Al ZN Coatings
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The Influence of Processing Parameters on the Coating Hardness/Ductility Behaviour of 55%Al-Zn Coated Steel *
Per Carlsson Carlsson and Mikael Olsson Dalarna Dalar na University SE-781 88 Borlänge, Sweden du.se se *E-mail: pca@du. *E-mail: pca@ 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 55%AlZn coated steel has been evaluated by statistical design of experiment, DOE. In these experiments the four response variables variables viz.; viz.; hardness, 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 sh ow 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 Th e hardness of the coating was significantly 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 t herefore decrease d ecreasess the coating coatin g hardness. hardn ess. The T he cracking crac king tenden te ndency cy was found foun d 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]. [1,2,3 ]. Observations of 55%Al-Zn coated steel strain strained ed 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 (in termetallic particles) or o r at pores p ores within with in the th e coatin coating g [4,5]. The T he individual ind ividual importance of these nucleation sites is difficult to proclaim.
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Due to ageing , i.e. precipitation hardening, the coating will obtain ob tain a relatively high high hardness (and consequently a low ductility) during room temperature storage. The maximum hardness will be obtained o btained approximately six weeks after coating deposition [6].
In order to obtain a planar and smoother surface of improved paintability paintability,, the coated co ated 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 sho wn that this treatment induces isolated cracks in the intermetallicc layer in connection to asperity indentations [7]. intermetalli
The coating thickness is thickness is controlled by the gas flow in the air jet knifes used for removal of superfluous melted metal. The thickness of the intermetallic intermetallic layer at the coating/steel substrate interface interf ace is mainly determined by the speed of the strip throw the bath. It is eexpected xpected 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 layer thicknesses within 1-6 µm showed sho wed similar cracking tendency.
By post By post heat treatment treat ment the 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 precipi tation reactions and particle coarsening. coarsenin g. 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 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 found on o n a sample deformed to 6%.
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There have been b een several previous investigations of the cracking behaviour (ductility (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 investigati on was carried out by varying varying all the selected factors simultaneously with the help of statistical statistical design of experiment (DOE). The factors were chosen on th thee basis of knowledge about the process, complemented by information found in the literature. 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 behavio ur of the th e coating. In these th ese experiments experime nts three response respon se variables viz.; viz.; area area fraction of cracks, the mean crack width, and cracking cracking inter distance are connected to the major process parameters; coating thickness, thickn ess, post heat treatment, temper rolling and ageing. Scanning Scannin g electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX), image analysis and micro hardness measurements were used to characterise the samples.
2
Statistica Statisticall 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-factorone-factorat-a-time experiments, statistical design results in reduced experimentation and thereby reduced resources such suc h as staff, time, etc. Besides, experimental design and statistical analysis analysis also give quantitative information information on the significance significance of each factor and their interactions on the measured response. Statistical design of experiments (DOE) also helps to develop a area fraction of cracks, crack regression function between the response variables η1→ l , (e.g., area mean inter distance, etc.) and the independent indepen dent variables x1, x2,.., x k (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, x 3 is given by the expression:
η = β0 + (β1 x1+ β2 x 2+ β3 x 3) + (β12 x 1 x 2 + β13 x 1 x3 + β23 x2 x3)
(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 η. The response at origin. The Th e 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 betwee n the variables variable s x1 x2, x1 x3, x2 x 3, respectively. respectively. The coeff co efficients icients 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 different coils of 55.0 wt% Al, 43.4 wt% Zn, 1.6 wt% Si coated steel produced produ ced in the continuou conti nuouss hot dip coating coatin g line at SSAB Tunnplåt Tun nplåt AB, Sweden, 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 substrate-coating interface 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 Fe-Zn-Al-Si compounds [13, 14] and acts to bond the t he coating coatin g metallurgically to the th e steel substrate. su bstrate.
4 4.1
Experimental Statistical design of experiments (DOE)
The list of factors investigated is presented in Table 2. The effects of the four factors: ageing ( xx 1) temper rolling ( x2) post heat treatment 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 fact or 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 h eat treated at a coil temperature of 260 °C. After 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. 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 o f the formed 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 available software, Quantimet 520.
