Abstract
It is known that the Al added to the Zn coating layer of hot-dip galvanized steel sheets (HDG) segregates on the surface of temper-rolled HDG as Al-based oxides with increasing aging time in air at room temperature. In this study, the surfaces of Zn-0.2mass%Al HDG with and without temper rolling were investigated to clarify the segregation mechanism. Specimens with a Zn coating weight of 55–57 g/m2 including 0.19–0.20 mass% of Al were used. The specimens were aged in air at 20°C or held in liquid nitrogen, and the surface and cross sections of the specimens were then observed and analyzed by XRF, SEM-EDX and EBSD. As a result, it was found that the velocity of Al-based oxide segregation on the surface of the temper-rolled HDG was much higher than that of the HDG without temper rolling. This was attributed to the difference in the area where formation of Al-based oxides was possible. It was also found that the Zn crystal grains in the coating layer were refined by recrystallization due to contact with the temper roll, resulting in an increased number of grain boundaries that can serve as Al diffusion paths. Some unrecrystallized grains also remained after temper rolling and could increase the number of formation sites for Al-based oxides, as they contain numerous dislocations that can serve as Al diffusion paths. These two different formation sites could lead to difference in the segregation rates observed in this study.
1. Introduction
Zn-based coated steel sheets are widely used in automotive body parts to meet the higher corrosion resistance requirements of recent years. Three kinds of Zn-based coated steel sheets are mainly used in automotive parts, galvannealed steel sheets (GA), hot-dip galvanized steel sheets (HDG) and electrogalvanized steel sheets (EG). In particular, HDG is used widely by automakers in Europe and the United States because it is easier to satisfy both high corrosion resistance and low manufacturing cost, as control of the Zn coating layer thickness is easy. HDG also has excellent anti-chipping properties.1)
Because automotive components are usually press-formed, frictional characteristics, which affect press formability, are recognized as an important property of automotive steel sheets. However, all three types of Zn-based coated steel sheets have a higher friction coefficient than cold-rolled steel sheets2,3) because the adhesion force between Zn and tool steel is higher than that between cold-rolled steel sheets and tool steel. In particular, because EG and HDG have a relatively soft Zn coating layer, galling and delamination of Zn following adhesion of Zn to the tool also causes instability in press forming.
In order to stabilize mass-production press forming and improve the press formability of HDG, technologies for control of surface roughness have been studied. Noro et al. reported that the surface roughness of HDG affects its frictional properties.4) Technologies for control of the surface texture of sheets by temper rolling have been demonstrated in many reports.5,6,7,8,9) The effect of surface roughness on frictional characteristics can be explained by the effect of oil trapping due to the remaining hollows on the HDG surface during sliding.10,11,12)
The effect of Al-based oxides, which naturally segregate on the HDG surface, on the friction coefficient also have been studied. Generally, less than 1 mass% of Al is added to the molten Zn bath in the HDG manufacturing process to decrease dross and prevent Zn from alloying with the substrate steel.13,14) Although the amount of added Al is small, it has been reported that this Al segregates on the HDG surface as oxides with increasing aging time in air at room temperature.15,16) Hoshino et al. investigated the effect of these Al-based oxides on the friction coefficient of HDG and reported that the friction coefficient decreased due to Al-based oxides on the surface.17,18,19,20) The Al-based oxides that originally existed on the HDG surface were removed by the sliding tool and adhered to the tool surface, and these adhering Al-based oxides prevented direct contact of the sliding tool with the Zn coating layer, thereby reducing the friction coefficient.18,19,20)
These segregated Al-based oxides presumably also affect other quality properties required in products, such as weldability. However, the segregation mechanism of Al-based oxides on the HDG surface with increasing aging time in air at room temperature has not yet been clarified. In order to understand this behavior, both the segregation behavior of the Al-based oxides and the microstructures of the Zn coating layer depending on temper rolling and aging time were investigated in this study, and the segregation mechanism was discussed from the viewpoint of diffusion of Al in the Zn coating layer.
