2. 2
Much of the success of coated sheet steel is due to a switch from uncoated
to coated sheet in the automotive industry and its increased use in the
building industry and household appliance manufacturing. By coating steel
via galvanizing or other coating technologies, the steel gains superior
corrosion resistance at a low cost without impacting the availability or
recycle-ability of the product. This has lead to its widespread use in
manufacturing. The session includes cosmetic and perforation corrosion
protection of sheet steel, the different types of metallic and organic
coatings for hot and cold rolled strip products, and the essentials of the
different coating technologies. Also included is the origin of the coating
microstructure and the importance of metallic and organic coatings on hot
and cold-rolled strip products.
Coating Technologies
Introduction to Coating Technologies for CRS
3. 3
Widespread use in manufacturing
Superior corrosion resistance
Low cost
Availability
Recycle-ability
Diversity of coating types
Favorable application
characteristics:
Excellent coating adhesion
Formability
Weldability
Phosphatability.
Much of the success of coated sheet
steel is due to a switch from uncoated
to coated sheet in the automotive
industry and its increased use in the
building industry and household
appliance manufacturing
Coated Sheet Steel
4. 4
Cosmetic corrosion on CRS
Filiform corrosion
Perforation corrosion on CRS
Corrosion of Uncoated Steel
5. 5
CRS
Red rust
Paint undercreep
Hot Dip Galvanised
No red rust
«white» rust
Zn-Fe Galvannealed
Low corrosion rate
Excellent paint adhesion
,
Corrosion of Coated Steel
6. 6
1. Cathodic disbondment:
Cathodic reduction of dissolved oxygen:
O2 + 2H2O + 4e- → 4OH-
Anodic reaction occurs at a coating defect:
Fe → Fe2+ +2e-
2. Oxide lifting: when anodic corrosion products accumulate under the
coating.
The cathodic reaction during anodic lifting:
O2 + 2H2O + 4e- → 4OH-
At the metal surface in the scribe or possibly at the outer magnetite interface:
8FeOOH + Fe2+ + 2e- → 3Fe3O4 + 4H2O
Uncoated steel
Corrosion of Coated Steel
8. 8
Zn Electro-coated coating thicknesses:
2.5 - 15µm/side or 20-105g/m2
The density of the Zn-coating is 7.14 g/cm3.
HDG Zn coatings:
Constructional steels:
Important: long term corrosion resistance requirements
Coating thickness (can be) high: 700 g/m2 or 49 µm/side.
Automotive steels:
Important: cosmetic and perforation resistance
GI Coating thickness: 6 - 20µm/side (40-140g/m2)
GA Coating thickness: 6 - 11µm/side (40-80g/m2)
Corrosion of Coated Steel
16. 16
Direct Fired Furnace
Hot Dip Galvanized Zn and Zn-alloy Coatings
Impingement burner panel of a Direct Fired Furnace.
Direct Fired Furnace characteristics:
1. Controlled air/gas ratio, with ~1000ppm of excess 02
2. Rapid and homogenous strip heating to 700ºC-750ºC
3. Strip surface cleaned by contact with combustion gas
4. Oxidizes the strip surface
5. Oxygen diffuses into the sheet
19. 19
DFF+RTF Annealing Furnace Equipment
Hot Dip Galvanized Zn and Zn-alloy Coatings
Burners
RTF with N2+5%H2 reducing gas atmosphereDFF
Pyrometer
Recuperative
Pre-heater
Heating
zone
Flue gas heats
incoming strip
Pyrometer
Heating
zone
Soaking
zone
Post-combustion
chamber
Pyrometer
Air inlet
Gas fired
radiant tubes
Transfer section with air-lock
Maximum strip
temperature 850ºC
Preheating strip
temperature 320ºC
DFF strip
temperature 750ºC
To the cooling sections…
20. 20
Furnace Cooling Section Equipment
Hot Dip Galvanized Zn and Zn-alloy Coatings
RTFDFF
Slow
cooling
Fast
gas-jet cooling
up to 100ºC/s, 1mm strip
To the Zn pot…
Snout dipped in Zn pot
Slow to fast cooling transfer zone
with hot bridle to increase the strip
tension before rapid cooling
Exit section equipment:
Small over-aging section
Post heating booster to reheat the strip for cooling below Zn pot temperature
Hot bridle to increase the strip tension (e.g. 10MPa to 25MPa)
22. 22
Hot Dip Galvanized Zn and Zn-alloy Coatings
During annealing the selective oxidation of alloying
elements such as Mn, Si, … takes place at the steel
surface and in the sub-surface
Dew Point
Furnace Gas Atmosphere
Weigth-%SiinSteel
T: 850ºC
N2+H2
-27ºC +14ºC
External
SiO2
Internal
SiO2
0% Si
1% Si
2% Si
3% Si
Me-oxide
Me-oxide
Internal
oxidation
External
oxidation
23. 23
Hot Dip Galvanized Zn and Zn-alloy Coatings
During annealing the selective oxidation of alloying
elements such as Mn, Si, … takes place at the steel
surface and in the sub-surface
Example for Fe-0.16%C-2%Mn-1%Al TRIP Steel Surface
Intercritical Annealing temperature: 827ºC
Furnace gas compositions N2+10%H2
Atmosphere dew point: -30ºC Oxide particles
Oxide film
Oxides730nm
Nitrides900nm
500nm
24. 24
“Heat-to-coat”: No Rex-annealing
Pickling Furnace
Rinsing, Drying
Cooling
Molten
Zn
Cooling
600oC
60s
Dipping
Time
Temperature
Pickling
Induction furnace
Zn pot
Hot Dip Galvanized Zn and Zn-alloy Coatings
Hot dip Galvanizing of Hot Rolled Strip
25. 25
Fe-oxide
reduction
Selective
oxidation
Snout: exposure
Zn metal vapor
Zn-pot:
1. Strip surface dissolution
2. Inhibition layer formation
Galvannealing:
1. Inhibition layer breakdown
2. Fe-Zn reaction
Gas wipers:
1. Final wetting
2. Solidification
Annealing furnace
Hot Dip Galvanized Zn and Zn-alloy Coatings
27. 27
• Zn bath management involves the control of the effective Al
content in the bath, the timing of the Al additions to the Zn
bath, the solute Fe content, the control of the bath hardware.
