Introduction to Steel Coating Technologies_BCDeCooman_2016

1

Coating Technologies

B.C. De Cooman

MDL

GIFT, POSTECH

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 TechnologiesIntroduction to Coating Technologies for CRS

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

Cosmetic corrosion on CRS

Filiform corrosionPerforation corrosion on CRS

Corrosion of Uncoated Steel

5

CRS

Red rustPaint undercreep

Hot Dip Galvanised

No red rust

«white» rust

Zn-Fe Galvannealed

Low corrosion rate

Excellent paint adhesion

,

Corrosion of Coated Steel

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


7


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)


9

Coating Weight, g/m2

Tim

e t

o 5

% R

ed

Ru

st

Fo

rmati

on

, h

ou

rs 500

400

300

200

100

0

0 20 40 60 80 100

Electro-galvanized

Hot Dip Galvanized

Zn-Ni Fe

Zn Zn

Galvanic corrosion

protection

Low atmospheric

corrosion rate of Zn

pH0 2 4 6 8 10 12 14

Co

rro

sio

n r

ate

, c

m/y 0.5

0.4

0.3

0.2

0.1

0


10

Electrolytic Zn Coatings of CRS

11

Sh

eet

Conductor

rolls

Electrolyte: sulphate

Cell volume: ~8m3

Current density: ~150A/dm2

Strip velocity: ~200m/min

Insoluble

Ti-IrO2 anodes

+ - +

Zn, Zn-alloy layer

Electrolytic Zn Coatings of CRS

12

pure Zn (ZE) Zn-Ni (ZNE) Zn-Fe (ZFE)

Electrolytic Zn, Zn-Alloy Coatings on CRS

13

Weight-% Zn

Te

mp

era

ture

, °C

500

800

700

600

300

400

200

60 70 80 90 Zn

Weight-% Zn

60 70 80 90 Zn5040

Al

Zn

95%

419°C

660°C

liquid

Weight-% Si

10 20 30 40Al

Al

660°C

11.7% Si

660°C

liquid

665°C

530°C550°C

425°C

Galvalume

Galfan

Galvanized

Aluminized

Galvannealed

Galvanized

Metallic Alloy Coatings on CRS

14

Hot Dip Galvanized Zn and Zn-alloy CoatingsGeneral Layout Continuous Hot Dip Galvanizing Line

Pay-off

reels

Tension

reels

Welder

Pre-cleaning

section

Cleaning

section

Entry

looper

Post-treatments

section

Delivery

looper

Tension

leveller

Skinpass

mill

Cooling

tower

Zn

pot

GA

furnace

Pre-heating

Heating

Soaking

Gas-jet cooling

Low-T holding

15

RTFDFF

Slow

Cooling

section

Fast

Cooling

section

Overaging

section Snout

Zn Pot

Cooling

Water

Quench

HDG line with over-aging section

Strip Width: 0.7-1.6m

Strip thickness: 0.25-2.5mm

Production: 350.000mtpy (~80tph)

T max: 820°C

HDG Line with Direct Fired Furnace + Radiant Tube Furnace

Hot Dip Galvanized Zn and Zn-alloy Coatings

Cooling tower

16

Direct Fired Furnace


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

17

Radiant tube

furnace

Cooling

section

Direct Fired Furnace + Radiant Tube Furnace


Maximum strip

temperature 850ºC

Cooling after Zn pot

220°C (to 45°C)

18

Cooling

section

Gas-jet cooling


Hot

Cooling gas

out

Cold

Cooling gas

out

19

DFF+RTF Annealing Furnace Equipment


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…

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Furnace Cooling Section Equipment


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)

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Hot

gauges

Galvannealing

furnace

An

ne

alin

g fu

rna

ce

Ga

lva

nn

ea

lin

g

furn

ac

e

Gas

wipers

Front view Side view Cross-sectional view

Zn pot

Cooling

section

Zn Pot Equipment


Snout

Zn pot

22

Hot Dip Galvanized Zn and Zn-alloy CoatingsDuring 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

Weig

th-%

Si

in S

teel

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

Hot Dip Galvanized Zn and Zn-alloy CoatingsDuring 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 particlesOxide film

Oxides730nm

Nitrides900nm

500nm

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“Heat-to-coat”: No Rex-annealing

Pickling Furnace

Rinsing, Drying

Cooling

Molten

Zn

Cooling

600oC

60s

Dipping

Time

Te

mp

era

ture

Pickling

Induction furnace

Zn pot

Hot Dip Galvanized Zn and Zn-alloy CoatingsHot dip Galvanizing of Hot Rolled Strip

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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


26

10µm

STEEL

Galvanized

Fe2Al5-xZnx

inhibition layer

(<0.5mm)

Zn solid solution

0.3-0.5 m-% Al

0.05 m-% Fe

Al2O3-rich

surface layer

mass-%

Al

30-50

0.1-0.2

Zn Bath Management: Inhibition layer formation


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• 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

