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In-situ observation in a scanning electron microscope on the exfoliation behavior of galvannealed Zn–Fe coating layers
Surface & Coatings Technology 201 (2007) 6261 – 6266 www.elsevier.com/locate/surfcoat
In-situ observation in a scanning electron microscope on the exfoliation behavior of galvannealed Zn–Fe coating layers Kyung-Keun Lee, In-Hwan Lee, Cheul-Ro Lee, Haeng-Keun Ahn ⁎ School of Advanced Materials Engineering, Research Center of Advanced Materials Development, Chonbuk National University, Chonbuk, 561-756, Republic of Korea Received 31 March 2006; accepted in revised form 19 November 2006 Available online 28 December 2006
Abstract Exfoliation behavior of galvannealed (GA) coating layers was continuously observed in-situ under 3-point bending load in a scanning electron microscope (SEM). Two kinds of substrate, Al-killed steel and Ti-stabilized interstitial free (IF) steel, were used. For GA coating layers formed on Al-killed steel, powdering was dominant at the region under compressive stress during the bending. Exfoliation occurred at the Γ/δ1 interface, and the amount of exfoliation decreased with an increase in Fe content in the coating layer. Meanwhile, for GA layers coated on IF steel substrate, flaking was dominant. One of the perpendicular cracks that were pre-induced during galvannealing grew and propagated up to the Γ/Fe matrix interface with increasing strain. On the other hand, the crack initiated at the Γ/Fe matrix interface grew and propagated along the interface. Then, this crack combined with a nearby perpendicular crack, resulted in flaking. © 2006 Elsevier B.V. All rights reserved. PACS: 62.20.Mk; 68.47.De; 68.37.Hk Keywords: C. Hot dip; D. Multilayer; D. Zinc alloy; X. Exfoliation
1. Introduction The coating layer of galvannealed (GA) steel consists of several Zn–Fe intermetallic compounds. In general, it has a complicated phase structure in the order of Γ (Fe3Zn10), Γ1 (Fe5Zn21), δ1(FeZn7), ζ(FeZn13) and η( Zn) [1–3]. The phase structure and the components of the Zn–Fe coating layer, which affect formability of the GA steel sheet, vary with the composition and temperature of the Zn bath, immersion period, annealing temperature and time, etc [4]. GA steel sheets have been widely used in the automotive and home appliance industries. The brittleness of Zn–Fe intermetallic compounds can lead to flake off or powder during presswork, which results in reducing corrosion resistance and paintability [5]. Powdering and flaking are two major fracture types of GA coating layer during presswork. Because it is important to form a coating layer with a good fracture
⁎ Corresponding author. Tel.: +82 63 270 2383; fax: +82 63 270 2376. E-mail address: [email protected] (H.-K. Ahn). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.11.021
resistance, many researches on the improvement of flaking and powdering resistance have been carried out [4,6–14]. However, research results for factors that affect formability of GA coating layer are inconsistent due to the unclearness of the fracture mechanism, and the differences in phase structure and components of the coating layer. Furthermore, researches so far reported on exfoliation behavior such as the crack initiation point and propagation path are not satisfactory. In addition, the relationship between exfoliation behavior and phase structure of the coating layer is unclear yet. In this study, in-situ observation in scanning electron microscope (SEM) on exfoliation behavior of the coating layer was performed for two types of GA steel sheets with different substrate. One was prepared on Al-killed steel substrate, and the other on Ti-stabilized interstitial free (IF) steel substrate. For the GA coatings on Al-killed steel substrate, we investigated the effects of Fe content in the coating layer on the exfoliation behaviors. In an attempt to study the fracture mechanisms of GA steel sheets and to examine the crack initiation point and propagation path on the compressive side of the coating layer, we applied a
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Table 1 Chemical composition (wt.%) of the substrate steel sheets
Specimen A Specimen B
C
Si
Mn
P
S
Al
Ti
Fe
0.04 0.02
0.01 0.008
0.2 0.1
0.01 0.01
0.01 0.006
0.04 0.024
– 0.051
Bal. Bal.
