FIELD OF THE INVENTION
The present invention relates to a coating process of Zn-Ni-Fe ternary alloy on
steel surface with improved paintability, weldability, corrosion and powdering
resistance properties for application in automotive components.
BACKGROUND OF THE INVENTION
For engineering applications, a material is normally selected to primarily satisfy
the strength requirement, although the selected material may not possess
satisfactory surface properties such as wear resistance, corrosion resistance,
abrasion resistance etc. To impart specific properties into the selected material,
different coatings are used. For long durability of a product, the top coating plays
a vital role. There are many coating technologies available in prior art which
enhances the life of the steel substrate, and which includes hot-dip coating,
electrodeposition, spraying etc.
Hot dip galvanising coating has many advantages over other metallic coating, in
terms of mode of application, longer protection and cost. The current
developments in the field of hot-dip galvanising include experimenting the
influence of the alloying elements present in the buffer layer and their impact on
the corrosion resistance, formability, coatability factors. Hot-dip galvanized steel
sheet is produced with molten zinc containing 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
the level of around 0.2 percent suppresses the growth of a brittle Fe-Zn alloy
layer at the interface between the coating and steel by forming Fe2AI5
intermetallic compound layer.
For multipurpose reasons, the automobile industry is under great pressure to
reduce vehicle body weight without compromising safety, cost competitiveness
or pollution. This led to demand of Advanced High Strength Steel (AHSS), such
as Transformation Induced Plasticity (TRIP), Dual Phase (DP) and High Strength
Steel (HSS) like interstitial free high strength (IFHS) etc. These grades of steel
have more strength and enhanced ductility. Hence, thinner gauge strips of HSS
and AHSS can be used in automobile sector that improve the fuel efficiency and
reduce pollution. These thin sheets are to be made corrosion resistant using the
most economical zinc coating technique. A significant technical hurdle for
implementation of AHSS is to achieve an improved quality coating while retaining
the targeted mechanical properties which is susceptible to formation of surface
oxides as a result of segregation of the constituent elements during high
temperature annealing. The thin layer of surface oxides thus formed affects the
wettability of the substrate by the 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
ofAHSSandHSS.
Conventional zinc coating is inferior in terms of payability and spot weldability.
To improve payability and weldibility performance of the coated steel, the prior
art adapts Zn-Fe alloy coating. On the other hand, Zn-Fe intermetallic phases
formed in the prior art, are brittle in nature and have a tendency for higher
powdering during forming.
Standard galvanization process on automotive components however poses
problems for example, alloying elements in AHSS, such as Mn and Si, are
preferentially oxidized on such steel surfaces during annealing prior to hot
dipping, forming surface oxide layer which reduces the wettability of the steel
and results in formation of bare spots at the time of welding.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a coating process of Zn-Ni-
Fe ternary alloy on steel surface with improved paintability, weldability, corrosion
and powdering resistance properties for application in automotive components.
Another object of the invention is to propose a coating process of Zn-Ni-Fe
ternary alloy on steel surface with improved paintability, weldability, corrosion
and powdering resistance properties for application in automotive components,
which produces automotive components mode of HSS or AHSS Steel having
higher corrosion resistance under chloride environment.
A still another object of the invention is to propose a coating process of Zn-Ni-Fe
ternary alloy on steel surface with improved paintability, weldability, corrosion
and powdering resistance properties for application in automotive components,
which decreases powdering tendency of the coating.
A further object of the invention is to propose a coating process of Zn-Ni-Fe
ternary alloy on steel surface with improved paintability, weldability, corrosion
and powdering resistance properties for application in automotive components,
which produces coated components with improved paintability and weldability.
SUMMARY OF THE INVENTION
The present invention describes a coating process comprising nickel coating on
steel surface by electroplating technique followed by hot dip zinc coating and
finally annealing operation to produce Zn-Ni-Fe ternary alloy coating on steel
surface with improved properties. The ternary alloy coated samples of HSS and
AHSS substrates showed 6 times improved performance against corrosion in
chloride environment, and a decrease of powdering tendency from 10 mg/m2 to
4 mg/m2 compared to the conventional Zn-10 wt% Fe galvannealed coating. The
ternary alloy coating of the invention further exhibited significant improvement in
paintability and weldability.
According to the invention, a ternary Zn-Ni-Fe alloy coating is proposed for
application on steel surface by the steps of electroplating followed by hot dipping
and further followed by galvannealing.
The present invention thus proposes a ternary Zn-Ni-Fe alloy coating on steel
substrate to improve coating performance. According to the invention, Zinc
coating on sheet steel surface is provided to protect against corrosion. Zinc
coated steel is used extensively in automobile sector.
The inventors observed that resistance to powdering can be improved by tenary
Zn-Ni-Fe alloy coating. The payability and weldability performance of this
ternary alloy coating get further improved. It is very difficult to obtain Zn-Ni-Fe
ternary alloy coating on steel surface by hot dip process. Thus, the invention
firstly implements Ni plating followed by hot dip galvanization and further
followed by galvannealing process to produce ternary alloy coating. The
inventive process produces alloy coating suitable for application on high-strength
steels (HSS) and advanced high-strength steels (AHSS) substrate. HSS and AHSS
allow automobile makers to substantially reduce vehicle weight without
compromising safety.
