FIELD OF INVENTION
The present invention relates to hot dip galvanized Zn-Mg-AI-Si-Sb coated steel
sheets with improved corrosion resistance, formability, coating adherence &
appearance and the method for producing the same.
BACKGROUND ART
The conventional hot dip galvanized zinc coated steel sheets are widely used in
Construction, Automotive and Appliance Industry. The demand for the
performance of the coated sheets with respect to corrosion protection, forrnability
and appearance is steadily increasing. To meet the demand for extended
longevity of the galvanized sheet, world wide efforts have been to develop new
types of zinc based coating, through addition of Al, Ni, Mg etc. in the zinc bath.
These are expected to achieve enhanced corrosion resistance along with
adequate forrnability and coating adherence properties. With this in view, efforts
made in the direction of developing Zn-Mg coated sheets have met with partial
success.
ATSUSHI KOMATSU in US patent document 6709770 discloses about "Steel
sheet hot dip coated with Zn-AI-Mg having high Al content". The invention relates
to high Al hot dip Zn-AI-Mg plated steel sheet whose plating layer has 4-10%AI
and 1-4%Mg. Despite being excellent in corrosion resistance, the plated steel
sheet has been slow to gain acceptance as industrial product due to ugly
appearance.
Further WILHELM WARENECKE in an European patent document
WO/2009/059950 discloses about "Flat steel product with an anti-corrosion
metallic coating and process for producing an anti-corrosion metallic Zn-Mg
coating on a flat steel product", wherein metallic coating consist of 4-8% Mg and
0.5-1.8% Al and the flat steel product according to the invention not only
afforded excellent protection against corrosion, but can also readily be welded
and coated with a subsequently applied organic layer.
Another patent document WO/2006/002843 is related to "Steel sheet with hot dip
galvanized zinc alloy coating and process to produce it", wherein zinc alloy
coating of 0.3-2.3% Mg, 0.6-2.3%AI has been applied on steel sheet. It has been
reported in the invention that though the high Mg additions improves the
corrosion resistance of the coated strip, it also leads to excessive oxidic dross
formation on the zinc bath and to brittle coatings.
Above inventions clearly highlight the improvements achieved in corrosion
resistance properties of Zn-Mg coated steel sheets w.r.t. conventional
galvanized sheets. However, formability, coating adherence and appearance of
such coated sheets were not found satisfactory.
OBJECTS OF INVENTION
Thus the primary objective of the present invention is to develop a unique Zn-Mg-
Al-Si-Sb based alloy coated steel sheets to achieve improved corrosion
resistance (more than 2 times w.r.t. conventional Gl sheets), formability at par
with substrate material, spangled appearance with bright & smooth finish.
Another object is to provide environment friendly coating by replacing Pb with Sb
in zinc bath for spangle control of galvanized sheet.
Therefore, such as herein described, there is provided a hot dip coated steel
sheets galvanized in molten zinc bath of composition Zn-(0.25-0.75%)Mg-(0.25-
0.30%)AI-(0.05-0.10%)Si-(0.08-0.12%)Sb having corrosion resistance about
three times with respect to conventional (Zn-0.16%AI-0.10%Pb coated) hot dip
galvanized sheets, formability at par with substrate steel, coating adherence as
per Lock Forming Quality (LFQ) standard, appearance bright & smooth.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention is described fully and particularly in an unrestricted manner with
reference to the accompanying drawings in which
Figure 1 illustrates a flow diagram illustrating the main steps followed in
sequence for processing the invented product in accordance with the present
invention.
Fig. 2 illustrates the Lock Forming Tested Specimen in accordance with the
present invention.
Fig. 3 illustrates the typical microstructure of (a) invented and (b) conventional
coated sheet in accordance with the present invention.
DETAILED DESCRIPTION
The making of the invented product comprises a process wherein Commercial
Quality Cold Rolled (CQCR) steel sheets are first annealed in a controlled
reducing atmosphere of N2+H2 by following pre-designed annealing cycle,
followed by cooling with N2 /air /natural upto the bath temperature, subsequent
dipping of the steel sheet as per specified duration in molten zinc bath of pre-
designed composition maintained at a particular temperature followed by wiping
of the coated sheet with N2 /air to wipe-out excess liquid zinc and to control the
coating thickness as per requirement. Following the wiping operation, the coated
sheets are cooled with the help of N2 /air /natural upto room temperature.
