FIELD OF THE INVENTION
The present invention relates a process for application of Zn-Fe binary coating on
steel substrate by a combination of hot dip galvanizing and annealing process
under varied process conditions to obtain coated steel sheets with improved
properties.
BACKGROUND OF THE INVENTION
For engineering applications a material is selected primarily to satisfy the
strength requirement. Accordingly, the selected material may not possess
satisfactory surface properties such as wear resistance, corrosion resistance,
abrasion resistance. Coatings are used to impart specific properties requirement
in final application of the steel sheet. For longer durability of the product, the top
coating plays a vital role. There are many coating technologies available to
enhance the life of a steel substrate which includes hot-dip coating, electro
deposition, and spraying.
Zinc coating on sheet steel surface is provided to protect against corrosion. Zinc
coated steel is extensively used in automobile sector. Conventional zinc coating is
found to be inferior in respect of paintability and spot weldability. To improve
paintability and weldibility performance of coated steel, Zn-Fe alloy coating was
developed. But, Zn-Fe intermetallic phases are brittle in nature and have a
tendency for higher powdering during forming. The resistance to powdering can
be improved by controlling Fe content and maintaining a right proportion of
different intermetallic phases in the coating. The right proportion of different
intermetallic phases is expected to further improve paintability and weldability
performance of this coating.
It is known that hot dip zinc 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 coating technology, still there is
tremendous scope for further improvement in this field. The developments
known in this field include addition of a small amount of elements like Al, Mg, Ni,
Si. Such added elements control alloying reaction, retard formation of suspended
dross 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 steel by forming thicker Fe2AI5 intermetallic compound
layer. Accordingly, the present inventors recognized that aluminium level in
molten zinc is a dominant factor to control the type of intermetallic phase
formation in the coating.
The depletion of fuel resources and environmental pollution has posed a real
challenge to the automobile industry to reduce vehicle body weight without
compromising safety, cost competitiveness or pollution. This led to demand to
Interstitial free high strength (IFHS) compared to interstitial free (IF) steel sheet.
The basic difference between IFHS and IF steel sheets are in the composition of
the two grades of steels. Alloying elements present in IF steel also play a
significant role in Fe diffusion and thereby control intermetallic phases in the
coating. The prior art available on this field disclose galvanization bath with
higher effective bath Al, higher gal annealing temperature and time,
(Ref: EP2009127A1, US20100139816A1, WO2000031311A1, WO2007109865A1,
W02009048838A1. The higher effective bath Al forms thicker innihibition layer
thereby need a higher galvannealing temperature and time for Fe-Zn alloying.
This process leads to a result where more effective is the Fe, more cavities are
formed in coating.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a process for production of
Zn-Fe binary coating on steel substrate by a combination of hot dip galvanizing
and annealing process under varied process conditions with improved properties,
which eliminates the disadvantages of prior art.
Another object of the invention is to propose a process for production of Zn-Fe
binary coating on steel substrate by a combination of hot dip galvanizing and
annealing process under varied process conditions with improved properties,
which improves powdering resistance, corrosion resistance, weldability and
paintability of the coated steel.
SUMMARY OF THE INVENTION
According to the invention, desirable proportion of different Zn-Fe intermetallics
in the coating are obtained by a process of:
Cold rolling? hot dip (developed bath)?developed galvannealing
cycle.
The galvannealing coating obtained under modified process condition is
superior in respect to powdering resistance than existing galvannealing product.
This inventive process is suitable for coating on IF and IFHS steel substrate also.
Conventional defects in galvannealed coating like bare spot, shiny spot expected
to be resolved by this process.
From the present invention, various grades (IF, IFHS, CQ, EDD and DP) of steel
can be galvanized and galvannealed through hot dip galvanizing and subsequent
annealing process with desirable Fe-Zn phases in coating. This galvanizing and
galvannealing process improves coating performance like, powdering including
printability and weldability.
