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The Effect of Mn Additions to Galvanizing Baths on the Formation of Intermetallic Compounds

N. Gao, Y.H. Liu and N.-Y. Tang TOFA 2010, Porto, PortugalSeptember 12-16, 2010

About Teck

• A leading producer of copper• The second largest exporter of

steelmaking coal• The second largest producer of zinc

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2009

AHSS

• Advanced high strength steels (AHSS) are increasingly required for the manufacture of light-weight vehicles due to their superior combination of strength and ductility.

North American Light Vehicle Metallic Material Trends*

*Courtesy of J.N. Hall, General Motors Manufacturing Engineering 3

Mn in AHSS

• Mn is the most important alloying element in AHSS.

• Most AHSS, such as TRIP (Transformation Induced Plasticity), DP (Dual Phase) and CP (Complex Phase) steels, contain Mn from 1.0 to (Complex Phase) steels, contain Mn from 1.0 to 2.5 wt.%.

• TWIP (Twinning Induced Plasticity) steels, a new generation of AHSS, contain Mn up to 25%.

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Continuous Galvanizing

• Continuous hot dip galvanizing provides AHSS with a zinc-based coating for corrosion protection.

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Galvanizing Bath

• During continuous galvanizing, the steel strip is dipped in a molten zinc bath at a temperature which is typically 460°C for 2 to 3 s.

• A galvanizing bath contains either 0.16 to 0.25% Al for galvanizing (GI) or 0.11 to 0.13% Al for galvannealing (GA).

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1 23

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Fe-Zn Binary System

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1

ζ no longer exists above 530°C

1 Steel substrate

2 Γ1-Fe5Zn21 layer

3 δ-FeZn10 layer

4 ζ-FeZn13 layer

5 Free Zn layer

ζζζζ-FeZn13δδδδ-FeZn10 δδδδ-FeZn10 + ηηηη-Fe2Al5

ηηηη-Fe2Al5

Zn-Rich Corner of the Zn-Fe-Al System at 460°C

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• Studies of the galvanizability of AHSS indicate: – Mn builds up in the bath following its dissolution from the

steel substrate. – A full inhibition of the growth of Fe-Zn alloys in the coating is

not materialized until the bath Al level reaches 0.17%.– The coatings on high Mn-containing AHSS are easily

Galvanizing of AHSS

– The coatings on high Mn-containing AHSS are easily galvannealed into full-δ alloy coatings even at a temperature below 530°C.

• During galvanizing, the existence of Mn in AHSS converts the Zn-Fe-Al system into a Zn-Fe-Al-Mn quaternary system in the vicinity of the coating/substrate interface.

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Mn-Zn Binary System

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• δ transforms into δ1 at low temperatures.• δ is considered the hexagonal MnZn9 phase.• ζ-MnZn13 is isostructural with ζ-FeZn13.• The ζ-MnZn13 phase is stable below 428°C while the ζ-FeZn13 phase is stable up to

530°C.

• Mn has a high solubility in ζ-FeZn13and δ-FeZn10 and can replace 75% and 90% of the Fe in the two compounds, respectively.

• When Mn content is above 2.4 at.%, the liquid phase is in

Zn-Rich Corner of Zn-Fe-Mn System at 450°C

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at.%, the liquid phase is in equilibrium with δ-MnZn9.

Reference: G. Reumont, G. Dupont and P. Perrot, The Fe-Zn-Mn System at 450°C,Z. Metallkd. 86(9), 1995, p 608-613.

• Three starting bath alloys were selected:

a Zn-0.14%Al-0.07%Feb Zn-0.16%Al-0.07%Fec Zn-0.27%Al-0.07%Fe

a b c

L + ηηηη

Studied Bath Alloys

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c Zn-0.27%Al-0.07%Fe• Four levels of Mn: 0.30%,

0.80%, 1.30% and 2.50%were added to the above baths, resulting in a total of 12 bath alloys.

L + δδδδL + δδδδ + ηηηη

Bath Temperature: 460°C

• The sample was taken at three depths of the bath using a quartz tube equipped with a sucking ball.

• The quartz tube was quenched into water immediately after being withdrawn from the bath.

Bath Alloy Sampling

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Top sample

Middle sample

Bottom sample

Sucking ball

Quartz tube Zn bath

Bath Sample Analysis

• The middle samples were analyzed using an inductively coupled plasma (ICP) spectrometer to determine the Al, Fe and Mn contents �close representations of their solubilities in the liquid phase. liquid phase.

• Top and bottom samples were examined using a scanning electron microscope equipped with an X-ray energy dispersive spectroscope (SEM-EDS) to determine intermetallic compounds co-existing with the liquid phase.

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Alloy Design composition (wt.%) Compound Fe Al Mn Zn

1 Zn-0.14Al-0.07Fe-0.30Mn δ-FeZn10

δ-MnZn9

6.6

0.8

0.9

0.9

2.8

7.3

89.8

91.0

2 Zn-0.14Al-0.07Fe-0.80Mn δ-FeZn10 5.6 1.4 3.0 90.0

Intermetallic Compounds in Alloys 1 to 4

2 Zn-0.14Al-0.07Fe-0.80Mn δ-FeZn10

δ-MnZn9

5.6

0.7

1.4

1.3

3.0

7.0

90.0

91.1

3 Zn-0.14Al-0.07Fe-1.30Mn δ-FeZn10

δ-MnZn9

4.2

0.8

0.6

0.2

5.8

6.9

89.4

92.1

4 Zn-0.14Al-0.07Fe-2.50Mn δ-MnZn9 0.3 0.8 9.1 89.9

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Intermetallic Compounds in Alloys 1 to 4 (continued)

SEM micrographs show δ-FeZn10 existing in the top sample (a) and δ-MnZn9 in thebottom sample (b) taken from Alloy 1.