2. Experimental Procedure
2.1. Test Specimens
An annealed steel sheet with a thickness of 0.7 mm was hot-dip galvanized, wiped with N2 gas to control the coating thickness and then cooled. The specimen was temper rolled with a dull roll immediately after galvanizing. Temper rolling was carried out in air at room temperature with lubricant water at a rolling force of 0.19 kN/mm and elongation of 0.7%. After temper rolling, the lubricant water was dried with hot air having a temperature of 60°C for 5 s. These processes were performed continuously, and the effect of the aging time after galvanizing and temper rolling on the HDG coating layer was investigated. A specimen without temper rolling was also prepared to investigate the effect of temper rolling. The time when hot-dip galvanizing or temper rolling was completed was defined as the starting point of aging time. The test specimens were aged for a certain time period in air at 20°C without oil. Specimens held in liquid nitrogen were also prepared to keep the same condition as just after galvanizing and temper rolling. The coating weight of Zn and other characteristics of the Zn coating layer of the evaluated side of the test specimens are summarized in Table 1.
Table 1. Test specimens.
| Sheet thickness (mm) | Surface roughness Ra (μm) | Zn coating weight (g/m2) | Total Al content including Fe–Al layer (mass%) | Al content in Zn coating layer (mass%) |
|---|
| Temper-rolled HDG | 0.7 | 0.85 | 57 | 0.51 | 0.20 |
| HDG without temper rolling | 0.7 | 0.31 | 55 | 0.49 | 0.19 |
The Al content of the Al-based oxides was measured with an X-ray fluorescence spectrometer (XRF; ZSX-101E, Rigaku). The amount of Al was calculated from the intensity of the Al peaks observed in the analysis of the specimens and the intensity observed in an analysis of reference steel sheets on which metallic Al had been vapor deposited, resulting in a known coating layer. The tube voltage, tube current and measured area were 45 kV, 45 mA and a circle with a diameter of 30 mm, respectively. Measurements were performed after aging for 24, 72, 120, 192, 360, 696, 1200 and 1680 h using the same specimens in the measurements at all time steps.
The surfaces of the specimens aged for 1680 h (70 d) were observed with a field emission scanning electron microscope (FE-SEM; ULTRA Plus, Carl Zeiss) and analyzed by an energy dispersive X-ray spectroscopy (EDX) device incorporated in the FE-SEM. The instrument was equipped with both secondary electron (SE) and backscattered electron (BSE) detectors. The acceleration voltages in the observation and analysis were 5 kV. The SE and BSE detectors were used as required, depending on the purpose.
2.3. Cross-sectional Observation and Analysis
As in the surface observation, cross sections of the Zn coating layer were observed with the above-mentioned FE-SEM and analyzed by EDX. The BSE detector was used for these observations. An acceleration voltage of 5 kV was employed in both the observation and analysis. 45° cross-sectional specimens were prepared with a focused ion beam (FIB) instrument (Quanta200 3D, FEI) for SEM observations.
The electron backscatter diffraction (EBSD) patterns of the Zn crystal grains of the HDG were analyzed with an EBSD detector (Hikari High Speed EBSD, EDAX) incorporated in an FE-SEM (SUPRA 40VP, ZEISS). A step size of 0.05 μm and acceleration voltage of 20 kV were employed in this measurement. 90° cross-sectional specimens were prepared with an Ar ion milling system (PECSII, GATAN) for EBSD analysis. The obtained data were analyzed with the dedicated processing software (OIM Matrix, EDAX).
The observation and analysis described above were conducted with the specimens aged for 1680 h (70 d).
3. Results
3.1. Segregation Behavior of Al-based Oxides on Surface of Hot-dip Galvanized Steel Sheets
The relationship between the amount of Al on the surface of the HDG without temper rolling measured by XRF and the aging time is shown in Fig. 1. When the specimen was aged in air at 20°C, the amount of Al increased slightly as the aging time increased. This suggests that the Al-based oxides segregated on the HDG surface during aging in air at 20°C. In contrast, the amount of Al of the specimen held in liquid nitrogen did not change as the aging time increased, indicating that it is possible to maintain the same surface condition as just after galvanizing.