3-point bending load in a SEM chamber and took an in-situ look at the vicinity of coating/substrate interface. 2. Experimental procedure Substrate materials used in this study were Al-killed steel sheets (specimen A) and Ti-stabilized IF steel sheets (specimen B). The chemical composition of the substrate sheets is listed in Table 1. As-received sheets were reductively cleaned at 800 °C for 30 s in a 10% H2–N2 atmosphere, and then hot-dipped in a molten Zn bath at 460 °C for 2 s. The galvanizing Zn bath had an Al content of 0.14 wt.% (specimen A) or 0.20 wt.% Al (specimen B). The hot-dipped (galvanized) steel sheets were annealed (galvannealed) at 500 °C, with a hold time of 15 s for specimen A and 50 s for specimen B. For this study, the Fe content in the coating layer of the GA steel sheets (specimen A) was controlled by continued annealing at 500 °C for 600 s, 1440 s and 2220 s. The total Fe content in the coating layer of specimen A was measured by inductively coupled high frequency plasma-atomic emission spectroscopy (ICP-AES, Shimazu, ICP-7500). For the ICP-AES measurement, a 1.0% H2SO4 solution was used to dissolve the coating from the substrate steel. The coating removal was completed when effervescing from the immersed sample was no longer observed. At this point, the solution containing the
Table 2 The phase thickness of the GA coating layers measured by electro-chemical stripping Fe content (at.%)
Phase thickness of coating layers (μm) Γ
δ1 + Γ
δ1(c)
δ1(p)
Total
13 15 21 24
0.60 0.80 1.30 1.87
0.69 0.85 0.96 1.03
0.41 0.38 0.28 0.23
5.24 5.00 4.42 3.87
6.94 7.03 6.96 7.00
dissolved coating was titrated. To analyze the phase structure of the coating layer, X-ray diffractometer (Rigaku, D/Max-IIA) was applied at 40 kV/30 mAwith a Cu–Kα target. The specimens were scanned at a rate of 2°/min in the 2θ range of 30°–90°. Also, the thickness of individual intermetallic compounds in the coating layer was measured by an electrolytic stripping method [15–17]. This method closely estimates the thickness of individual layers of a multilayered coating and the electrochemical potential differences between the individual layers while being anodically stripped at constant current density. This test was conducted in a dissolved 200 g/l NaCl and 100 g/l ZnSO4 solution at a constant current density of 2.5 mA/cm2, using a Potentiostat/Galvanostat (EG and G, model 273A). The total thickness of the Zn–Fe coating of the GA steel sheets were determined using a measuring microscope (Nikkon, MM-40/L3u). The specimens for total thickness measurement were sectioned (0.75 × 5 × 30 mm3) with a micro cutter, perpendicularly to the coating layer, and then mechanically polished with alumina powder. On each specimen, thickness measurements were performed at twenty different positions across the coating layer, separated by distances of 0.5 mm, and the obtained twenty values were averaged. Before SEM analysis, all specimens were cleaned with acetone in an ultrasonic bath for 10 min followed by rinsing in distilled water and dried. In-situ observation on the exfoliation behavior of the GA coating layer in SEM was performed with a micro tensile/ bending tester (Micro-test tensile and bending stage for SEM, Debon Co.). A polished specimen was prepared and a 3-point bending test was performed at a loading rate of 0.1 mm/min. The crack initiation and the exfoliation processes of the coating layer during bending were observed in-situ. The strain was calculated using the curvature radius from the bending test. A tape-stripping test was also performed to investigate the cracking and exfoliation of the coating surface. 3. Results and discussion 3.1. Phase structure of the coating layer
Fig. 1. XRD patterns with various Fe contents in the GA coating layer of the hotdip galvanized Al-killed steel annealed at 500 °C for 15 s, 600 s, 1440 s and 2220 s, respectively.
The total thickness of the Zn–Fe coating layer of GA steel sheets used in this study were 7 μm (specimen A) and 14 μm (specimen B) on an average. In addition, the Fe content in the coating layer of specimen A varied 13, 15, 21, 24 at.% with the annealing time, and the Fe content of specimen B was 9 at.%. The X-ray diffraction analysis was performed and the results showed that the Zn–Fe coating layer of GA steel sheets (specimen A and B) mainly consisted of δ1 and Γ phases. Next
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Fig. 2. Cross sectional views of GA coating layers in a compressive stress state during bending (specimen A).
to the steel substrate was the brittle Γ phase, followed by the δ1 phase. The X-ray diffraction pattern for the coating layer with the Fe content of GA steel (specimen A) is shown in Fig. 1. Since the diffraction peaks of the Γ and the Γ1 phase were duplicated, only the Γ phase diffraction peak was analyzed. Comparing the peak intensity of (330)δ1 and (330)Γ in the X-ray diffraction pattern as a function of the Fe content in the coating layer, the peak intensity of (330)δ1 slightly increased with the
increase in Fe content up to the 15 at.% Fe content [10,14]. However, it decreased rapidly at 21 at.% Fe content and there was no variation afterwards. The peak intensity of (330)Γ with Fe content showed a slower increase. The increase of the (330)δ1 peak intensity with the increase in Fe content up to the 15 at.% can be explained by the transformation of the ζ phase in the coating layer into the phase by continued annealing [20]. Also, the (330)δ1 peak intensity decreased and the (330)Γ peak
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intensity increased due to the transformation of the δ1 phase in the coating layer into the Γ phase by continued annealing above 21 at.% Fe content. The ζ phase was not detected by X-ray diffraction due to its insignificant amount [19]. In order to measure the thickness of individual Zn–Fe intermetallic phases which affect the exfoliation characteristics of GA coating layer, the electrolytic stripping test of the coating layer of various Fe contents was performed and electric potentials corresponding to δ1( p), δ1(c), (δ1 + Γ ), Γ phase, and Fe matrix were detected. The thickness of each intermetallic compound was measured from the variation of electric potentials [11]. The thickness of each intermetallic compound obtained from the electrolytic stripping curve is shown in Table 2. After continued annealing, the total thickness of the coating layer did
Fig. 3. Cross sectional views of GA coating layers in a compressive stress state during bending (specimen B).
