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The Influence of Processing Parameters on the Coating

Hardness/Ductility Behaviour of 55%Al-Zn Coated Steel

Per Carlsson* and Mikael Olsson

Dalarna University

SE-781 88 Borlänge, Sweden *E-mail: pca@du.se

Telephone: +46 (0) 23 77 86 26

Fax: +46 (0) 23 77 86 01

Abstract

The influence of different processing parameters on the coating cracking behaviour of

55%Al-Zn coated steel has been evaluated by statistical design of experiment, DOE. In these

experiments the four response variables viz.; hardness, area fraction of cracks, the mean crack

width, and cracking inter distance are connected to the major process parameters; coating

thickness, temper rolling, post heat treatment and ageing. Scanning electron microscopy

(SEM), energy dispersive x-ray spectroscopy (EDX), image analysis and micro hardness

measurements were used to characterise the coated samples.

The results show that statistical design of experiments provides a good method of quantifying

the effects of various process parameters on the coating cracking behaviour of 55%Al-Zn

coated steel. The hardness of the coating was significantly influenced by temper rolling, post

heat treatment and coating thickness. Temper rolling gives a small deformation hardening

effect, while heat treatment transforms coherent Guinier-Preston zones to greater and softer

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phases and therefore decreases the coating hardness. The cracking tendency was found to be

significantly decreased by heat treatment as a result of the increasing ductility.

Keywords: Hot-dip coated steel sheet; 55%Al-Zn; statistical design of experiment (DOE);

ductility; cracking characteristics.

1 Introduction

Hot-dip zinc and zinc-aluminium alloy coated steel are today frequently used in a large

number of industrial applications, e.g. in the building and automotive industry. In many of

these applications the performance of the coated steel is controlled by its formability,

weldability, paintability, surface finish and corrosion resistance. Unfortunately many of the

forming operations may result in severe cracking of the coating and exposure of the steel

substrate and thus a reduced corrosion resistance of the product. Consequently, it is of

outmost importance to understand the effect of different process parameters on the coating

cracking behaviour of the material in order to avoid extensive cracking during forming.

The ductility and coating cracking behaviour of 55%Al-Zn coating has been investigated in

previous works [1,2,3]. Observations of 55%Al-Zn coated steel strained in uniaxial and planar

tension have shown that the coating has a relatively low ductility with crack initiation at

tensile strains as low as 2-5 %. Cracks may nucleate in the intermetallic layer, at silicon

particles, at dross (intermetallic particles) or at pores within the coating [4,5]. The individual

importance of these nucleation sites is difficult to proclaim.

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Due to ageing, i.e. precipitation hardening, the coating will obtain a relatively high hardness

(and consequently a low ductility) during room temperature storage. The maximum hardness

will be obtained approximately six weeks after coating deposition [6].

In order to obtain a planar and smoother surface of improved paintability, the coated sheet is

temper rolled, frequently using sand blasted rolls, at reductions of less than 1% true strain, in

a continuous rolling mill. It has been shown that this treatment induces isolated cracks in the

intermetallic layer in connection to asperity indentations [7].

The coating thickness is controlled by the gas flow in the air jet knifes used for removal of

superfluous melted metal. The thickness of the intermetallic layer at the coating/steel substrate

interface is mainly determined by the speed of the strip throw the bath. It is expected that an

increased coating thickness as well as an increased intermetallic layer thickness will increase

the cracking tendency. However, Willis et al. [2] observed that intermetallic layer thicknesses

within 1-6 µm showed similar cracking tendency.

By post heat treatment the strength (ductility) of the coating may be decreased (increased).

The use of heat treatment to improve the ductility of 55% Al-Zn coating has been

demonstrated in previous investigations [2,4,8,9]. The improved ductility is mainly due to

precipitation reactions and particle coarsening. Willis et al. [2] found that heat treatment

significantly reduces the crack severity if the coating is heat treated at 200 °C for 30 minutes

followed by furnace cooling resulting in a slow cooling rate of 5 °C/min. By this kind of heat

treatment the level of cracking found on a sample deformed to 18% can be reduced to that

normally found on a sample deformed to 6%.