Figure 2 shows the relationship between the amount of Al on the surface of the temper-rolled HDG measured by XRF and the aging time. The amount of Al just after galvanizing and temper rolling was about 2.5 mg/m2 and increased to approximately 7.0 mg/m2 after aging for 1680 h in air at 20°C. This behavior was similar to that observed in previous reports16,17,18,19,20) investigating the segregation behavior of the Al-based oxides on the temper-rolled HDG surface. As in the result for HDG without temper rolling, the amount of Al increased with increasing aging time in air at 20°C, but the rate of increase was much larger than in the HDG without temper rolling (Fig. 1). The amount of Al of the specimen held in liquid nitrogen did not change with aging time. This means that the surface condition of the temper-rolled specimens can also be kept in the same condition as just after galvanizing and temper rolling.
The surfaces of the test specimens without temper rolling after aging for 1680 h in air at 20°C and holding in liquid nitrogen for the same time were observed with the FE-SEM, and their Al and O intensities were analyzed by EDX. The results are shown in Fig. 3. As shown in the SE images in Figs. 3(a) and 3(d), the surface morphology did not change depending on the aging condition. However, a significant difference was observed in the analytical results for Al (Figs. 3(b) and 3(e)) and O (Figs. 3(c) and 3(f)). Although the intensities of the Al and O on the HDG held in liquid nitrogen were low, higher intensities of Al and O on the grain boundary of Zn were observed on the HDG aged in air at 20°C. Because the distributions of Al and O were substantially the same, as shown in Figs. 3(e) and 3(f), respectively, these are Al-based oxides that segregated on the HDG surface with increasing aging time.
Figure 4 shows the results of surface observation with the FE-SEM and analysis of the Al and O intensities by EDX for the temper-rolled HDG surfaces after aging for 1680 h in air at 20°C and holding in liquid nitrogen for the same time. No change was seen in the surface morphology depending on the aging condition, as shown in Figs. 4(a) and 4(d), but a difference was observed in the analytical results of Al (Figs. 4(b) and 4(e)) and O (Figs. 4(c) and 4(f)). After aging in air at 20°C, Al-based oxides were observed in the hollows formed by contact with the temper roll, as well as in the area on the grain boundaries of the Zn, whereas the intensities of Al and O on the HDG held in liquid nitrogen for the same time were low. This behavior was also similar to that observed in previous reports16,17,18,19,20) investigating the segregation behavior of the Al-based oxides on the temper-rolled HDG surface.
In order to understand the segregation behavior of the Al-based oxides on the area of contact with the temper roll, the surfaces of the HDG without temper rolling and the temper-rolled HDG after aging for 1680 h in air at 20°C were observed with the BSE detector. The results are shown in Fig. 5. In this observation, areas of bright contrast show metallic Zn and areas of dark contrast show Al-based oxides. As shown in Figs. 5(a) and 5(b), Al-based oxides were observed on the grain boundaries of Zn on the surface of the HDG without temper rolling. However, on the temper-rolled HDG surface, in addition to the Al-based oxides on the grain boundaries, Al-based oxides were also observed on the area of contact with the temper roll, as shown in Figs. 5(c) and 5(d). The morphologies of the Al-based oxides can be divided into two types. One is a long linear formation, which is similar to the formation on the grain boundaries (although the grains are smaller than in the specimen without temper rolling), and the other is an aggregation of short linear formations, which is different from the formation on the grain boundaries. This difference in the formation site of the Al-based oxides could lead to the difference in the segregation rates of the Al-based oxides on HDG without temper rolling and temper-rolled HDG shown in Figs. 1 and 2.
To identify the change of the microstructure of the Zn coating layer as a result of temper rolling and aging in air at 20°C, 45° cross sections of the HDG without temper rolling and temper-rolled HDG were obtained by FIB. The BSE images of the HDG without temper rolling and temper-rolled HDG are shown in Figs. 6 and 7, respectively, and the results of the EDX analysis of the area in the center of the white boxes from (1) to (8) in Figs. 6 and 7 are shown in Table 2.