not vary, but the Fe content and thickness of each intermetallic compound varied with annealing time. Particularly, the thickness of the δ1 phase decreased with an increase in Fe content in the coating layer, while that of (δ1 + Γ) phase and Γ phase increased. The ζ phase was not detected from electrolytic stripping as well. It seems that the amount of the ζ phase is very small and exists randomly on the coating surface. 3.2. Exfoliation behavior of the coating layer in a cross sectional area Exfoliation aspects of the GA coating layer (specimen A) with various Fe contents in a cross sectional area that was in a compressive stress state during bending are shown in Fig. 2. As shown in Fig. 2, even before the bending load was applied, cracks perpendicular to the steel substrate existed in the coating layer. These cracks are most likely due to the volume change caused by phase transformation or the outburst reactions that occurred within the coating during annealing [4,8]. In this study, considerable cracks were observed in the δ1 phase of the coating layer. Since these pre-induced cracks grew rapidly under very low strain, it was very difficult to detect the crack initiation point. But, exfoliation behavior could be observed with the increase in strain. For the coating layer with 13 at.% Fe content, the crack propagated in the direction of the Fe matrix at 0.04 strain, after which swelling was observed at 0.08 strain. Such swelling of the coating layer occurred severely at 0.12 strain, resulting in powdering. This finding is consistent with the research results [8] that powdering occurs mainly above 10 at.% Fe content in the coating layer. The thickness of swelled thin film was approximately 6.5 μm. Considering the thickness of the intermetallic compounds measured by electrolytic stripping (Table 2), it is believed that the exfoliation occurred at the δ1/Γ interface. In the case of 15 at.% Fe content, considerable swelling occurred at 0.04 strain and exfoliation occurred at 0.08 strain. This exfoliation occurred at a lower strain and the exfoliation degree was higher than that of 13 at.% Fe content. The thickness of the swelled thin film was about 6.3 μm. In the case of 21 at.% Fe content in the coating layer, similar behavior was observed, but the exfoliation occurred at a higher strain than that of 13 at.% and 15 at.% Fe content, and the exfoliation degree was lower. In the case of 21 at. % and 24 at.% Fe content in the coating layer, the thickness of each swelled thin film was 5.4 μm and 4.8 μm respectively. For the coating layers with 21 at.% and 24 at.% Fe content, the exfoliation occurred at the δ1/Γ interface as well. The thickness of the exfoliation decreased with an increase in Fe content above 21 at.%. Fukuzuka and others [18] reported that powdering resistance decreased with the increase in the Γ phase, but the results of this study suggested the opposite. The results of the thickness of intermetallic compounds measured by the electrolytic stripping test of this study showed that the Γ phase was the thickest at 24 at.% Fe content, and the exfoliation degree decreased. Therefore, the results of this study suggest that the thickness of the Γ phase has little influence on the powdering resistance above 21 at.% Fe content in the coating layer. It is believed that
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Fig. 4. The exfoliation aspects with various Fe contents in a compressive coating surfaces (specimen A).
the exfoliation degree decreased with the increase in Fe content because of the amount of the δ1 phase in the coating layer. The δ1 phase has a low fracture resistance compared with other phases of the coating layer due to the prevalence of a large number of cracks, as shown in Fig. 2. Therefore, it is considered that the exfoliation degree decreased with the increase in Fe content in the coating layer because the δ1 phase decreased with the increase in Fe content. Likewise, as shown in Fig. 2, close adhesion of the coating layer/Fe matrix interface increased with the increase in Fe content in the coating layer. This result is consistent with Nakamori [19], who reported that in the case of the galvannealing temperature of 500 °C, close adhesion of the coating layer/Fe matrix interface increased at above 15 at.% Fe content. It is considered that such a close adhesion of the coating layer/Fe matrix interface also affected the exfoliation degree. Exfoliation aspects of the GA coating layer (specimen B) with strain in a cross sectional area that was in a compressive stress state during bending is shown in Fig. 3. Fig. 3(c) indicates the observed part of the specimen, and Fig. 3(a) and (b) shows the crack initiation and the propagation paths with the increase in strain at each position in the same specimen. Fig. 3(a) shows remarkable exfoliation and damage of the coating layer at initial bending. One of the pre-induced perpendicular cracks grew and propagated to the coating layer/Fe matrix interface with the