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There have been several previous investigations of the cracking behaviour (ductility) of 55%

Al-Zn coating. Nevertheless, all these studies were done by the classical method of

experimentation, which allowed variation of only one factor at a time. The present

investigation was carried out by varying all the selected factors simultaneously with the help

of statistical design of experiment (DOE). The factors were chosen on the basis of knowledge

about the process, complemented by information found in the literature.

In the present investigation statistical design of experiments has been used to develop

regression equations illustrating the influence of process parameters on the cracking

behaviour of the coating. In these experiments three response variables viz.; area fraction of

cracks, the mean crack width, and cracking inter distance are connected to the major process

parameters; coating thickness, post heat treatment, temper rolling and ageing. Scanning

electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX), image analysis and

micro hardness measurements were used to characterise the samples.

2 Statistical design of experiments

There are a lot of benefits of using statistical design of experiments (DOE) in the development

and optimisation of materials and processes [10]. Compared with commonly used one-factor-

at-a-time experiments, statistical design results in reduced experimentation and thereby

reduced resources such as staff, time, etc. Besides, experimental design and statistical analysis

also give quantitative information on the significance of each factor and their interactions on

the measured response. Statistical design of experiments (DOE) also helps to develop a

regression function between the response variables η1→ l, (e.g., area fraction of cracks, crack

mean inter distance, etc.) and the independent variables x1, x2,.., xk (e.g., post heat treatment,

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coating thickness etc.). The most common, as well as the simplest, form of regression

function is a polynomial of order 1, which for 3 independent variables x1, x2, x3 is given by

the expression:

η = β0 + (β1x1+ β2x2+ β3x3) + (β12x1x2 + β13x1x3 + β23x2x3) (1)

where β0, β1, β2, β3, β12, β13, β23 are regression coefficients of the function. The first

coefficient, β0, is the overall average effect of all factors and corresponds to the level of

response at origin. The coefficients β1, β2, β3 represent the linear effect on the response η. The

coefficients β12, β13, β23 represent the effect on the response η as explained by the interaction

between the variables x1x2, x1x3, x2x3, respectively. The coefficients are calculated on the basis

of the least square method by fitting equation (1) to a number of observations, N, which is

determined by varying all the factors simultaneously.

3 Materials

In the present study four different coils of 55.0 wt% Al, 43.4 wt% Zn, 1.6 wt% Si coated steel

produced in the continuous hot dip coating line at SSAB Tunnplåt AB, Sweden, were

investigated, see Table 1. The role of Si in the alloy coating is to prevent a strong exothermic

reaction between the Al-Zn bath and the steel substrate [11, 12, 13].

Viewed in a plane parallel to the steel sheet surface, see Fig. 1, the coating is seen to consist

of aluminium-rich dendrites and zinc-rich interdendritic regions. The extension of these

regions is also seen in cross-section, see Fig. 2, where also silicon particles, 5-20 µm in size,

can be seen in the interdendritic regions. At the substrate-coating interface a thin, 0.5-2 µm,

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intermetallic layer is formed by solid-state diffusion of aluminium, zinc and silicon into the

steel surface. This layer consists of Fe-Zn-Al and Fe-Zn-Al-Si compounds [13, 14] and acts to

bond the coating metallurgically to the steel substrate.

4 Experimental

4.1 Statistical design of experiments (DOE)

The list of factors investigated is presented in Table 2. The effects of the four factors: ageing

(x1) temper rolling (x2) post heat treatment (x3) and coating thickness (x4) were studied at two

levels, whereas the effect of deformation (x5) was evaluated at eight different levels. The

samples were tested in accordance with the treatment combinations given in the design

matrixes in Tables 3 and 4. Each trail was repeated three times, i.e. three replicates of each

factor combination were made.