Table 2. Results of EDX analysis of areas in center of white boxes (1)–(8) in
Figs. 6 and
7.
| No. | Analytical results (at%) |
|---|
| O | Al | Zn |
|---|
| 1 | 11.7 | 5.7 | 82.6 |
| 2 | 2.9 | 0.2 | 96.8 |
| 3 | 2.7 | 19.6 | 77.7 |
| 4 | 26.2 | 15.1 | 58.6 |
| 5 | 1.1 | 50.0 | 48.9 |
| 6 | 1.9 | 31.9 | 66.2 |
| 7 | 23.6 | 15.4 | 61.1 |
| 8 | 1.8 | 79.1 | 19.1 |
The cross sections of the HDG without temper rolling after holding in liquid nitrogen are shown in Figs. 6(a) and 6(b). Grain boundaries of Zn were observed in the cross section, and a small area of dark contrast at the top surface on a grain boundary (white box (1) in Fig. 6(b)) was observed. The area of dark contrast can be identified as an Al-based oxide because higher Al and O contents were detected in this area than in the bulk (white box (2) in Fig. 6(b)).
The cross sections of the HDG without temper rolling after aging in air at 20°C are shown in Figs. 6(c)–6(f). As in the HDG held in liquid nitrogen, an area of dark contrast located on a grain boundary at the top surface was also observed in this specimen and can be identified as an Al-based oxide (white box (4) in Fig. 6(e)), although the amount of the Al-based oxide is larger than that after holding in liquid nitrogen. In addition, areas of dark contrast were also observed inside the grains (white box (3) in Fig. 6(d)) and on the Zn grain boundaries (white box (5) in Fig. 6(f)). Because a higher Al content was detected in these areas, while O was not detected, these areas can be identified as metallic Al (although possibly including some amount of metallic Zn).
If it is assumed that the same condition as just after galvanizing can be maintained in the HDG without temper rolling held in liquid nitrogen, Al-based oxides could grow on the top surface of the grain boundaries of Zn. At the same time, metallic Al precipitated in the grains and grain boundaries of Zn as aging time increased. This precipitation of metallic Al could occur more readily at the grain boundary because the size of the metallic Al is larger in the grain boundaries than in the grains.
The cross sections of the temper-rolled HDG after holding in liquid nitrogen are shown in Figs. 7(a)–7(c). Obvious changes in the microstructure were observed after temper rolling. Lines which appear to be grain boundaries were observed in the Zn coating layer. Al-based oxides were not observed on the top surface of these lines which appear to be grain boundaries (Fig. 7(c)), while metallic Al precipitates were observed in both the matrix and the grain boundaries (Figs. 7(a) and 7(b)).
The cross sections of the temper-rolled HDG after aging in air at 20°C are shown in Figs. 7(d)–7(f). Similar lines which appear to be grain boundaries were also observed in this condition. Al-based oxides were observed on the top surface of these apparent grain boundaries (Fig. 7(e)), and metallic Al precipitates were observed in both the matrix and the grain boundaries (Figs. 7(d) and 7(f)). However, the size of the Al precipitates was much larger in this case than that in the temper-rolled HDG after holding in liquid nitrogen (Figs. 7(a) and 7(b)).
Because these metallic Al precipitates were not observed before temper rolling, these results suggest that their precipitation was stimulated by temper rolling, and the precipitates then grew as aging time increased. Moreover, Al-based oxides were detected on the top surface of the lines which appear to be grain boundaries, suggesting that the segregation mechanism of the Al-based oxides could be strongly related to these apparent grain boundaries in the microstructure.
To clarify the microstructure of the Zn coating layer, 90° cross sections of the HDG without temper rolling and temper-rolled HDG were obtained by using an Ar ion milling system, and the EBSD patterns of the Zn crystal grains of the HDG were obtained. Figure 8 shows the inverse pole figure (IPF) maps of these cross sections. In the HDG without temper rolling held in liquid nitrogen, a single Zn grain existed from the top surface to the substrate steel, and two different grains were observed laterally in a view (Fig. 8(a)). The result for the HDG without temper rolling aged in air at 20°C (Fig. 8(b)) was similar to that for the specimen held in liquid nitrogen. Since the HDG without temper rolling held in the liquid nitrogen is substantially the same as the HDG just after galvanizing, this similarity suggests that the microstructural change due to aging is small. In contrast, the temper-rolled HDG held in liquid nitrogen contained several Zn grains in a view (Fig. 8(c)), and the temper-rolled HDG aged in air at 20°C showed a similar result (Fig. 8(d)). These results suggests that the Zn grains were refined by temper rolling, and the lines which appear to be grain boundaries observed in Fig. 7 are in fact grain boundaries.