5
Results and Discussion
Tables 5 and 6 give the results concerning co ncerning the micro hardness and cracking characteristics of the samples investigated. The matrices were treated mathematically by performing multiple linear regression (MLR). The regression coefficients 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. 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 insignifi cant and therefore negligible. Thus, the regression equation obtained is given as:
Coating hardness HV15g [kg/mm2] = 89.1 + 3.4 x 3.4 x2 – 16.9 x3 – 2.4 x4 where
(1)
89.1 = Mean coating hardness x2 = Temper T emper rolling x3 = Post heat treatment x4 = Coating thickness x5 = Deformation
When one is studying equation (1) it is important to remember that temper rrolling olling ( x x2) and variables, which describe variation variation at post heat he at treatment ( x3) are discrete and qualitative variables, fixed fix ed levels (-1 or +1), see Table 5. Thus, Th us, equation (1) reveals that the use of post heat 2
treatment decreases the coating hardness by 33.8 [kg/mm ]. This can be explained by the fact that the post po st heat treatment transforms transforms coherent Guinier-Preston 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 deformation hardening. Equation (1) also shows that thinner coatings have a higher hardness as compared with thicker coatings. However, However, this effect is probably due to the fact fact that the in indentation dentation load was to high, and consequently consequ ently the harder underlying steel substrate wi will ll 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 significant regression equations are given as:
where
Area fraction of cracks [%] = 2.8 – 1.7 x 3 + 2.1 x 5
(2)
Crack mean width [µm] = 6.8 – 1.5 x 3 + 0.5 x4 + 2.0 x5
(3)
Mean Crack interdistance [µm] = 638 + 502 x 3 – 908 x 5
(4)
x = Post heat treatment 3 x4 = Coating thickness x5 = Deformation
As can be seen, post heat treatment ( xx3) has a significant decreasing effect on the area fraction of cracks (eq. 2), the mean crack width (eq. 3) and a significant significant increasing effect on the mean crack inter distance (eq. 4). Furthermore, the coating thickness ( x 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 eff ects on the response variables. variables. As expected, the th e area fraction 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 c oncluded as follows:
(1) The statistical design of experiments provides a good method of quantifying the effects effects of various factors factors on the coating cracking c racking 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 significant increasing effect 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 Influence of the Interaction Interaction Between the Coating and the Sheet Steel Base on the Formability of Aluminium--Zinc Coated Steel, ASM International, Interna tional, 1988, pp. p p. 31-41.
2
D.J. Willis, Z.F. Zhou, Factors influencing influenci ng the ductility ductil ity of 55% Al-Zn coatings, coatings , Iron and Steel Society/AIME (USA), (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, material, Strength of Metals and th
Alloys (ICSMA6), Proceedings in the 6 Int. Conf., Melbourne, ed. R C Gifkins, Pergamon Pergam on Press, 1982, 1982 , Vol. I, pp. 247-252.
5
D.J. Willis, Cracking characteristics 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 structure and deformation deformation behaviour of zincrich 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 Metho d of imroving the ductility ductilit y of the coating of an aluminium-zinc alloy coated ferrous ferrous product, US Patent No 4,287,008, Sep 1, 1 , 1981.
9
E. Aguirre, B. Fernandez, Fernande z, J.M. Puente, Puente , Post-Annealed Post-Anneale d 55% Al--Zn Al--Zn Alloy Coated Steel Sheets: Microstructural Microstructural Characterization and Ductility Properties, Properties, The Th e Minerals, Metals & Materials Society (USA), (USA), 1993, pp. pp . 137-152. 137 -152.
10
G.E.P. Box, W.G. Hunter, J.S. Hunter, Statistics for for Experimenters, Experiment ers, 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, 1988 , pp. 1193-1203 1193 -1203..
13
J.H. Selverian, A.R. Marder, M.R. Notis, Metall. Trans. A., A., 20A, 1989, 1989 , pp. 543-55.
14
J.H. Selverian, A.R. Marder, M.R. Notis, J. Electron Micro. Tech., Tech ., 5(3), 1987, 1987 , pp. 22326.