4.2 Sample preparation

The cold rolled strip was processed in the Aluzink® line, at SSAB Tunnplåt AB, using an

annealing temperature of 700-800 °C and a metal bath temperature of 600 °C. To achieve

desired coating thickness values the pressure in the air jet knifes were modulated. After

coating deposition the strip was post heat treated at a coil temperature of 260 °C. After

reaching the annealing temperature, the cooling starts immediately, i.e. there is no holding

time. Temper rolling was performed to reductions of approximately 0.7-1.0%. The ageing

process was performed for 7 weeks at room temperature. Samples (5 cm × 5 cm) were

deformed either by plain strain bending or biaxially strain forming.

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4.3 Micro Hardness

The hardness of the coatings was obtained for a load of 15 g using a conventional Vickers

micro hardness indenter. The hardness measurements were performed on undeformed

samples.

4.4 Coating Ductility Characterisation

Cracks on the tension side of the formed specimens were thoroughly examined by using SEM

and EDS (Fig. 3). Coating damage parameters, such as area fraction of cracks, mean crack

width and mean crack inter distance, were obtained by performing image analysis on

thresholded (Fig 4a) SEM images (Fig. 4b). Digital image processing operations and image

measurements were performed using the commercial available software, Quantimet 520.

5 Results and Discussion

Tables 5 and 6 give the results concerning the micro hardness and cracking characteristics of

the samples investigated. The matrices were treated mathematically by performing multiple

linear regression (MLR). The regression coefficients and corresponding limits of significance

are presented in Tables 7 and 8. The significance of each coefficient can be determined by

studying the confidence limits in comparison with the value of each coefficient. If the value of

a regression coefficient is inside the confidence interval then the regression coefficient is

insignificant at the 5% level. In the following sections the results from the micro hardness

measurements and the cracking characterisation of the samples investigated will be

statistically treated in detail.

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5.1 Micro Hardness

From Table 7, it can be seen that the ageing coefficient and all interaction coefficients are

insignificant and therefore negligible. Thus, the regression equation obtained is given as:

Coating hardness HV15g [kg/mm2] = 89.1 + 3.4 x2 – 16.9x3 – 2.4x4 (1)

where 89.1 = Mean coating hardness

x2 = Temper rolling

x3 = Post heat treatment

x4 = Coating thickness

x5 = Deformation

When one is studying equation (1) it is important to remember that temper rolling (x2) and

post heat treatment (x3) are discrete and qualitative variables, which describe variation at

fixed levels (-1 or +1), see Table 5. Thus, equation (1) reveals that the use of post heat

treatment decreases the coating hardness by 33.8 [kg/mm2]. This can be explained by the fact

that the post heat treatment transforms coherent Guinier-Preston zones to larger and more

stable phases, which are less effective to prevent deformation by slip of dislocations. It can

also be seen that temper rolling increases the coating hardness due to deformation hardening.

Equation (1) also shows that thinner coatings have a higher hardness as compared with

thicker coatings. However, this effect is probably due to the fact that the indentation load was

to high, and consequently the harder underlying steel substrate will contribute to the

measured hardness value.

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5.2 Cracking characteristics

Table 6 was analysed in order to get the effects of the main factors and the interactions listed

in Table 8.

The resulting significant regression equations are given as:

Area fraction of cracks [%] = 2.8 – 1.7x3 + 2.1x5 (2)

Crack mean width [µm] = 6.8 – 1.5x3 + 0.5x4 + 2.0x5 (3)

Mean Crack interdistance [µm] = 638 + 502x3 – 908x5 (4)

where x3 = Post heat treatment

x4 = Coating thickness

x5 = Deformation

As can be seen, post heat treatment (x3) has a significant decreasing effect on the area fraction

of cracks (eq. 2), the mean crack width (eq. 3) and a significant increasing effect on the mean

crack inter distance (eq. 4). Furthermore, the coating thickness (x4) has a significant effect on

the mean crack width. For example, if the coating thickness is increased by approximately 5

µm, the crack width is increased by 1 µm. Finally, the forming operations have very strong

effects on the response variables. As expected, the area fraction of cracks and the mean crack

width will increase while the crack inter distance will decrease during forming operations.

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6 Conclusions

In the present investigation, the influence of different processing parameters on the coating

hardness/ductility behaviour of 55%Al-Zn coated steel has been evaluated by statistical

design of experiment. The results can be concluded as follows:

(1) The statistical design of experiments provides a good method of quantifying the effects of

various factors on the coating cracking behaviour of 55%Al-Zn coated steel.