The grain boundaries were identified from the obtained EBSD patterns and divided into low angle and high angle grain boundaries. A low angle grain boundary was defined as a grain boundary having a grain orientation difference of between 5° and 15°, and a high angle grain boundary was defined as one having a difference of between 15° and 180°. The results are shown in Fig. 9. The grain boundaries of the HDG without temper rolling held in liquid nitrogen and aged in air at 20°C consisted of high angle grain boundaries (Figs. 9(a) and 9(b)). In contrast, the grain boundaries of the temper-rolled HDG held in liquid nitrogen and aged in air at 20°C consisted of both high angle and low angle grain boundaries (Figs. 9(c) and 9(d)), although the majority were high angle grain boundaries. This means that the grain boundaries formed by temper rolling were not caused by rotation of crystal grains due to plastic deformation of Zn.
However, as shown in the IPF maps (Fig. 8), some grains of temper-rolled HDG showed a deviation of orientation in a grain. This result suggests that the some grains were plastically deformed by temper rolling. In order to estimate the amount of plastic deformation of the crystal grains, Fig. 10 shows a grain reference orientation deviation (GROD) map. In the GROD technique, the misorientation of a measurement grid from the average orientation of a grain is shown as a GROD value. The GROD of the HDG without temper rolling were small for both holding in liquid nitrogen and aging in air at 20°C (Figs. 10(a) and 10(b)). However, with the temper-rolled HDG, some grains with larger GROD values were observed in under both aging conditions (Figs. 10(c) and 10(d)). Although the majority of these grains were observed on the steel substrate side of the Zn coating layer, some grains were exposed at the top surface of the HDG.
4. Discussion
4.1. Precipitates of Metallic Al
A Zn–Al phase diagram was calculated using Thermo-Calc software21) and thermodynamic parameters assessed by one of the authors.22) The calculated phase diagram for the Zn-rich portion is shown in Fig. 11. While it is generally thought that the accuracy of calculated phase diagrams at around room temperature is not particularly high, it can be said that the solubility of Al in Zn at 20°C is less than 0.2 mass% because this was shown by the calculated diagram and has also been demonstrated by reports investigating the solubility of Al in Zn at around room temperature.23,24) Therefore, the equilibrium state of the Zn coating layer including 0.2 mass% Al should be separated into two phases, η-Zn (hcp) phase and α-Al (fcc) phase, in the temperature region lower than 150°C. However, the cross sections of the HDG without temper rolling held in liquid nitrogen are uniform and do not show the α-Al (fcc) phase (Figs. 6(a) and 6(b)), suggesting that Al is supersaturated in the η-Zn (hcp) phase (Zn coating layer) just after galvanizing. This supersaturated Al could precipitate as the α-Al (fcc) phase in the η-Zn (hcp) phase (Zn coating layer) as aging time increases, resulting in the metallic Al precipitates that were observed after aging in air at 20°C (Figs. 6(c), 6(d) and 6(f)). Since deformation of the Zn coating layer by temper rolling could promote precipitation of the α-Al (fcc) phase, many Al particles were observed in the cross sections of the temper-rolled HDG held in liquid nitrogen (Figs. 7(a) and 7(b)), and a larger size and larger amount of metallic Al precipitates were also observed in the temper-rolled HDG aged in air at 20°C (Figs. 7(d) and 7(f)).