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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 pa rameters invest i nvestigated igated together togeth er with their thei r experimental experi mental levels l evels.. Process parameters
Variable
Level (-)
Level (+)
Ageing
x1
1 week
4 weeks
Temper rolling
x2
No
Yes
Post heat treatment
x3
No
Yes 2
2
Nominal coating tthickness hickness x4
100-120g/mm (13-16 µm)
150-185g/mm (20-2 (20-25 5 µm)
Deformation Defor mation mode
4 levels plain strain
4 levels biaxially strain forming
Effective strain
x5
11%
- 13 -
21%
30%
35 %
19%
36%
52%
60%
Table 3 Matri Matrixx of experimenta expe rimentall design for coating coat ing hardness ha rdness evaluation evalu ation.. Trial
Process parameters
Interactions
x1
x2
x3
x4
x1 x2
x1 x3
x1 x4
x2 x3
x2 x4
Ageing
Temper rolling
Post heat treatm.
Coating thickness
1 2
-1 1
-1 -1
-1 -1
3
-1
1
4
1
5
-1 -1
1 -1
1 -1
1 -1
1 1
1 1
1 1
-1
-1
-1
1
1
-1
-1
1
1
-1
-1
1
-1
-1
-1
-1
1
-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
- 14 -
x3 x4
Table 4 Matri Matrixx of experiment expe rimental al design desi gn for cracking cra cking behav b ehaviour iour evaluat ev aluation. ion. Trial
Process parameter parameterss x1
x2
Ageing
Temper rolling
x3 Post heat treatment
x4 Coating thickness
x5 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 26
-
-
+ +
+ +
19 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 parameter parameterss x1
x2
Ageing
x3
Temper rolling
Response x4
Post heat treatment
Coating thickness
Vickers Hardness
Standard deviation
[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 14
-1 1
-1 -1
1 1
1 1
69.6 62.1
8.0 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 fract ion of cracks, crack s, mean me an crack c rack width and a nd mean m ean crack inter distance dista nce for different dif ferent paramete p arameterr combinations. combina tions. Trial
Process parameters x1
Ageing
Responses
x2
x3
x4
x5
Area fraction of cracks
Mean crack width
Mean crack inter distance
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 5 590
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 23
+
+ +
-
+ +
30 11
6.2 0.2
10.3 3.9
168 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 Regres sion coeffi c oefficient cientss and correspo c orresponding nding confident confi dent limits l imits as a s obtained obta ined in in the micro hardness test. Regression Coefficient
Confident limit
Process parameters
Regression coefficient
for Micro hardness [HV15g]
(P=0.05)
x0
β 0
89.14
2.21
x1
β 1
-1.20
2.21
x2
β 2
3.34
2.21
x3
β 3
-16.89
2.21
x4
β 4
-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 Regre ssion coeffi c oefficient cientss and correspondin corres ponding g confident conf ident limits as obtained obt ained in the coating ductility test. Confident limit
(P=0.05)
Mean crack interdist. [µm]
6.82
±0.29
638
±385
±0.38
0.07
±0.29
53.5
±385
0.09
±0.38
0.07
±0.29
-117
±385
β3
-1.69
±0.36
-1.49
±0.28
502
±365
x4
β4
0.03
±0.36
0.54
±0.28
-61
±365
x5
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
Parameters
Regression coefficient
Area fraction of cracks [%]
Confident limit
x0
β0
2.75
±0.38
x1
β1
0.06
x2
β2
x3
(P=0.05)
Mean crack width [µm]
- 18 -
Confident limit
(P=0.05)
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.
c oating. I - Al-rich dendrite arm, II Figure 2 Cross-section view of an as received coating. Zn-rich interdendritic interden dritic region, III - Si-particle Si-particle,, IV - intermetallic intermet allic layer and V - steel substrate.
Figure 3 SEM micrograph (a) and elemental maps recorded from corresponding surface surfa ce (b) of a typical typi cal crack c rack formed f ormed on o n bended bend ed 55%Al-Zn coated c oated steel. st eel.
Binaryy image (a) used for fo r coating coatin g cracking cracki ng evaluatio evalu ation n after treshold tre sholding ing of Figure 4 Binar image (b). SEM micrograph of a typical crack pattern formed on bended 55%Al-Zn coated steel (b).
- 19 -
Figures
(a)
(b) Figure 1
(a)
(b) Figure 2
- 20 -
(a)
(b) Figure 3
(a)
(b) Figure 4
- 21 -
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