accordingly, there is provided an improved process of galvanizing high strength
steel substrate to enhance zinc coatability and zinc adhesion, comprising the
steps of: depositing an intermediate metallic layer by one of an electro plating
and an electroless plating technique, wherein one of a solution containing nickel
ions and a solution containing nickel-phosphorus alloys including a reducing
agent, a complexing agent, and a PH-adjusting alkali compound is supplied from

One of external power source and the reducing agent; and hot-dip galvanizing of
the substrate having said intermediate metallic layer formed on the surface in a
galvanizing bath.
BRIEF DESCRIPTION OF THE ACCOM PING DRAWINGS
Figure 1-shows a process chart according to the present invention
Figure 2 (a) - shows the pictorial view of zinc coating on DP-surface without
forming an intermetallic layer of the invention.
Figure 2 (b) - Shows a pictorial view of zinc coating on DP- surface with an
intermetallic layer formed of pure nickel according to the present invention
Figure 2 (c) - shows a pictorial view of zinc coating on DP-surface with an
intermetallic layer formed of nickel-phosphorous alloy according to the invention.
DETAIL DESCRIPTION OF THE INVENTION
According to the invention, and as shown in fig 1, a pure nickel or nickel-
phosphorous alloy layer is first deposited on high strength steel surface by
electro or electroless plating process. The pure nickel-phosphorous alloy coated
steel substrates, are then dipped in the molten zinc. Subsequently, the zinc
coatability and zinc adhesion of the steel surface with nickel or nickel-
phoaphorous intermediate layer is compared to the steel surface without nickel
or nickel-phosphorous intermediate metallic layer. The high strength steel

substrates with nickel and nickel-phosphorus intermediate metallic shows a
better zinc coatability and zinc adhesion.
Figs 2 (a),(2), and 2(c) respectively shows pictorial views, of zinc coating on DP
steel surface (a) without any intermediate metallic layer (b)with pure nickel
intermetallic layer, and (c) with nickel- phosphorous alloy as intermediate layer.
The intermediate metallic layer of the invention is deposited on different steel
substrates before hot dip galvanization to improve zinc coatability and adhesion,
the intermediate metallic layer being a pure Ni or alloy of Ni-P.The intermediate
metallic layer is deposited by electro or electroless plating process on the steel
substrate (cathode) by consumption of electron being supplied from an external
power source for electro plating, or the electron being supplied by a reducing
element added in the solution for electroless plating.
i
The electro or electroless plating process is conducted in a solution containing
nickel ions under varied concentration depending on requirement. In case of
pure nickel plating on the steel surface the same is conducted in a solution
containing only nickel metallic ion under varied concentrations and varied pH
conditions at room temperature. The nickel ion includes in the concentration is in
a range from 30 gm/ltr. to 70 gm/ltr. The pH range of the solution is between 3
to 4, and being maintained at a temperature between 5 to 45 degree c.
When a Ni-P alloy coating is deposited on the steel surface by electroless plating,
the solution consists of different nickel-phosphorous alloys, and further includes

different reducing agents, complexing and PH adjusting agent's alkali compounds
under varied solution temperature. The reducing agents include sodium
hypophosphite and hydrazine under varied concentration. The complexing agent
is selected from a group consisting of sodium citrate acid, glycolic acid and citric
acid under varied concentration. The alkali compounds comprise one of sodium
hydroxide, potassium hydroxide and hydrated ammonia. The solution
temperature for electroless plating is maintained between 60 to 95° C
wherein; the phosphorous content in the different nickel-phosphorous alloy
varies in the range of 5 to 16 wt%, the preferred value being 12wt%
phosphorous which gives best zinc coatability performance.
Hot dip galvanization is thereafter done under varied bath temperature on the
steel substrate having said intermediate metallic layer. The galvanizing bath can
be one of a pure zinc bath as well as different Zn-AI alloy baths. The Zn-AI alloy
baths contains Al content in the range of 0.08 to 5 wt%.
The galvanization bath temperature is varied between 415 to 490° c, preferably
460° for pure zinc bath, and 423°C for Zn-5wt% alloy bath.
After galvanizing process, the coated steel surface is cooled in different medium
for example, normal air cooling, or water quenching.

WE CLAIM:
1. An improved process of galvanizing high strength steel substrate to
enhance zinc coatability and zinc adhesion, comprising the steps of:
depositing an intermediate metallic layer by one of an electro plating
and an electroless plating technique, wherein one of a solution
containing nickel ions and a solution containing nickel-phosphorus
alloys including a reducing agent, a complexing agent, and PH-
adjusting alkali compound is supplied from, one of an external power
source and the reducing agent; and hot-dip galvanizing of the
substrate having said intermediate metallic layer in a galvanizing bath.
2. The process as claimed in claim 1, wherein the electro plating
technique is implemented at room temperature with the concentration
of nickel ions in the solution ranging from 30 gm/ltr.
to 70 gm./ltr. for electro deposition of NL on steel substrate.
3. The process as claimed in claim 2, wherein the PH range of the
solution containing nickel is between 3 to 4, and maintained at a
temperature between 5 to 45°c.
4. The process as claimed in claim l,wherein the reducing agent consists
of hypophosphate hydrazine
5. The process as claimed claim 1 or 4, wherein the complexing agent is
selected from a group consisting of sodium citrate acid, glycolic acid
and citric acid.

6. The process as claimed in one of claims 1, 4, and 5, wherein the PH-
adjusting alkali compound comprises one of sodium hydroxide,
potassium hydroxide, and hydrate ammonia.
7. The process as claimed in claim 1, wherein the phosphorous is
between 5 to 16% wt%, preferably 12wt%.
8. The process as claimed in claim 1, wherein the galvanizing bath is one
of pure zinc bath or Zn-AI alloy bath, and w,herein the Zn-AI alloy bath
contains aluminum in the range of 0.08 to 5 wt%.
9. The process as claimed in claims 1 or 8, wherein the bath temperature
is maintained between 415° to 490°, preferably at 460° for Zn-AI alloy
bath.
10. An improved process of galvanizing high strength steel substrate to
enhance zinc coatability and zinc adhesion, as substantially described
and illustrated herein with reference to the accompanying drawings.

An improved process of galvanizing high strength steel substrate to enhance zinc
coatability and zinc adhesion, comprising the step of: depositing an intermediate
metallic layer by one of an electro plating and an electroless plating technique,
wherein one of a solution containing nickel ions and a solution containing nickel-
phosphorus alloys including a reducing agent, a complexing agent, and PH-
adjusting alkali compound is supplied from one of an external power source and
the reducing agent; and hot-dip galvanizing of the substrate having said
intermediate metallic layer in a galvanizing bath.
{FIGURE 1}