Fe2Al5Top dross

Bottom dross

Zn

Hot Dip Galvanized Zn and Zn-alloy CoatingsZn Bath Management

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Zn-Fe-Al diagram at 465°C

Liquid

+

Liquid

+

Liquid

+

Fe2Al5Liquid Zn

+

Fe, Al in solution

Liquid+

+Fe2Al5

Al content, mass-%

Fe

co

nte

nt,

mas

s-%

0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.200

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

Liquid+

+

50mm

Fe2Al5Top dross

Bottom dross

Zn


29

Liquid

+

Liquid

+

Liquid

+

Fe2Al5Liquid Zn

+

Fe, Al in solution

Al content, mass-%

Fe c

on

ten

t, m

ass-%

0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.200

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 Fe2Al5Changes the GA kinetics


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

+

Fe2Al5Liquid Zn

+

Fe, Al in solution

Liquid+

+Fe2Al5

Al content, mass-%

Fe c

on

ten

t, m

ass-%

0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.200

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


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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


32

Liquid

+

Liquid

+

Liquid

+

Fe2Al5Liquid Zn

+

Fe, Al in solution

Al content, mass-%

Fe

co

nte

nt,

mass

-%

0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.200

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

Bath-T: 461°CSurface 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


33

Position

Zn

th

ick

ne

ss

Air-knive

<10 to >100mm

Zn thickness control

Air Knife

Hot Dip Galvanized Zn and Zn-alloy CoatingsZn Pot Equipment: the air knife

34

0 100 200 300 400

0

100

200

300

400

500

600

700

800

Te

mp

era

ture

, °C

Time, s

Soaking

Hot dipping in Zn bath

Galvannealing

Hot Dip Galvanized Zn and Zn-alloy CoatingsThe Thermal Cycle

35

Hot Dip Galvanized Zn and Zn-alloy CoatingsThe 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

Tem

pera

ture

, C

Time, s0 100 200 300 400 500

Time, s0 100 200 300 400 500

Time, s0 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

36

Zn-pot

Time

Tem

pera

ture

Ac3

Ac1

Ms

TRIP Grades

DP Grades

Q&P Grades

Hot Dip Galvanized Zn and Zn-alloy CoatingsNew Thermal Cycles for Galvanized AHSS

DP, TRIP and Q&P Grades

Strip reheating (inductors)

37

0 100 200 300 400 500 600

0

200

400

600

800

280C

350C

GA 510C

-0.4 C/s

DP steel processing

Line speed : 100mpm

-3 C/s

-7.6 C/s

650C

Tem

pera

ture

(C

)

Time (sec)

3 C/s

790C

-6.5 C/s

GI: 460C

560C

0 100 200 300 400 500 600 700 800 900 1000

0

200

400

600

800

DP Steel processing

Line speed : 150mpm

-2.5 C/s

270C

-19C/s

Tem

pera

ture

(C

)

Time (sec)

2.5 C/s

790C

-3 C/s

650C

0 100 200 300 400 500

0

200

400

600

800

280C

350C

GA 510C

DP Steel processing

Line speed: 100mpm

-3 C/s

-9.2 C/s

650C

Tem

pera

ture

(C

)

Time (sec)

3 C/s

790C

-6.5 C/s

GI: 460C

560C

10 vol-% g

a‘

Ms

Mf

~400s ~150s

Examples of Processing of DP in HDG Lines

38

STEEL

Galvannealed

-phase FeZn3

-phase FeZn8

-phase FeZn13 5.7-6.3

7.9-10.9

20-28

STEEL

Galvanized

Fe2Al5-xZnx

inhibition layer

(<0.5mm)

Zn solid solution

0.3-0.5 m-% Al

0.05 m-% Fe

Al2O3-rich

surface layer

mas

s-%

Al

30-50

0.1-0.2

mass-%

Fe


39

outbursts

10 µm

Hot Dip Galvanized Zn and Zn-alloy CoatingsThe Galvannealing Reaction

40

Hot Dip Galvanized Zn and Zn-alloy CoatingsThe Galvannealing Reaction

0 2 4 6 8 10 12 0 2 4 6 8 10 120 2 4 6 8 10 12

0

10

20

30

40

50

60

70

80

90

100

Fe in coating, m-%

0 2 4 6 8 10 12

%

: Zn : FeZn13 : FeZn7 : FeZn3

~10% Fe

41

Fe in coating, m-%

0 2 4 6 8 10 12 14

Po

wd

eri

ng

, g

/m2

0

2

4

6

8

10

12

14

16

18

20

Hot Dip Galvanized Zn and Zn-alloy CoatingsPowdering of Galvannealed Coatings

~10% Fe

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

43

Hot Dip Galvanized Al-10% Si Alloy Coatings

44

Sheet steel substrate

Metallic coating

(20-120 g/m2 Zn, Zn-alloy, …)

Cr-free pre-treatment3-35 mm Organic primer

15-200mm Organic topcoat

Organic backcoat

Organic primerPre-treatmentMetallic coating

Organic Coatings

45

Capacity: 192,000 t/a

Organic Coatings

46

20 µm

Zn particle

Organic CoatingsNew Coatings

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

• 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

Documents

Introduction to Steel Coating Technologies_BCDeCooman_2016