(a) (b)

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Intermetallic Compounds in Alloys 5 to 8

Alloy Design composition (wt.%) Compound Fe Al Mn Zn

5 Zn-0.16Al-0.07Fe-0.30Mn δ-FeZn10

η-Fe2Al5

7.7

36.3

1.8

45.3

1.5

0.6

88.9

17.82 5

6 Zn-0.16Al-0.07Fe-0.80Mn δ-FeZn10

δ-MnZn9

4.9

1.0

1.4

1.9

4.4

7.1

89.2

90.1

7 Zn-0.16Al-0.07Fe-1.30Mn δ-FeZn10

δ-MnZn9

3.7

0.5

1.1

1.1

6.1

7.8

89.1

90.6

8 Zn-0.16Al-0.07Fe-2.50Mn δ-MnZn9 0.4 0.9 8.8 90.0

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Intermetallic Compounds in Alloys 5 to 8 (continued)

Backscattered electron (BSE) micrographs: (a) co-existence of δ-FeZn10 and η-Fe2Al5 in the top sample of Alloy 5 and (b) δ-MnZn9 existing as the soleintermetallic compound in the bottom sample taken from Alloy 8.

(a) (b)

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Alloy Design composition (wt.%) Compound Fe Al Mn Zn

9 Zn-0.27Al-0.07Fe-0.30Mn η-Fe2Al5 32.0 54.0 0.6 13.4

10 Zn-0.27Al-0.07Fe-0.80Mn δ-MnZn9 0.7 4.7 7.5 87.0

Intermetallic Compounds in Alloys 9 to 12

10 Zn-0.27Al-0.07Fe-0.80Mn δ-MnZn9

η-Fe2Al5

0.7

30.5

4.7

54.8

7.5

1.8

87.0

13.0

11 Zn-0.27Al-0.07Fe-1.30Mn δ-MnZn9 0.4 2.6 8.0 89.0

12 Zn-0.27Al-0.07Fe-2.50Mn δ-MnZn9 0.4 1.2 8.3 90.1

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Intermetallic Compounds in Alloys 9 to 12 (continued)

Top sample from Alloy 9 Top sample from Alloy 10

Top sample from Alloy 11 Bottom sample from Alloy 11

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Liquid Domain of the Zn-Fe-Al-Mn System at 460°C

Al Fe Mn

1 0.136 0.030 0.00

2 0.100 0.031 0.00

3 0.000 0.041 0.00δ1

4 0.000 0.025 1.72

5 0.000 0.011 2.15

N.-Y. Tang, Journal of Phase Equilibria, 2000, 21(1), p 70-77.G. Reumont, G. Dupont and P. Perrot, Z. Metallkd., 86(9), 1995, p 608-613.

ζ: FeZn13

δ1: FeZn10

δ: MnZn9

η: Fe2Al5

δ1+δ+η+L

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Conclusions

• Four intermetallic compounds, δ-MnZn9, δ1-FeZn10, ζ- FeZn13 and η-Fe2Al5, co-existed with the liquid metal in the composition range of interest to the galvanizing operation.

• Low Mn additions stabilize the δ-FeZn10 phase while • Low Mn additions stabilize the δ-FeZn10 phase while high Mn additions favour the formation of δ-MnZn9.

• Both δ phases develop at the expense of η-Fe2Al5. With increasing Mn content in the system, the ζ/δtransition occurs at an ever decreasing effective Al level, and the knee point shifts to a higher Al level.

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Conclusions (continued)

Important implications for galvanizing applications:• Mn additions to the bath promote the formation of δ phases. • A full inhibition effect requires a higher bath Al content when

galvanizing Mn-containing AHSS. • A small addition of Mn to the bath can facilitate the • A small addition of Mn to the bath can facilitate the

galvannealing process to readily produce a full-δ coating. • It is necessary to monitor the Mn content in the bath during

the galvanizing of high Mn-containing AHSS to ensure sufficient inhibition.

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The authors are grateful to Fred N. Coady for carrying out part of the experimental work.

Thank you for your kind attention!

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Intermetallic Compounds of Interest

Phase Pearson

symbol

Space

group

Lattice parameter, nm Zn (at.%)

a b c

δ-FeZn10 hP555 P63mmc 1.2820 - 5.7040 86.5-91.8

ζ-FeZn13 mC28 C2/m 1.3424 0.7608 0.5061 92.8-94.0

η-Fe Al oC14 Cmcm 7.633 6.470 4.229

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η-Fe2Al5 oC14 Cmcm 7.633 6.470 4.229

ζ-MnZn13 mC28 C2/m 1.3483 0.7663 0.5134 92.6 to 92.9

δ-MnZn9 hP* - 86.5 to 90.6

δ1 hP555 P63/mmc - 88.9 to 90.9