FeZn13, FeZn7
« Top Dross »
Fe2Al5-xZnx
solute Fe
Aleff
« Bottom Dross »
Annealing Furnace
Stabilizing Rolls
Air-knives
Touch Rolls
GA- Furnace
Snout
50mm
Fe2Al5
Top dross
Bottom dross
Zn
Hot Dip Galvanized Zn and Zn-alloy Coatings
Zn Bath Management
29. 29
Liquid
+
Liquid
+
Liquid
+
Fe2Al5
Liquid Zn
+
Fe, Al in solution
Al content, mass-%
Fecontent,mass-%
0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10 Increase of the Al content:
Lowers the Fe solubility
Changes of compound from to to Fe2Al5
Changes the GA kinetics
Hot Dip Galvanized Zn and Zn-alloy Coatings
Zn Bath Management
30. 30
Al content control:
1. Low, i.e. 0.12 mass-%, for galvannealing (weak inhibition Fe-Zn reactions)
2. High, i.e. 0.2 mass-%, for galvanizing (strong inhibition Fe-Zn reactions by Fe2Al5 formation
FeZn13, FeZn7
« Top Dross »
Fe2Al5-xZnx
solute Fe
Aleff
« Bottom Dross »
Annealing Furnace
Stabilizing Rolls
Air-knives
Touch Rolls
GA- Furnace
Snout
Liquid
+
Liquid
+
Liquid
+
Fe2Al5
Liquid Zn
+
Fe, Al in solution
Liquid+
+
Fe2Al5
Al content, mass-%
Fecontent,mass-%
0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
Liquid+
+
Al addition:
1. Pre-melting in separate induction melting unit
2. Direct addition of small Zn-Al alloy bars
Hot Dip Galvanized Zn and Zn-alloy Coatings
Zn Bath Management
31. 31
Al content distribution:
1. The Al content is homogeneous, due to the efficient mixing by the strip motion and the
turbulence caused by the induction heating.
2. The temperature distribution is the most important factor; it is also homogeneous.
451°C
450°C
447°C
0.13%
0.126%
0.14%
Temperature distribution Solute Al distribution
Hot Dip Galvanized Zn and Zn-alloy Coatings
Zn Bath Management
32. 32
Liquid
+
Liquid
+
Liquid
+
Fe2Al5
Liquid Zn
+
Fe, Al in solution
Al content, mass-%
Fecontent,mass-%
0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
Bath-T: 461°C
Surface sampling
Bottom sampling
Solute Al content :
The solute Al content, Al effective, is not the same as
the total Al content
Determination of the effective Al content requires an
accurate temperature control
Hot Dip Galvanized Zn and Zn-alloy Coatings
Zn Bath Management
34. 34
0 100 200 300 400
0
100
200
300
400
500
600
700
800
Temperature,°C
Time, s
Soaking
Hot dipping in Zn bath
Galvannealing
Hot Dip Galvanized Zn and Zn-alloy Coatings
The Thermal Cycle
35. 35
Hot Dip Galvanized Zn and Zn-alloy Coatings
The Thermal Cycle: CA vs HDG Line Cycles
0 200 400 600 800
0
100
200
300
400
500
600
700
800
CA Line
with overaging
Temperature,C
Time, s
0 100 200 300 400 500
Time, s
0 100 200 300 400 500
Time, s
0 100 200 300 400 500
Time, s
HDG Line
no overaging
Advanced
HDG Line
HDG Line
with overaging
Al-killed
Low C grades
Interstitial-free
grades
AHSS
grades
AKLC and IF
grades
41. 41
Fe in coating, m-%
0 2 4 6 8 10 12 14
Powdering,g/m2
0
2
4
6
8
10
12
14
16
18
20
Hot Dip Galvanized Zn and Zn-alloy Coatings
Powdering of Galvannealed Coatings
~10% Fe
42. 42
10 µm 10 µm
Steel
Coating
Zn-5%Al Galfan coating. The matrix is primary Zn and the darker areas are Al-rich.
Note that the eutectic solidification leads to a lamellar or a rod-like microstructure.
Hot Dip Galvanized Zn-5% Al Alloy Coatings
Zn-Al eutectic solidification
47. 47
Phosphate layer on galvanised, galvannealed and electroplated pure Zn coatings
Phosphate crystal types on cold rolled steel
and pure Zn electroplated coatings.
Substrate Phosphate crystals
CRS
Electroplated Zn
(ZE)
Zn2(Fe,Mn)(PO4)2.4H2O:
phosphophyllite
Zn3(PO4)2.4H2O: hopeite
Mn2Zn(PO4)2.4H2O
Zn3(PO4)2.4H2O: hopeite
Mn2Zn(PO4)2.4H2O
Phosphating
48. 48
• The cosmetic and perforation corrosion protection of
sheet steel, the different types metallic and organic
coatings for (hot and) cold rolled strip products, and
the essentials of the different coating technologies,
were presented.
• The origin of the coating microstructure, in particular
the structure of galvanized and galvannealed coatings,
was discussed.
• The importance of metallic and organic coatings on
the continue use of (hot and) cold rolled strip products
was emphasized.
Session Conclusions