According to the present invention a bath composition and galvannealing cycle
have been identified to obtain hot dip zinc coated IF and IHFS steel sheets with
improved powdering resistance, weldability and paintability properties.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - shows a process flow sheet according to the invention.
Figure 2 - shows a galvannealing cycle of the process of the invention.
Figure 3 - shows a microstructure of the coated steel sheet according to the
invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention identifies a lower aluminium (0.10-0. 128 wt%) level in
bath compared to the existing of 0.134wt% to achieve right kind of phases in the
coating. Further, a lower GA power and higher line speed has been identified to
additionally improve the coating properties. With the optimized process
conditions as optimized, obtain coating with improved powdering, weldability and
paintability properties. Figure 1 shows a modified galvannealing cycle for a sheet
sample with dimension say, (thickness: 0.75 and width: 1405 mm) and a
desirable coating microstructure as shown in Figure 2 with improved powdering
weldability, paintability performances is obtained. The process flow is illustrated
in Figure 1.
Thermal calculations are needed to obtain same coating in other dimension of
steel sheet as mentioned below.
1. GA power and mass through put dictate the annealing temperature
(TO C).
2. Line speed and furnace length dictate residence time in seconds (t).
% Fe in the coating = f {T,t}, a model has been developed based on the
following assumptions
1. Temperature of the strip at the entry of GA furnace is 4500 C.
2. Induction furnace efficiency is approximately 26 % (based on back
calculation of 1000 data of Gal strip Tern, GA power and mass through put
calculation)
3. No change in strip temperature in mob soak and fix soak zone.
i. T=450 + ?T
ii. Mass through put * Cp* ?T = power applied * efficiency.
WE CLAIM:
1. A process for application of Zn-Fe binary coating on steel substrate by a
combination of hot dip galvanizing and annealing process under varied
process conditions to obtain coated steel sheets with improved
properties, the process comprising:
- implementing the hot dip galvanizing process under varied conditions
including variable bath temperature, dipping time, and bath aluminium;
and
- conducting the galvannealing process after galvanization under variable
power of induction heater and resident time.
2. The process as claimed in claim 1, wherein the steel substrate includes IF,
IFHS, EDD and HSS steel.
3. The process as claimed in claim 2, wherein the HSS includes DP500 to
DP5900.
4. The process as claimed in any of the preceding claim, wherein the
thickness of the steel substrate is in the range of 0.3 to 2.5 mm.
5. The process as claimed in claim 1, wherein the variable galvanizing bath
temperature is between 445 to 490 degree C.
6. The process as claimed in claim 1, wherein the variable dipping time of
the galvanizing step is 2 to 6 seconds.
7. The process as claimed in claim 1, wherein the variable bath aluminium
comprises 0.10 to 0.128wt%.
8. The process as claimed in claim 1, wherein the power of induction heater
is between 700 to 1400 kW.
9. The process as claimed in claim 1, wherein the resident time is 12 to 60
seconds.
10. The process as claimed in claim 1, wherein the Zn-Fe binary alloy coating
contains varied amount of Zn and Fe.
11.The process as claimed in claim 10, wherein the Zn content is 88 to
92wt%.
12. The process as claimed in claim 10, wherein the Fe content is 8 to
12wt%.
13. The process as claimed in claim 1, wherein the intermetallic phases in
the coating exhibits improved powdering resistance, corrosion, weld-
ability and paintability.

ABSTRACT

The present invention describes a process of hot dip zinc coating in molten Zn-AI
bath followed by annealing cycle to get improved powdering performances of
galvannead steel sheet. The bath consists of less Al (0.1-0.128wt%) compared to
existing practice of higher Al and the developed annealing cycle consist of lower
GA power and higher line speed. The galvannealed product obtained under
modified process condition contains less iron compared to the earlier
galvannealed material. The tendency to powering is decreased from 10 mg.m2 to
4 mg/m2 under the modified process conditions.