increase of strain. On the other hand, the crack initiated at the Γ/ Fe matrix interface grew and propagated along the interface, and then combined with a nearby perpendicular crack. Flaking occurred as a result. In the case of Fig. 3(b), although the development of a crack at the initial stage was delayed in comparison with Fig. 3(a), and the crack propagated along the Γ/Fe matrix interface deflected toward another crack, the overall fracture aspect was similar to the case of Fig. 3(a). In general, it is easy for flaking to occur with low Fe content in the coating layer or high Al content in the Zn bath [4]. Fe content in the coating layer of specimen B used in this study was relatively low, 9 at.%, and the Al content in the Zn bath was high, 0.2 wt.%. Therefore flaking was dominant in this type of coating layer. 3.3. Exfoliation behavior at the coating surface After a certain strain was applied to the GA steel sheet (specimen A) with various Fe contents, a tape-stripping test was performed. The exfoliation aspects at compressive surfaces are shown in Fig. 4. Under 0.08 strain, considerable exfoliation occurred at 13−21 at.% Fe content. Little exfoliation, however, occurred at 24 at.% Fe content. It is believed that the exfoliation did occur at the δ1/Γ interface, because of the dense Γ phase remaining when exfoliation occurred, and a columnar crystal δ1 phase around the Γ phase. This result also is consistent with
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cross sectional views of the GA coating layer in Fig. 2. Under 0.15 strain, a similar exfoliation aspect was shown. The exfoliation degree reached its maximum at 15 at.% Fe content, and decreased above 21 at.% Fe content, but did not occur at 24 at.% Fe content. Under 0.2 strain, a similar exfoliation aspect was shown, but in the case of 24 at.% Fe content, some exfoliation was shown, which differed from the 0.15 strain. On the other hand, the amount of exfoliation with various Fe contents was measured, and it varied with the Fe content in the coating layer. The amount of exfoliation was large at 15 at.% Fe content, decreased remarkably above 15 at.% Fe content, and exfoliation did not occur up to the 0.15 strain at 24 at.% Fe content. This is why the δ1 phase, formed during galvannealing, decreased with the increase of Fe content. 4. Conclusions (1) The Zn–Fe coating layer of GA steel sheets used in this study mainly consisted of δ1 and Γ phases. The total thickness of the coating layer did not vary, but the Fe content and the thickness of each intermetallic compound varied with the annealing time at 500 °C. (2) For the exfoliation of the GA coating layer formed on Alkilled steel substrate, powdering was dominant at the region under compressive stress during the bending test. Exfoliation occurred at the Γ/δ1 phase interface, regardless of the Fe content in the coating layer. Whereas, the exfoliation degree decreased with an increase in the Fe content in the coating layer. (3) For the exfoliation of the GA coating layer formed on Tistabilized IF steel substrate, flaking was dominant at the region under compressive stress during the bending test. One of the perpendicular cracks pre-induced during galvannealing grew and propagated up to the Γ/Fe matrix interface with increased strain. On the other hand, the crack initiated at the Γ/Fe matrix interface grew and
propagated along the interface. Then, this crack combined with a nearby perpendicular crack, resulted in flaking. Acknowledgment This work was supported by grant No. R05-2001-00000831-0 from the Basic Research Program of the Korea Science and Engineering Foundation. References [1] M.A. Ghoniem, K. Lonhberg, Metall 26 (1972) 1026. [2] T.B. Massalski, Binary Alloy Phase Diagrams, ASM, Metals Park, Ohio, 1986, p. 1128. [3] A.R. Marder, Prog. Mater. Sci. 45 (2000) 196. [4] M. Urai, M. Arimura, J. Iron Steel Inst. Jpn. 81 (1995) 70. [5] J. Yu, J. Liu, J. Zhang, J. Wu, Mater. Lett. 59 (2005) 2698. [6] S. Okada, K. Sakata, M. Imanaka, K. Kyono, T. Hanazawa, JSAE Rev. 7 (1996) 313. [7] Y. Adachi, T. Nakamori, J. Iron Steel Inst. Jpn. 80 (1994) 225. [8] C.E. Jordan, K.M. Goggins, A.R. Mader, Metall. Mater. Trans., A Phys. Metall. mater. sci. 25A (1983) 2101. [9] S. Chang, Y.M. Choi, Inst. Met. Mat. 31 (1993) 1241. [10] M. Urai, M. Arimura, M. Terada, M. Yamacuchi, H. Sakai, S. Nomura, J. Iron Steel Inst. Jpn. 77 (1991) 971. [11] Y. Tokunaga, M. Yamada, T. Hada, J. Iron Steel Inst. Jpn. 72 (1986) 997. [12] T. Nakamori, A. Shibuya, Proc. on World Material Cong. of Corrosion Resistant Automotive Sheet Steels, ASM, 1988, p. 135. [13] C.S. Lim, M. Meshii, Proc. of Galvatech 95, ISS, 1995, p. 477. [14] M. Sakurai, L.W. Zhang, Y. Tajiri, T. Kondo, J. Iron Steel Inst. Jpn. 77 (1991) 979. [15] W. Katz, Arch. Eisenhuttenwes. 25 (1954) 307. [16] E. Almeida, M. Morcillo, Surf. Coat. Technol. 124 (2000) 180. [17] A. Besseyrias, F. Dalard, J.J. Rameau, H. Baudin, Corros. Sci. 39 (1997) 1883. [18] T. Fukuzuka, M. Urai, K. Wakayama, Kobe Steel Engineering Report, vol. 30, KOBELCO, Kobe, Japan, 1980, p. 77. [19] T. Nakamori, T. Sakane, C. Sudoh, A. Shibuya, J. Iron Steel Inst. Jpn. 77 (1991) 964. [20] J. Inagaki, M. sakurai, T. Watanabe, J. Iron Steel Inst. Jpn. 79 (1993) 57.