(2) The Vickers hardness of the coating was found to be significantly influenced by temper

rolling, post heat treatment and coating thickness. Temper rolling gives a small hardening

effect, while heat treatment transforms coherent Guinier-Preston zones to greater and

softer phases.

(3) Post heat treatment has a significant decreasing effect on the area fraction of cracks, the

mean crack width and a significant increasing effect on the mean crack inter distance.

Acknowledgements

SSAB Tunnplåt AB is gratefully acknowledged for the financial support and for delivering

the test samples. Dr. Göran Engberg, Dr. Hans Klang and Dr. Sven Erik Hörnström, SSAB

Tunnplåt AB, are all recognized for valuable discussions.

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References

q 1 D.J. Willis, J.S.H. Lake, The Influence of the Interaction Between the Coating and the

Sheet Steel Base on the Formability of Aluminium--Zinc Coated Steel, ASM

International, 1988, pp. 31-41.

2 D.J. Willis, Z.F. Zhou, Factors influencing the ductility of 55% Al-Zn coatings, Iron

and Steel Society/AIME (USA), 1995, pp. 455-462.

3 V. Rangarajan, N.M. Giallourakis, D.K. Matlock, G.V. Krauss, The effect of texture

and Microstructure on Deformation of Zinc Coatings, J. Mater. Shaping Technol. 6 (4)

1989, pp. 218-227.

4 D.J. Willis, Coated sheet steel viewed as a composite material, Strength of Metals and

Alloys (ICSMA6), Proceedings in the 6th Int. Conf., Melbourne, ed. R C Gifkins,

Pergamon Press, 1982, Vol. I, pp. 247-252.

5 D.J. Willis, Cracking characteristics of zinc and zinc-aluminium alloy coatings,

International Conference on Zinc and Zinc Alloy Coated Steel Sheet, GALVATECH

'89, 1989, pp. 351-358.

6 G.Engberg, SSAB Tunnplåt AB, private communication.

7 S.R. Shah, J.A. Dilewijns, R.D. Jones, The structure and deformation behaviour of zinc-

rich coatings on steel sheet, Journal of Materials Engineering and Performance, 5 (5)

1996, pp. 601-608.

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q 8 T.E. Torok, P.W. Shin, A.R. Borzillo, Method of imroving the ductility of the coating of

an aluminium-zinc alloy coated ferrous product, US Patent No 4,287,008, Sep 1, 1981.

9 E. Aguirre, B. Fernandez, J.M. Puente, Post-Annealed 55% Al--Zn Alloy Coated Steel

Sheets: Microstructural Characterization and Ductility Properties, The Minerals, Metals

& Materials Society (USA), 1993, pp. 137-152.

10 G.E.P. Box, W.G. Hunter, J.S. Hunter, Statistics for Experimenters, John Wiley & Sons,

Inc., New York (1978).

11 A.R. Borzillo, J.B. Horton, U.S. patent #3343930, September 26, 1967.

12 J.H. Selverian, A.R. Marder, M.R. Notis, Metall. Trans. A., 19A, 1988, pp. 1193-1203.

13 J.H. Selverian, A.R. Marder, M.R. Notis, Metall. Trans. A., 20A, 1989, pp. 543-55.

14 J.H. Selverian, A.R. Marder, M.R. Notis, J. Electron Micro. Tech., 5(3), 1987, pp. 223-

26.

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Tables

Table 1 Coating chemical composition of the coils investigated.

Coil Zn Al Si

[wt %] [wt %] [wt %] 1 44.0 53.0 1.7 2 43.7 53.6 1.7 3 42.6 53.9 1.9 4 43.1 54.2 1.9

Table 2 Process parameters investigated together with their experimental levels.