Referring to Fig.1, the process according to the present invention comprises the
following steps in sequence:
(a) Annealing of the CQCR sheets in N2+H2 atmosphere, where %H2
and dew point of the atmosphere varies from 10 to 20% and -20 to -
30°C respectively, based on the cleanliness level of the steel
substrate. Depending on the sheet thickness, annealing cycle
comprises heating of the CQCR sheets at the rate of 20-30°C/s
upto annealing temperature of 700-750°C and holding at this
temperature for 45-60 sec. The Annealing of cold rolled steel sheet
in the atmosphere of (1-x)N2+(x)H2 atm where (x=0.1 to 0.2)
(b) cooling of the annealed sheet with N2 gas upto near bath
temperature (5° - 10°C more than the bath temperature) at the rate
of 3-5°C/s
(c) Dipping of steel sheet in molten zinc bath of a given composition
comprising the following : 0.25-0.75%Mg, 0.25-0.30%AI, 0.05-
0.10%Si, 0.08-0.12%Sb, maintained at temperature of 420-460°C
and dipped for 1.5-3.5 sec.
(d) Wiping of the molten zinc coated steel sheet by N2 gas, wherein
wiping delay time, wiping gas flow rate and nozzle distance kept in
the range of 0.5-2.0 sec, 250-300 litre/min. and 16-18 mm
respectively.
(e) Cooling of the zinc coated steel sheet with N2 gas at the rate of 4-
8°C/s upto room temperature
Typical bath composition, coating thickness, corrosion resistance, formability and
adherence properties of the invented and conventional coated sheets are given
in Table I.
Table I: Typical properties of invented (Zn-Mg-AI-Si-Sb Coated) Sheet vis-a-vis
conventional (Zn-AI-Pb Coated) hot dip galvanized sheet
The corrosion resistance of the invented and conventional galvanized sheets has
been evaluated in the known Potentiodynamic method and Salt Spray Test
(shown in Table II). The comparative test results obtained for the invented (Zn-
Mg-AI-Si-Sb coated) sheets and conventional (Zn-AI-Pb) Gl sheets are shown in
Table I from which it is noted that the corrosion resistance of invented coated
sheets is higher than the conventional Gl sheets by 3 times. Further, salt spray
test results (Table II) also show that resistance to white rust formation of the
coated sheets galvanized in Zn-Mg-AI-Sb bath is much superior compared to
those galvanized in Zn-AI-Pb zinc bath. In case of Zn-0.16-0.1 OPb coated sheets,
100% white rust formation takes place within 18 hrs. while in case of Zn-Mg-AI-
Si-Sb coated 100% white rust coverage on the sheet surface happens only after
36 hrs. of exposure. This is because zinc carbonate hydroxide and zinc oxide
observed in Zn-0.16AI-0.10Pb coatings are suppressed in the presence of
magnesium in the coating, and the whole surface is covered with zinc chloride
hydroxide, which is protective for the coating layer.
Table II: Salt Spray Test Results
IRR: Initiation of Red Rust, NRR: No Red Rust Formation
Formability property of the coated sheets has been evaluated through Erichsen
cup tester. During Erichsen cup test, the point at which the cracks/peel-off of the
coating begins, punch movement at that point was taken as the Erichsen cup
value of the coated sheet. From the Table I, it can be noted that the Erichsen cup
value (measured for the sheet thickness of 0.8 mm) of the invented coated sheet
is 10.4 mm as compared to 7.8 mm for the conventional coated sheets.
Coating adherence of the coated sheets has been evaluated by Lock Forming
Tester. Coating adherence, as shown in the tested specimens in Fig.2 through
absence of any cracks contrary to the fine cracks present in the conventional Gl
sheets, is much superior in the invented coated sheets as compared to
conventional Gl sheets.
Microstructure of invented coated sheets, as shown in Fig.3, comprises
prominent and uniform formation of Zn/Mg/AI ternary eutectic phase mixture at
the grain boundaries leading to slower anodic dissolution, formation of protective
MgO & protective corrosion products of zinc and thereby resulting in superior
corrosion resistance of the invented coated sheets. Further, it may be noted in
micrograph of the invented coated sheets (Fig.3), that there is no formation of
Fe-Zn intermetallics at the steel-coating interface due to presence of adequate Al
& Si level in the galvanizing bath, resulting in superior formability and coating
adherence of coated sheets. However, outburst of Fe-Zn intermetallics can be
clearly seen in the micrographs of the conventional Gl sheets leading to its
inferior formability and coating adherence.
Coating appearance of the invented sheets has also been found bright and
smooth contrary to the dull appearance reported for the Zn-Mg coated sheets
developed elsewhere. This is due to higher level of Al: 0.25-0.30% maintained in
the zinc bath during the processing of the developed coated sheets.
HIGHLIGHT OF THE INVENTION
It is thus possible by way of the present invention to provide hot dip galvanized
Zn-Mg-AI-Si-Sb coated sheets with following attributes:
• Corrosion rate: 4.7 mpy (1/3rd w.r.t. corrosion rate of ~15 mpy of
conventional Gl sheets) as measured through polarization resistance
using electrochemical measurements and salt spray test.