In-situ observation in a scanning electron microscope on the exfoliation behavior of galvannealed Zn–Fe coating layers Kyung-Keun Lee, In-Hwan Lee, Cheul-Ro Lee, Haeng-Keun Ahn ⁎ School of Advanced Materials Engineering, Research Center of Advanced Materials Development, Chonbuk National University, Chonbuk, 561-756, Republic of Korea Received 31 March 2006; accepted in revised form 19 November 2006 Available online 28 December 2006
Abstract Exfoliation behavior of galvannealed (GA) coating layers was continuously observed in-situ under 3-point bending load in a scanning electron microscope (SEM). Two kinds of substrate, Al-killed steel and Ti-stabilized interstitial free (IF) steel, were used. For GA coating layers formed on Al-killed steel, powdering was dominant at the region under compressive stress during the bending. Exfoliation occurred at the Γ/δ1 interface, and the amount of exfoliation decreased with an increase in Fe content in the coating layer. Meanwhile, for GA layers coated on IF steel substrate, flaking was dominant. One of the perpendicular cracks that were pre-induced during galvannealing grew and propagated up to the Γ/Fe matrix interface with increasing strain. On the other hand, the crack initiated at the Γ/Fe matrix interface grew and propagated along the interface. Then, this crack combined with a nearby perpendicular crack, resulted in flaking. © 2006 Elsevier B.V. All rights reserved. PACS: 62.20.Mk; 68.47.De; 68.37.Hk Keywords: C. Hot dip; D. Multilayer; D. Zinc alloy; X. Exfoliation
1. Introduction The coating layer of galvannealed (GA) steel consists of several Zn–Fe intermetallic compounds. In general, it has a complicated phase structure in the order of Γ (Fe3Zn10), Γ1 (Fe5Zn21), δ1(FeZn7), ζ(FeZn13) and η( Zn) [1–3]. The phase structure and the components of the Zn–Fe coating layer, which affect formability of the GA steel sheet, vary with the composition and temperature of the Zn bath, immersion period, annealing temperature and time, etc [4]. GA steel sheets have been widely used in the automotive and home appliance industries. The brittleness of Zn–Fe intermetallic compounds can lead to flake off or powder during presswork, which results in reducing corrosion resistance and paintability [5]. Powdering and flaking are two major fracture types of GA coating layer during presswork. Because it is important to form a coating layer with a good fracture
⁎ Corresponding author. Tel.: +82 63 270 2383; fax: +82 63 270 2376. E-mail address: [email protected] (H.-K. Ahn). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.11.021
resistance, many researches on the improvement of flaking and powdering resistance have been carried out [4,6–14]. However, research results for factors that affect formability of GA coating layer are inconsistent due to the unclearness of the fracture mechanism, and the differences in phase structure and components of the coating layer. Furthermore, researches so far reported on exfoliation behavior such as the crack initiation point and propagation path are not satisfactory. In addition, the relationship between exfoliation behavior and phase structure of the coating layer is unclear yet. In this study, in-situ observation in scanning electron microscope (SEM) on exfoliation behavior of the coating layer was performed for two types of GA steel sheets with different substrate. One was prepared on Al-killed steel substrate, and the other on Ti-stabilized interstitial free (IF) steel substrate. For the GA coatings on Al-killed steel substrate, we investigated the effects of Fe content in the coating layer on the exfoliation behaviors. In an attempt to study the fracture mechanisms of GA steel sheets and to examine the crack initiation point and propagation path on the compressive side of the coating layer, we applied a
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Table 1 Chemical composition (wt.%) of the substrate steel sheets
Specimen A Specimen B
C
Si
Mn
P
S
Al
Ti
Fe
0.04 0.02
0.01 0.008
0.2 0.1
0.01 0.01
0.01 0.006
0.04 0.024
– 0.051
Bal. Bal.