Process parameters Variable Level (-) Level (+)

Ageing x1 1 week 4 weeks

Temper rolling x2 No Yes

Post heat treatment x3 No Yes

Nominal coating thickness x4 100-120g/mm2 (13-16 µm) 150-185g/mm2 (20-25 µm)

Deformation mode 4 levels plain strain 4 levels biaxially strain forming

Effective strain x5 11% 21% 30% 35% 19% 36% 52% 60%

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Table 3 Matrix of experimental design for coating hardness evaluation.

Trial Process parameters Interactions

x1 x2 x3 x4 x1 x2 x1 x3 x1 x4 x2 x3 x2 x4 x3 x4

Ageing Temper rolling

Post heat treatm.

Coating thickness

1 -1 -1 -1 -1 1 1 1 1 1 1 2 1 -1 -1 -1 -1 -1 -1 1 1 1 3 -1 1 -1 -1 -1 1 1 -1 -1 1 4 1 1 -1 -1 1 -1 -1 -1 -1 1 5 -1 -1 1 -1 1 -1 1 -1 1 -1 6 1 -1 1 -1 -1 1 -1 -1 1 -1 7 -1 1 1 -1 -1 -1 1 1 -1 -1 8 1 1 1 -1 1 1 -1 1 -1 -1 9 -1 -1 -1 1 1 1 -1 1 -1 -1

10 1 -1 -1 1 -1 -1 1 1 -1 -1 11 -1 1 -1 1 -1 1 -1 -1 1 -1 12 1 1 -1 1 1 -1 1 -1 1 -1 13 -1 -1 1 1 1 -1 -1 -1 -1 1 14 1 -1 1 1 -1 1 1 -1 -1 1 15 -1 1 1 1 -1 -1 -1 1 1 1 16 1 1 1 1 1 1 1 1 1 1

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Table 4 Matrix of experimental design for cracking behaviour evaluation.

Trial Process parameters

x1 x2 x3 x4 x5

Ageing Temper rolling

Post heat treatment

Coating thickness

Strain

[%]

1 - - - - 19 2 - - - - 35 3 + - - - 60 4 + - - - 21 5 - + - - 60 6 - + - - 21 7 + + - - 19 8 + + - - 35 9 - - + - 11

10 - - + - 52 11 + - + - 36 12 + - + - 30 13 - + + - 36 14 - + + - 30 15 + + + - 11 16 + + + - 52 17 - - - + 11 18 - - - + 52 19 + - - + 36 20 + - - + 30 21 - + - + 36 22 - + - + 30 23 + + - + 11 24 + + - + 52 25 - - + + 19 26 - - + + 35 27 + - + + 60 28 + - + + 21 29 - + + + 60 30 - + + + 21 31 + + + + 19 32 + + + + 35

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Table 5 Vickers hardness for different parameter combinations.

Trial Process parameters Response

x1 x2 x3 x4 Vickers Hardness

Standard deviation

Ageing Temper rolling

Post heat treatment

Coating thickness [HV15g] [HV15g]

1 -1 -1 -1 -1 108.3 11.0 2 1 -1 -1 -1 106.0 9.3 3 -1 1 -1 -1 115.1 13.0 4 1 1 -1 -1 106.3 10.4 5 -1 -1 1 -1 70.4 2.8 6 1 -1 1 -1 72.5 6.0 7 -1 1 1 -1 78.4 4.0 8 1 1 1 -1 75.3 3.6 9 -1 -1 -1 1 96.6 7.3

10 1 -1 -1 1 100.9 6.6 11 -1 1 -1 1 107.3 10.9 12 1 1 -1 1 107.7 6.1 13 -1 -1 1 1 69.6 8.0 14 1 -1 1 1 62.1 4.3 15 -1 1 1 1 77.0 4.3 16 1 1 1 1 72.7 5.9

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Table 6 Area fraction of cracks, mean crack width and mean crack inter distance for different parameter combinations.

Trial Process parameters Responses

x1 x2 x3 x4 x5 Area fraction of

cracks

Mean crack width

Mean crack inter

distance Ageing Temper

rolling Post heat

treatm.

Coating thickn.