As shown in Fig. 9, the grain boundaries caused by temper rolling mainly consist of high angle grain boundaries, suggesting the possibility of recrystallization. Similar recrystallization of Zn and Zn alloys at lower temperatures has also been observed in some studies.25,26,27,28,29) The effect of temperature on the recrystallization behavior of Zn and Zn alloys was investigated by Kyotani. He reported that recrystallization of Zn begins from around −70°C to 200°C, depending on the purity of the Zn and the cold rolling reduction ratio.29) When the cold rolling reduction ratio is from 50% to 90%, the beginning and finishing temperatures of recrystallization of 99.98 mass% Zn are 10°C and 80°C, respectively, and the beginning and finishing temperatures of recrystallization of 99.1 mass% Zn are 50°C and 100°C, respectively.29) As shown in Table 1, the purity of the Zn in the Zn coating layer used in this study was around 99.8 mass%, which is between 99.98 and 99.1 mass%. Although the composition of the Zn alloy was different from that in the previous reports, it is assumed that the beginning temperature of recrystallization of the Zn coating layer would be between 10°C and 50°C, and the finishing temperature would be between 80°C and 100°C. Although the actual temperature of the top surface during temper rolling is unknown, it is reasonable to think that the temperature would be higher than that of the substrate due to the heat caused by friction and deformation during temper rolling. Thus, the refined Zn grains observed in Figs. 8, 9, 10 can be explained by the recrystallization of Zn, assuming the temperature of the Zn coating layer during temper rolling was between the beginning and finishing temperatures of recrystallization. Although the solidified Zn grains after galvanizing were recrystallized by temper rolling, the temperature could be below the recrystallization finishing temperature, which would explain the coexistence of both unrecrystallized (plastically-deformed) grains and recrystallized grains in the Zn coating layer of the temper-rolled HDG as shown in Figs. 10(c) and 10(d). Here, it may be noted that a relatively larger number of unrecrystallized grains were observed on the steel substrate side of the Zn coating layer (Figs. 10(c) and 10(d)), suggesting that the temperature at the top surface was slightly higher than that on the steel substrate side.
4.3. Segregation Models of Al-based Oxides on Surface of Hot-dip Galvanized Steel Sheets
Figure 12 shows the amounts of Al measured by XRF on the surface of HDG without temper rolling and temper-rolled HDG, which were shown in Figs. 1 and 2, as function of the square root of aging time. In both cases, the amount of Al increased linearly against the square root of aging time. This suggests that the rate of segregation of the Al-based oxides was controlled by the diffusion of Al in the Zn coating layer.30,31,32,33) The segregation rate of the Al-based oxides of the temper-rolled HDG was larger than that of the HDG without temper rolling. This can be explained by the difference of the area where the Al-based oxides were able to segregate due to the different density of grain boundaries caused by recrystallization during temper rolling. Although Al in the Zn coating layer could diffuse to the top surface mainly through the grain boundaries in the Zn coating layer, the dislocations that could remain in the unrecrystallized grains would also provide a diffusion path for Al in the Zn coating layer.
Schematic images of the possible segregation models of the Al-based oxides on the HDG surface are presented in Fig. 13. After galvanizing, the Zn coating layer has a solidification microstructure of Zn. At solidification, Al-based oxides can form slightly on the surface, including on grain boundaries. After aging in air at 20°C without temper rolling, supersaturated Al precipitates in the grain boundaries and grains as the α-Al (fcc) phase. At the same time, Al diffuses to the top surface of the HDG through the grain boundaries, and then segregates as Al-based oxides.
Temper rolling after galvanizing also causes precipitation of the α-Al (fcc) phase because recrystallization of Zn occurs during temper rolling. After aging in air at 20°C, the supersaturated Al which remained in the Zn coating layer precipitates and grow as the α-Al (fcc) phase in grains and grain boundaries, in the same way as in the HDG without temper rolling. While the diffusion rate of Al in the temper-rolled HDG could be similar to HDG without temper rolling, the concentration of grain boundaries was increased by recrystallization. Moreover, some grains remained unrecrystallized, and these grains contained a high concentration of dislocations, which could increase the number of formation sites for Al-based oxides. Since both grain boundaries and dislocations can provide diffusion paths for Al in the Zn coating layer to top surface of the HDG, it is reasonable to think that the long linear oxides (Fig. 5(d)) correspond to the grain boundaries introduced by recrystallization, and aggregations of short linear oxides (Fig. 5(d)) probably correspond to unrecrystallized grains, considering the high concentration of dislocations in those grains. Supersaturation of Al in the Zn coating layer, as mentioned in section 4.1, could also affect the diffusion rate, and faster diffusion of Al than expected could have occurred even at 20°C owing to this supersaturated condition.
5. Conclusions
The segregation behavior of Al-based oxides and the microstructures of Zn-0.2mass%Al hot-dip galvanized steel sheets (HDG) depending on temper rolling and aging time were investigated in order to clarify the segregation mechanism of Al-based oxides on the surface of HDG in air at room temperature.