3-point bending load in a SEM chamber and took an in-situ look at the vicinity of coating/substrate interface. 2. Experimental procedure Substrate materials used in this study were Al-killed steel sheets (specimen A) and Ti-stabilized IF steel sheets (specimen B). The chemical composition of the substrate sheets is listed in Table 1. As-received sheets were reductively cleaned at 800 °C for 30 s in a 10% H2–N2 atmosphere, and then hot-dipped in a molten Zn bath at 460 °C for 2 s. The galvanizing Zn bath had an Al content of 0.14 wt.% (specimen A) or 0.20 wt.% Al (specimen B). The hot-dipped (galvanized) steel sheets were annealed (galvannealed) at 500 °C, with a hold time of 15 s for specimen A and 50 s for specimen B. For this study, the Fe content in the coating layer of the GA steel sheets (specimen A) was controlled by continued annealing at 500 °C for 600 s, 1440 s and 2220 s. The total Fe content in the coating layer of specimen A was measured by inductively coupled high frequency plasma-atomic emission spectroscopy (ICP-AES, Shimazu, ICP-7500). For the ICP-AES measurement, a 1.0% H2SO4 solution was used to dissolve the coating from the substrate steel. The coating removal was completed when effervescing from the immersed sample was no longer observed. At this point, the solution containing the
Table 2 The phase thickness of the GA coating layers measured by electro-chemical stripping Fe content (at.%)
Phase thickness of coating layers (μm) Γ
δ1 + Γ
δ1(c)
δ1(p)
Total
13 15 21 24
0.60 0.80 1.30 1.87
0.69 0.85 0.96 1.03
0.41 0.38 0.28 0.23
5.24 5.00 4.42 3.87
6.94 7.03 6.96 7.00
dissolved coating was titrated. To analyze the phase structure of the coating layer, X-ray diffractometer (Rigaku, D/Max-IIA) was applied at 40 kV/30 mAwith a Cu–Kα target. The specimens were scanned at a rate of 2°/min in the 2θ range of 30°–90°. Also, the thickness of individual intermetallic compounds in the coating layer was measured by an electrolytic stripping method [15–17]. This method closely estimates the thickness of individual layers of a multilayered coating and the electrochemical potential differences between the individual layers while being anodically stripped at constant current density. This test was conducted in a dissolved 200 g/l NaCl and 100 g/l ZnSO4 solution at a constant current density of 2.5 mA/cm2, using a Potentiostat/Galvanostat (EG and G, model 273A). The total thickness of the Zn–Fe coating of the GA steel sheets were determined using a measuring microscope (Nikkon, MM-40/L3u). The specimens for total thickness measurement were sectioned (0.75 × 5 × 30 mm3) with a micro cutter, perpendicularly to the coating layer, and then mechanically polished with alumina powder. On each specimen, thickness measurements were performed at twenty different positions across the coating layer, separated by distances of 0.5 mm, and the obtained twenty values were averaged. Before SEM analysis, all specimens were cleaned with acetone in an ultrasonic bath for 10 min followed by rinsing in distilled water and dried. In-situ observation on the exfoliation behavior of the GA coating layer in SEM was performed with a micro tensile/ bending tester (Micro-test tensile and bending stage for SEM, Debon Co.). A polished specimen was prepared and a 3-point bending test was performed at a loading rate of 0.1 mm/min. The crack initiation and the exfoliation processes of the coating layer during bending were observed in-situ. The strain was calculated using the curvature radius from the bending test. A tape-stripping test was also performed to investigate the cracking and exfoliation of the coating surface. 3. Results and discussion 3.1. Phase structure of the coating layer
Fig. 1. XRD patterns with various Fe contents in the GA coating layer of the hotdip galvanized Al-killed steel annealed at 500 °C for 15 s, 600 s, 1440 s and 2220 s, respectively.
The total thickness of the Zn–Fe coating layer of GA steel sheets used in this study were 7 μm (specimen A) and 14 μm (specimen B) on an average. In addition, the Fe content in the coating layer of specimen A varied 13, 15, 21, 24 at.% with the annealing time, and the Fe content of specimen B was 9 at.%. The X-ray diffraction analysis was performed and the results showed that the Zn–Fe coating layer of GA steel sheets (specimen A and B) mainly consisted of δ1 and Γ phases. Next
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Fig. 2. Cross sectional views of GA coating layers in a compressive stress state during bending (specimen A).
to the steel substrate was the brittle Γ phase, followed by the δ1 phase. The X-ray diffraction pattern for the coating layer with the Fe content of GA steel (specimen A) is shown in Fig. 1. Since the diffraction peaks of the Γ and the Γ1 phase were duplicated, only the Γ phase diffraction peak was analyzed. Comparing the peak intensity of (330)δ1 and (330)Γ in the X-ray diffraction pattern as a function of the Fe content in the coating layer, the peak intensity of (330)δ1 slightly increased with the
increase in Fe content up to the 15 at.% Fe content [10,14]. However, it decreased rapidly at 21 at.% Fe content and there was no variation afterwards. The peak intensity of (330)Γ with Fe content showed a slower increase. The increase of the (330)δ1 peak intensity with the increase in Fe content up to the 15 at.% can be explained by the transformation of the ζ phase in the coating layer into the phase by continued annealing [20]. Also, the (330)δ1 peak intensity decreased and the (330)Γ peak
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intensity increased due to the transformation of the δ1 phase in the coating layer into the Γ phase by continued annealing above 21 at.% Fe content. The ζ phase was not detected by X-ray diffraction due to its insignificant amount [19]. In order to measure the thickness of individual Zn–Fe intermetallic phases which affect the exfoliation characteristics of GA coating layer, the electrolytic stripping test of the coating layer of various Fe contents was performed and electric potentials corresponding to δ1( p), δ1(c), (δ1 + Γ ), Γ phase, and Fe matrix were detected. The thickness of each intermetallic compound was measured from the variation of electric potentials [11]. The thickness of each intermetallic compound obtained from the electrolytic stripping curve is shown in Table 2. After continued annealing, the total thickness of the coating layer did
Fig. 3. Cross sectional views of GA coating layers in a compressive stress state during bending (specimen B).