The steel substrate useable for the process comprises one of IF, IFHS, EDD HSS
and AHSS steel. The HSS includes DP500 to DP800. The AHSS includes different
grades of TRIP, complex phase and TWIP steel. According to the invention, cold
rolled steel is first subjected to alkali cleaning followed by rinsing and
electroplating. The Ni plating is conducted under varied concentration of Ni ions,
bath pH and solution temperature. The Ni ions concentration is in the range of
0.1 to 2 moles. The pH of the plating solution is maintained between 5 to 2. The
solution temperature is in the range of 15 to 60 degree C. The annealing
operation is conducted under varied temperatures, times and environments prior
to hot dipping, the varied temperatures for annealing is between 600 to 850
degree C, and the varied time for annealing is between 100 seconds to 300
seconds. The varied environments for annealing include different level of
hydrogen and nitrogen mixture even 100% nitrogen atmosphere is possible. The
galvanizing operation is conducted under varied process conditions which include
variable bath temperature, dipping time and bath aluminium. The variable
galvanizing bath temperature is in the range of 430 to 490 degree C. The
variable dipping time is in the range of 2 to 6 seconds. The variable bath
aluminium includes 0.11 to 0.14 wt% Al. The galvanneaning operation is done
after galvanization, and implemented under variable power of induction heater
and resident time. The power of induction heater is between 700 to 1400 kW,
the resident time is in between 12 to 60 seconds. The Zn-Ni-Fe ternary alloy
coating contain varied Zn, Ni and Fe, in which the Zn content is 80 to 90wt%,
the Ni content 5 to 12 wt%, and the Fe content 4 to 8wt%.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Shows a process flow chart according to the invention.
Figure 2(a) - Shows a pictorial view of zinc coating on a steel surface
without intermetallic larger according to prior art.
Figure 2(b) - Shows a pictorial view of Zn-Ni-Fe coating on HSS or AHSS
substrate according to the invention.
DETAIL DESCRIPTION OF THE INVENTION
As shown in figure 1, pure nickel is deposited on HSS or AHSS steel surface by
electro plating technique. The pure nickel coated steel substrate is then dipped in
molten zinc. Subsequently, the zinc coated steel is annealed in an induction
heating furnace to obtain ternary Zn-Fe-Ni alloy coating. The performance of the
developed ternary alloy coating when compared with the conventional Zn-Fe
binary alloy coating, exhibited improved zinc coatability and zinc adhesion of the
steel substrate provided with nickel plating prior to hot dipping.
Figure 2 (a) and 2(b) respectively shows the conventional Zn-Fe alloy coating
and Zn-Ni-Fe alloy coating of the invention implemented by electrode plating of
Ni on steel surface followed by galvanization and galvannealing operation.
The invention has following advantages.
(1) Automobile fuel consumption will be less.
(2) Less consumption of natural minerals.
(3) Less pollution.
(4) Defect free coating.
Reference US patent: 4913785A, 5677005, 6797411, 6911268, 6902829 B2,
7413780 B2, and 7608155.
WE CLAIM :
1. A coating process of Zn-Ni-Fe ternary alloy on steel surface with improved
paintability, weldability, corrosion and powdering resistance properties for
application in automotive components, comprising :
- subjecting a cold rolled steel sheet to alkali cleaning followed by rinsing
and electroplating, wherein the Ni-plating is conducted under varied
concentrations of Ni-ions, bath pH, and solution temperature.
- conducting a step of hot dipping in molten zinc - under variable bath
temperature, dipping time and bath aluminium;
- implementing a galvannealing step under variable power of indication
heater and resident time.
2. The process as claimed in claim 1, wherein the Ni-ion concentration in the
range of 0.1 to 2 mol., wherein the pH of the plating solution is between 5 to
2 at a solution temperature in the range of 15 to 60°C, wherein the varied
time for annealing is in between 100 seconds to 300 seconds, and wherein
the variable environments for annealing include different levels of mixture of
hydrogen and nitrogen, preferably 100% nitrogen atmosphere.
3. The process as claimed in claim 1, wherein the variable bath temperature
during galvanizing is in between 430 to 490°C with a variable dipping time is
in between 2 to 6 seconds, and wherein the variable bath aluminium includes
0.11to0.14wt%of Al.
4. The process as claimed wherein the galvannealing is done in an induction
furnace at a power between 700 to 1400 kW, the resident time is in between
12 to 60 seconds.
5. The process as claimed in claim 1, wherein Zn-Ni-Fe ternary alloy coating
comprises Zn in the range of 80 to 90 wt%, Ni in the range of 5 to 12 wt%,
and Fe in the range of 4 to 8 wt%.
6. A coating process of Zn-Ni-Fe ternary alloy on steel surface with improved
paintability, weldability, corrosion and powdering resistance properties for
application in automotive components as substantially described and
illustrated herein with reference to the accompanying drawings.

The invention relates to a coating process of Zn-Ni-Fe ternary alloy on steel surface with improved paintability, weldability, corrosion and powdering resistance properties for application in automotive components, comprising :subjecting a cold rolled steel sheets to alkali cleaning followed by rinsing and electroplating, wherein the Ni-plating is conducted under varied concentrations of Ni-ions, bath pH, and solution temparture, conducting a step of hot dipping in molten zinc - under variable bath temperature, dipping time and bath aluminium; and implementing a galvannealing step under variable power of indication heater and induction time.