(1) The segregation rate of Al-based oxides on the surface of the temper-rolled HDG was much larger than that of the HDG without temper rolling. The Al-based oxides on the HDG without temper rolling existed on the grain boundaries of Zn, whereas those on the temper-rolled HDG existed at the area of contact with the temper roll in addition to grain boundaries of Zn. This difference in the formation site of the Al-based oxides could lead to the difference in the segregation rates. The segregation rate of the Al-based oxides could be controlled by the diffusion of Al in the Zn coating layer, as the amount of Al increased linearly against the square root of aging time. Two types of morphologies were observed in the Al-based oxides at the area of contact with the temper roll; one was a long linear type and the other was an aggregation of short linear oxides.
(2) The concentration of Al in the Zn coating layer was supersaturated just after galvanizing, considering the fact that the HDG held in liquid nitrogen was substantially the same as HDG just after galvanizing. The supersaturated Al precipitated as the α-Al (fcc) phase in the grain boundaries and grains of the Zn coating layer as a result of temper rolling and with increasing aging time. This supersaturated condition could lead to fast diffusion of the Al in the Zn coating layer at room temperature.
(3) Due to the low beginning temperature of Zn recrystallization, the grains of the Zn coating layer were refined by temper rolling, but some grains remained unrecrystallized. This increased segregation site of the Al-based oxides on the surface of the temper-rolled HDG, because both the grain boundaries and the dislocations in the unrecrystallized Zn grains provide diffusion paths for Al in the Zn coating layer to the surface.
(4) Unrecrystallized Zn grains remained mainly on the steel substrate side of the Zn coating layer. However, some unrecrystallized Zn grains were also exposed at the top surface. This suggests that the long linear Al-based oxides correspond to grain boundaries introduced by recrystallization, and aggregations of short linear Al-based oxides correspond to unrecrystallized Zn grains.
References
- 1) H. Kawaguchi, A. Tanaka, S. Sudo and H. Nakano: Proc. 3rd Int. Conf. on Zinc and Zinc Alloy Coated Steel Sheet (GALVATECH ‘95), ISS, Warrendale, PA, (1995), 605.
- 2) J. Iwatanai and M. Miyahara: Kobe Steel Eng. Rep., 41 (1991), 83 (in Japanese).
- 3) M. Ejima: J. Jpn. Soc. Tribol., 36 (1991), 755 (in Japanese).
- 4) H. Noro, Y. Kimura and K. Yanagai: JFE Giho, 13 (2006), 9 (in Japanese).
- 5) G. R. Payen, E. Felder, M. Repoux and J. M. Mataigne: Wear, 276–277 (2012), 48. https://doi.org/10.1016/j.wear.2011.12.007
- 6) C. Gachot, A. Rosenkranz, S. M. Hsu and H. L. Costa: Wear, 372–373 (2017), 21. http://doi.org/10.1016/j.wear.2016.11.020
- 7) Y. Kimura, M. Ueno and Y. Mihara: Tetsu-to-Hagané, 95 (2009), 399 (in Japanese). https://doi.org/10.2355/tetsutohagane.95.399
- 8) S. Costil, A. Lamraoui, C. Langlade, O. Heintz and R. Oltra: Appl. Surf. Sci., 288 (2014), 542. https://doi.org/10.1016/j.apsusc.2013.10.069
- 9) O. Pawelski, W. Rasp, W. Zwick, H. J. Nettelbeck and K. Steinhoff: J. Mater. Process. Technol., 45 (1994), 215. https://doi.org/10.1016/0924-0136(94)90343-3
- 10) Y. Yamasaki, A. Azushima and Y. Tokita: J. Jpn. Soc. Technol. Plast., 46 (2005), 957 (in Japanese).
- 11) C. Sudo, Y. Hayashi and M. Nishihara: J. Jpn. Soc. Technol. Plast., 17 (1976), 867 (in Japanese).
- 12) H. Ike: J. Jpn. Soc. Technol. Plast., 27 (1986), 63 (in Japanese).