• Superior formability (Erichsen cup value : 10.4 mm for sheet thickness
of 0.8 mm) and coating adherence (as per LFQ standard) of the
developed coated sheets as compared to conventional Gl sheets
• Coating appearance bright and smooth, better than that reported for
the Zn-Mg coatings developed elsewhere
• Developed coating is environment friendly as Pb has been replaced
with Sb (for spangle control) in the zinc bath
• Processing control steps to achieve above attributes:
- Zinc bath composition ¦
Zn-{0.25-0.75)%Mg-(0.25-0.30)%AI-(0.05-0.10)%Si-f0 08-
0.12)%Sb
- Coating parameters:
Bath temperature: 420-460°C, Dipping time: 1 5-3.5 sec
Wiping gas flow rate: 250-300 Ipm
Nozzle Distance: 16-18 mm
Wiping Delay Time: 0.5-2.0 sec.
Although the foregoing description of the present invention has been shown and
described with reference to particular embodiments and applications thereof, it
has been presented for purposes of illustration and description and is not
intended to be exhaustive or to limit the invention to the particular embodiments
and applications disclosed. It will be apparent to those having ordinary skill in the
art that a number of changes, modifications, variations, or alterations to the
invention as described herein may be made, none of which depart from the spirit
or scope of the present invention. The particular embodiments and applications
were chosen and described to provide the best illustration of the principles of the
invention and its practical application to thereby enable one of ordinary skill in the
art to utilize the invention in various embodiments and with various modifications
as are suited to the particular use contemplated. All such changes, modifications,
variations, and alterations should therefore be seen as being within the scope of
the present invention as determined by the appended claims when interpreted in
accordance with the breadth to which they are fairly, legally, and equitably
entitled.
We claim:
1. A hot dip galvanized steel sheet with improved corrosion resistance
comprising a peripheral coating of composition 0.25-0.75%Mg, 0.25-0.30%AI,
0.05-0.10%Si, 0.08-0.12%Sb and rest zinc.
2. A hot dip galvanized steel sheet with improved corrosion resistance as
claimed in claim 1, wherein the coating thickness is around 23.8^m.
3. A hot dip galvanized steel sheet with improved corrosion resistance as
claimed in claim 1, wherein the Formability (Ecv) is around 10.4 mm (t . 0.8mm)
or more depending upon the formability of the substrate steel sheet.
4. A method for producing hot dip galvanized Zn-Mg-AI-Si-Sb coated steel
sheets comprising the steps of:
annealing of the CQCR sheets in N2+H2 atmosphere;
cooling of the annealed sheet with N2 gas upto near bath temperature;
dipping of steel sheet in molten zinc bath of composition 0.25-0.75%Mg,
0.25-0.30%AI, 0.05-0.10%Si and 0.08-0.12%Sb;
wiping of the molten zinc coated steel sheet by N2 gas; and
cooling of the zinc coated steel sheet with N2 gas.
5. A method for producing steel sheets as claimed in claim 4, wherein the
atmosphere for the purpose of annealing comprises of H2 which varies from 10 to
20% and dew point varies from -20 to -30°C respectively, based on the
cleanliness level of the steel substrate.
6. A method for producing steel sheets as claimed in claim 4, wherein
depending on the sheet thickness the annealing cycle comprises heating of the
CQCR sheets at the rate of 20-30°C/s upto annealing temperature of 700-750°C
and holding at the said temperature for 45-60 sec.
7. A method for producing steel sheets as claimed in claim 4, wherein the
cooling of the annealed sheets is carried out at temperature of 5oC - 10°C more
than the bath temperature and at the rate of 3-5°C/s
8. A method for producing steel sheets as claimed in claim 4, wherein the
dipping of the steel sheet in molten zinc bath is carried out at temperature of 420-
460°C and the steel sheet is dipped for 1.5-3.5 sec in the said bath.
9. A method for producing steel sheets as claimed in claim 4, wherein the
wiping delay time, wiping gas flow rate and nozzle distance kept in the range of
0.5-2.0 sec, 250-300 litre/min. and 16-18 mm respectively.
10. A method for producing steel sheets as claimed in claim 4, wherein the
cooling of the zinc coated steel sheet is carried out at the rate of 4-8°C/s upto
room temperature

The present invention relates to hot dip galvanized Zn-Mg-AI-Si-Sb coated steel
sheets with improved corrosion resistance, comprising a peripheral coating of
composition 0.25-0.75%Mg, 0.25-0.30%Al, 0.05-0.10%Si, 0.08-0.12%Sb and
rest zinc and the method for producing the same.