Strain

[%] [%] [µm] [µm]

1 - - - - 19 1.5 5.8 393 2 - - - - 35 4.7 6.9 149 3 + - - - 60 9.1 10.7 118 4 + - - - 21 2.5 5.5 231 5 - + - - 60 10.0 12.1 122 6 - + - - 21 2.1 5.0 249 7 + + - - 19 1.5 5.9 382 8 + + - - 35 5.2 7.6 148 9 - - + - 11 0.1 3.7 5590

10 - - + - 52 0.9 5.9 694 11 + - + - 36 0.2 5.5 3176 12 + - + - 30 1.5 4.5 325 13 - + + - 36 0.4 4.5 1186 14 - + + - 30 1.7 4.6 277 15 + + + - 11 0.4 4.1 1565 16 + + + - 52 0.7 5.6 840 17 - - - + 11 0.5 4.3 1068 18 - - - + 52 6.5 12.0 187 19 + - - + 36 2.6 7.9 303 20 + - - + 30 5.3 9.5 212 21 - + - + 36 2.6 8.8 336 22 - + - + 30 6.2 10.3 168 23 + + - + 11 0.2 3.9 1766 24 + + - + 52 7.6 12.2 163 25 - - + + 19 0.3 4.8 1737 26 - - + + 35 2.1 5.7 281 27 + - + + 60 1.4 6.8 527 28 + - + + 21 1.0 4.6 517 29 - + + + 60 1.4 6.6 457 30 - + + + 21 0.5 4.1 864 31 + + + + 19 0.2 4.5 3814 32 + + + + 35 2.6 7.0 273

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Table 7 Regression coefficients and corresponding confident limits as obtained in the micro hardness test.

Process parameters Regression coefficient Regression Coefficient

for Micro hardness [HV15g]

Confident limit (P=0.05)

x0 β0 89.14 2.21 x1 β1 -1.20 2.21 x2 β2 3.34 2.21 x3 β3 -16.89 2.21 x4 β4 -2.40 2.21

x1 x2 β12 -0.78 2.21 x1 x3 β13 -0.40 2.21 x1 x4 β14 0.31 2.21 x2 x3 β23 0.26 2.21 x2 x4 β24 1.10 2.21 x3 x4 β34 0.50 2.21

Table 8 Regression coefficients and corresponding confident limits as obtained in the coating ductility test.

Param-eters

Regression coefficient

Area fraction of cracks [%]

Confident limit

(P=0.05)

Mean crack width [µm]

Confident limit

(P=0.05)

Mean crack interdist.

[µm]

Confident limit

(P=0.05) x0 β0 2.75 ±0.38 6.82 ±0.29 638 ±385 x1 β1 0.06 ±0.38 0.07 ±0.29 53.5 ±385 x2 β2 0.09 ±0.38 0.07 ±0.29 -117 ±385 x3 β3 -1.69 ±0.36 -1.49 ±0.28 502 ±365 x4 β4 0.03 ±0.36 0.54 ±0.28 -61 ±365

5x β5 2.08 ±0.52 1.98 ±0.41 -908 ±531 x1 x3 β13 0.02 ±0.36 0.15 ±0.28 -23 ±370 x1 x4 β14 0.04 ±0.36 -0.04 ±0.28 140 ±370 x2 x3 β23 -0.07 ±0.36 -0.12 ±0.28 -137 ±370 x2 x5 β25 0.00 ±0.36 0.03 ±0.28 288 ±370

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Figure captions

Figure 1 SEM micrograph (a) of 55%Al-Zn coated steel viewed in a plane parallel to the surface. (b) elemental maps recorded from the surface.

Figure 2 Cross-section view of an as received coating. I - Al-rich dendrite arm, II - Zn-rich interdendritic region, III - Si-particle, IV - intermetallic layer and V - steel substrate.

Figure 3 SEM micrograph (a) and elemental maps recorded from corresponding surface (b) of a typical crack formed on bended 55%Al-Zn coated steel.

Figure 4 Binary image (a) used for coating cracking evaluation after tresholding of image (b). SEM micrograph of a typical crack pattern formed on bended 55%Al-Zn coated steel (b).

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Figures

(a)

(b)

Figure 1

(a)

(b)

Figure 2

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(a)

(b)

Figure 3

(a)

(b)

Figure 4