not vary, but the Fe content and thickness of each intermetallic compound varied with annealing time. Particularly, the thickness of the δ1 phase decreased with an increase in Fe content in the coating layer, while that of (δ1 + Γ) phase and Γ phase increased. The ζ phase was not detected from electrolytic stripping as well. It seems that the amount of the ζ phase is very small and exists randomly on the coating surface. 3.2. Exfoliation behavior of the coating layer in a cross sectional area Exfoliation aspects of the GA coating layer (specimen A) with various Fe contents in a cross sectional area that was in a compressive stress state during bending are shown in Fig. 2. As shown in Fig. 2, even before the bending load was applied, cracks perpendicular to the steel substrate existed in the coating layer. These cracks are most likely due to the volume change caused by phase transformation or the outburst reactions that occurred within the coating during annealing [4,8]. In this study, considerable cracks were observed in the δ1 phase of the coating layer. Since these pre-induced cracks grew rapidly under very low strain, it was very difficult to detect the crack initiation point. But, exfoliation behavior could be observed with the increase in strain. For the coating layer with 13 at.% Fe content, the crack propagated in the direction of the Fe matrix at 0.04 strain, after which swelling was observed at 0.08 strain. Such swelling of the coating layer occurred severely at 0.12 strain, resulting in powdering. This finding is consistent with the research results [8] that powdering occurs mainly above 10 at.% Fe content in the coating layer. The thickness of swelled thin film was approximately 6.5 μm. Considering the thickness of the intermetallic compounds measured by electrolytic stripping (Table 2), it is believed that the exfoliation occurred at the δ1/Γ interface. In the case of 15 at.% Fe content, considerable swelling occurred at 0.04 strain and exfoliation occurred at 0.08 strain. This exfoliation occurred at a lower strain and the exfoliation degree was higher than that of 13 at.% Fe content. The thickness of the swelled thin film was about 6.3 μm. In the case of 21 at.% Fe content in the coating layer, similar behavior was observed, but the exfoliation occurred at a higher strain than that of 13 at.% and 15 at.% Fe content, and the exfoliation degree was lower. In the case of 21 at. % and 24 at.% Fe content in the coating layer, the thickness of each swelled thin film was 5.4 μm and 4.8 μm respectively. For the coating layers with 21 at.% and 24 at.% Fe content, the exfoliation occurred at the δ1/Γ interface as well. The thickness of the exfoliation decreased with an increase in Fe content above 21 at.%. Fukuzuka and others [18] reported that powdering resistance decreased with the increase in the Γ phase, but the results of this study suggested the opposite. The results of the thickness of intermetallic compounds measured by the electrolytic stripping test of this study showed that the Γ phase was the thickest at 24 at.% Fe content, and the exfoliation degree decreased. Therefore, the results of this study suggest that the thickness of the Γ phase has little influence on the powdering resistance above 21 at.% Fe content in the coating layer. It is believed that
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Fig. 4. The exfoliation aspects with various Fe contents in a compressive coating surfaces (specimen A).
the exfoliation degree decreased with the increase in Fe content because of the amount of the δ1 phase in the coating layer. The δ1 phase has a low fracture resistance compared with other phases of the coating layer due to the prevalence of a large number of cracks, as shown in Fig. 2. Therefore, it is considered that the exfoliation degree decreased with the increase in Fe content in the coating layer because the δ1 phase decreased with the increase in Fe content. Likewise, as shown in Fig. 2, close adhesion of the coating layer/Fe matrix interface increased with the increase in Fe content in the coating layer. This result is consistent with Nakamori [19], who reported that in the case of the galvannealing temperature of 500 °C, close adhesion of the coating layer/Fe matrix interface increased at above 15 at.% Fe content. It is considered that such a close adhesion of the coating layer/Fe matrix interface also affected the exfoliation degree. Exfoliation aspects of the GA coating layer (specimen B) with strain in a cross sectional area that was in a compressive stress state during bending is shown in Fig. 3. Fig. 3(c) indicates the observed part of the specimen, and Fig. 3(a) and (b) shows the crack initiation and the propagation paths with the increase in strain at each position in the same specimen. Fig. 3(a) shows remarkable exfoliation and damage of the coating layer at initial bending. One of the pre-induced perpendicular cracks grew and propagated to the coating layer/Fe matrix interface with the