- 13) H. Yamaguchi and Y. Hisamatsu: Tetsu-to-Hagané, 59 (1973), 131 (in Japanese). https://doi.org/10.2355/tetsutohagane1955.59.1_131
- 14) H. Nitto, T. Yamazaki, N. Morita, K. Yabe and S. Bandoo: Tetsu-to-Hagané, 70 (1984), 1719 (in Japanese). https://doi.org/10.2355/tetsutohagane1955.70.14_1719
- 15) A. Rodnyansky, Y. J. Warburton and L. D. Hanke: Surf. Interface Anal., 29 (2000), 215. https://doi.org/10.1002/(SICI)1096-9918(200003)29:3%3C215::AID-SIA703%3E3.0.CO;2-X
- 16) M. Dubois: Proc. 5th Int. Conf. on Zinc and Zinc Alloy Coated Steel Sheet (GALVATECH ‘2001), Verlag Stahleisen, Düsseldorf, (2001), 409.
- 17) K. Hoshino, M. Nagoshi, Y. Yamasaki, S. Taira, N. Yoshimi and W. Tanimoto: CAMP-ISIJ, 28 (2015), 222 (in Japanese).
- 18) K. Hoshino, Y. Yamasaki and S. Taira: IOP Conf. Ser. Mater. Sci. Eng., 651 (2019), 012041. https://doi.org/10.1088/1757-899X/651/1/012041
- 19) K. Hoshino, M. Nagoshi, W. Tanimoto, Y. Yamasaki, S. Furuya, A. Matsuzaki and N. Yoshimi: Tetsu-to-Hagané, 102 (2016), 507 (in Japanese). https://doi.org/10.2355/tetsutohagane.TETSU-2016-016
- 20) K. Hoshino, M. Nagoshi, W. Tanimoto, Y. Yamasaki, S. Furuya, A. Matsuzaki and N. Yoshimi: ISIJ Int., 57 (2017), 895. https://doi.org/10.2355/isijinternational.ISIJINT-2016-704
- 21) J. O. Andersson, T. Helander, L. Höglund, P. F. Shi and B. Sundman: Calphad, 26 (2002), 273.
- 22) K. Oikawa and N. Ueshima: Collected Abstracts of the 2017 Spring Meeting of the Japan Institute of Metals and Materials, JIM, Sendai, (2017), No. 143, DVD (in Japanese).
- 23) J. L. Murray: Bull. Alloy Phase Diagr., 4 (1983), 55. https://doi.org/10.1007/BF02880321
- 24) Y. Mae: J. Met. Finish. Soc. Jpn., 31 (1980), 328 (in Japanese). https://doi.org/10.4139/sfj1950.31.328
- 25) S. K. Sahoo, R. K. Sabat, S. Panda, S. C. Mishra and S. Suwas: Mater. Charact., 123 (2017), 218. http://doi.org/10.1016/j.matchar.2016.11.030
- 26) Z. Shi, J. Yu and X. Liu: Mater. Charact., 137 (2018), 9. https://doi.org/10.1016/j.matchar.2018.01.021
- 27) G. Dirras, J. Gubicza, H. Couque, A. Ouarem and P. Jenei: Mater. Sci. Eng. A, 564 (2013), 273. http://doi.org/10.1016/j.msea.2012.12.010
- 28) D. E. Solas, C. N. Tome, O. Engler and H. R. Wenk: Acta Mater., 49 (2001), 3791. https://doi.org/10.1016/S1359-6454(01)00261-0
- 29) M. Kyotani: J. Jpn. Inst. Met., 23 (1959), 79 (in Japanese). https://doi.org/10.2320/jinstmet1952.23.1_79
- 30) M. Sasabe: Tetsu-to-Hagané, 81 (1995), N84 (in Japanese). https://doi.org/10.2355/tetsutohagane1955.81.3_N84
- 31) M. Sasabe: Tetsu-to-Hagané, 81 (1995), N214 (in Japanese). https://doi.org/10.2355/tetsutohagane1955.81.5_N214
- 32) M. Sasabe: Tetsu-to-Hagané, 81 (1995), N277 (in Japanese). https://doi.org/10.2355/tetsutohagane1955.81.6_N277
- 33) Y. Okumura, M. Tanaka, Y. Fushiwaki and Y. Nagataki: ISIJ Int., 58 (2018), 1629. https://doi.org/10.2355/isijinternational.ISIJINT-2018-213