increase of strain. On the other hand, the crack initiated at the Γ/ Fe matrix interface grew and propagated along the interface, and then combined with a nearby perpendicular crack. Flaking occurred as a result. In the case of Fig. 3(b), although the development of a crack at the initial stage was delayed in comparison with Fig. 3(a), and the crack propagated along the Γ/Fe matrix interface deflected toward another crack, the overall fracture aspect was similar to the case of Fig. 3(a). In general, it is easy for flaking to occur with low Fe content in the coating layer or high Al content in the Zn bath [4]. Fe content in the coating layer of specimen B used in this study was relatively low, 9 at.%, and the Al content in the Zn bath was high, 0.2 wt.%. Therefore flaking was dominant in this type of coating layer. 3.3. Exfoliation behavior at the coating surface After a certain strain was applied to the GA steel sheet (specimen A) with various Fe contents, a tape-stripping test was performed. The exfoliation aspects at compressive surfaces are shown in Fig. 4. Under 0.08 strain, considerable exfoliation occurred at 13−21 at.% Fe content. Little exfoliation, however, occurred at 24 at.% Fe content. It is believed that the exfoliation did occur at the δ1/Γ interface, because of the dense Γ phase remaining when exfoliation occurred, and a columnar crystal δ1 phase around the Γ phase. This result also is consistent with
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cross sectional views of the GA coating layer in Fig. 2. Under 0.15 strain, a similar exfoliation aspect was shown. The exfoliation degree reached its maximum at 15 at.% Fe content, and decreased above 21 at.% Fe content, but did not occur at 24 at.% Fe content. Under 0.2 strain, a similar exfoliation aspect was shown, but in the case of 24 at.% Fe content, some exfoliation was shown, which differed from the 0.15 strain. On the other hand, the amount of exfoliation with various Fe contents was measured, and it varied with the Fe content in the coating layer. The amount of exfoliation was large at 15 at.% Fe content, decreased remarkably above 15 at.% Fe content, and exfoliation did not occur up to the 0.15 strain at 24 at.% Fe content. This is why the δ1 phase, formed during galvannealing, decreased with the increase of Fe content. 4. Conclusions (1) The Zn–Fe coating layer of GA steel sheets used in this study mainly consisted of δ1 and Γ phases. The total thickness of the coating layer did not vary, but the Fe content and the thickness of each intermetallic compound varied with the annealing time at 500 °C. (2) For the exfoliation of the GA coating layer formed on Alkilled steel substrate, powdering was dominant at the region under compressive stress during the bending test. Exfoliation occurred at the Γ/δ1 phase interface, regardless of the Fe content in the coating layer. Whereas, the exfoliation degree decreased with an increase in the Fe content in the coating layer. (3) For the exfoliation of the GA coating layer formed on Tistabilized IF steel substrate, flaking was dominant at the region under compressive stress during the bending test. One of the perpendicular cracks pre-induced during galvannealing grew and propagated up to the Γ/Fe matrix interface with increased strain. On the other hand, the crack initiated at the Γ/Fe matrix interface grew and
propagated along the interface. Then, this crack combined with a nearby perpendicular crack, resulted in flaking. Acknowledgment This work was supported by grant No. R05-2001-00000831-0 from the Basic Research Program of the Korea Science and Engineering Foundation. References [1] M.A. Ghoniem, K. Lonhberg, Metall 26 (1972) 1026. [2] T.B. Massalski, Binary Alloy Phase Diagrams, ASM, Metals Park, Ohio, 1986, p. 1128. [3] A.R. Marder, Prog. Mater. Sci. 45 (2000) 196. [4] M. Urai, M. Arimura, J. Iron Steel Inst. Jpn. 81 (1995) 70. [5] J. Yu, J. Liu, J. Zhang, J. Wu, Mater. Lett. 59 (2005) 2698. [6] S. Okada, K. Sakata, M. Imanaka, K. Kyono, T. Hanazawa, JSAE Rev. 7 (1996) 313. [7] Y. Adachi, T. Nakamori, J. Iron Steel Inst. Jpn. 80 (1994) 225. [8] C.E. Jordan, K.M. Goggins, A.R. Mader, Metall. Mater. Trans., A Phys. Metall. mater. sci. 25A (1983) 2101. [9] S. Chang, Y.M. Choi, Inst. Met. Mat. 31 (1993) 1241. [10] M. Urai, M. Arimura, M. Terada, M. Yamacuchi, H. Sakai, S. Nomura, J. Iron Steel Inst. Jpn. 77 (1991) 971. [11] Y. Tokunaga, M. Yamada, T. Hada, J. Iron Steel Inst. Jpn. 72 (1986) 997. [12] T. Nakamori, A. Shibuya, Proc. on World Material Cong. of Corrosion Resistant Automotive Sheet Steels, ASM, 1988, p. 135. [13] C.S. Lim, M. Meshii, Proc. of Galvatech 95, ISS, 1995, p. 477. [14] M. Sakurai, L.W. Zhang, Y. Tajiri, T. Kondo, J. Iron Steel Inst. Jpn. 77 (1991) 979. [15] W. Katz, Arch. Eisenhuttenwes. 25 (1954) 307. [16] E. Almeida, M. Morcillo, Surf. Coat. Technol. 124 (2000) 180. [17] A. Besseyrias, F. Dalard, J.J. Rameau, H. Baudin, Corros. Sci. 39 (1997) 1883. [18] T. Fukuzuka, M. Urai, K. Wakayama, Kobe Steel Engineering Report, vol. 30, KOBELCO, Kobe, Japan, 1980, p. 77. [19] T. Nakamori, T. Sakane, C. Sudoh, A. Shibuya, J. Iron Steel Inst. Jpn. 77 (1991) 964. [20] J. Inagaki, M. sakurai, T. Watanabe, J. Iron Steel Inst. Jpn. 79 (1993) 57.