2020 年 61 巻 1 号 p. 142-149
This study investigates the effect of surface texture on the crystal orientation relation between Fe and Zn on steel sheets. The Zn was deposited onto Al-killed and IF steel sheets in an agitated sulfate solution at 313 K (under deposition conditions of 1500 A·m−2 and 1.48 × 104 C·m−2). Chemically polishing the steel simplifies the epitaxial deposition of Zn by decreasing the strain on the steel surface. For Zn deposited on polished Al-killed steel, the Burgers’ orientation relation {110}Fe//{0001}Zn was observed. In this case, the {111}Fe orientation was increased while the {0001}Zn orientation was reduced. In contrast, when Zn was deposited on IF steel, the preferred relation was {111}Fe//{0001}Zn. Because IF steel has a larger crystal-grain size than Al-killed steel, the epitaxial growth of Zn is easier on IF steel than on Al-killed steel. Meanwhile, the {111}Fe orientation was more prominent on IF steel than on Al-killed steel. Thus, the orientation relationship {111}Fe//{0001}Zn is probably driven by the enhanced {111}Fe orientation. The strain, crystal orientation, and grain size of the steel influenced the orientation relation between the deposited Zn and steel, and hence the crystal orientation of the deposited Zn.
This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 83 (2019) 363–371.
Fig. 7 Crystal orientation mapping images and {110} and {0001} pole figures of (a) IF steel after chemical polishing for 10 s and (b) Zn deposited on the IF steel.
Owing to their excellent corrosion resistance and small fingerprints, electrogalvanized steel sheets coated with silica-containing thin organic films are widely applied as the functional conversion-coated steel sheets in electrical appliances. The lightness,1,2) glossiness,1) surface roughness,3,4) press formability,4,5) and chromate treatment6) of the electrogalvanized steel sheets depend on the crystal orientation of Zn. Therefore, controlling the crystal orientation of the deposited Zn is very important. Numerous studies have reported the effects of the electrolysis conditions,3,7–10) electrolyte type,11,12) addition of inorganic13–15) and organic16–19) compounds to the solution, and the substrate pre-adsorption20–23) of organic additives, on the crystal orientation of the deposited Zn.
When Zn is deposited on an α-Fe substrate, the initial Zn deposits are reported to grow epitaxially. The growth follows Burgers’ orientation relationship, described as {110}Fe//{0001}Zn and $[\bar{1}11]$Fe//$[11\bar{2}0]$Zn.11,24) Another epitaxial growth relationship, {111}Fe//{0001}Zn, has also been observed,25–27) indicating that the relationship is affected by the surface texture of the Fe substrate and the electrolysis conditions. The Burgers’ orientation relationship is reportedly completed on high-purity electrolytic Fe with large crystal-grain size, whereas Zn deposited at low deposition overpotential on cold rolled steel sheets with a small grain size preferentially adopts the {0001} orientation, regardless of the Fe orientation.28,29) However, the effect of the surface texture of the steel substrate on the orientation relationship between Zn and Fe remains ambiguous. To better clarify this relationship, we deposited Zn onto steel sheets with various crystal orientations, grain sizes and surface strains, and examined the Zn–Fe orientation relationship by the electron back scatter diffraction (EBSD) technique.
The electrolytic solution was prepared by dissolving reagent-grade ZnSO4·7H2O (1.2 mol·dm−3) and Na2SO4 (0.56 mol·dm−3) in distilled and deionized water. The pH was adjusted to 2.0 with sulfuric acid (H2SO4). The cathode substrates were polycrystalline Al-killed and interstitial-free (IF) steel sheets. Additional IF steel sheets were rolled by 1% and 5%. Prior to electrodeposition, each substrate was polished with emery papers (Nos. 600, 1500, and 2000) and chemically polished to mirror smoothness. The chemical polishing was performed at 313 K in a solution containing 2.0 mol·dm−3 H2O2 and 1.0 mol·dm−3 HF. The chemical polishing time was controlled at 10 s and 60 s, yielding approximately 1.7 µm and 10 µm of etching thickness, respectively.30) Each polished substrate was washed in a 20% (by mass) H2SO4 solution for 10 s, then electrolytically degreased in an alkaline solution for 10 s. The Zn electrodeposition was performed in solutions agitated by stirring at 400 rpm under coulostatic (1.48 × 104 C·m−2) and galvanostatic (1500 A·m−2) conditions at 313 K. The targeted surface density of the Zn coating was 5 g·m−2. The Al-killed and IF steel sheets (measuring [3 × 3] cm2) were used as cathodes, and a platinum mesh (measuring [8 × 12] cm2) was the anode.
The surface morphology of the deposited Zn was imaged by scanning electron microscopy (SEM). The crystal orientations of the Fe substrate and deposited Zn were determined by the method of Wilson and Rogers.31) The X-ray diffraction intensity was 110 for the 222 reflection and 0002 for the $10\bar{1}4$ reflection. The crystal orientations of the deposited Zn, and the Fe substrate after removing the Zn, were analyzed at the same location by EBSD. The correct location was identified by finding the previously made indentation on the Fe substrate. The Zn was removed by argon (Ar)-ion milling etching. To evaluate the strain distribution in the Fe crystals of the steel sheets, the kernel average misorientation (KAM) value,32) which quantifies the local misorientation, was measured by EBSD. Prior to EBSD, the steel sheets were polished to mirror smoothness with emery paper and 10 s of chemical polishing.
The KAM maps of the chemically polished Al-killed and IF steel sheets are shown in Fig. 1. After chemical polishing for 10 s, the strain remained on the steel surfaces. The average KAMs32) of the Al-killed and IF steel sheets (Fig. 1(a) and (b)) were very similar (0.97 and 0.86 respectively). Meanwhile, the average KAMs of the IF steel sheets subjected to 1% and 5% rolling (Fig. 1(c) and (d)) were 0.97 and 1.41 respectively, confirming that 5% rolling increased the residual strain on the steel surface.
KAM maps of the Al-killed and IF steels after chemical polishing for 10 s. [(a) Al-killed steel, (b) IF steel without rolling, (c) IF steel with 1% rolling, (d) IF steel with 5% rolling]
Figure 2 shows the SEM images of the Zn deposited on Al-killed and IF steel sheets without chemical polishing. On both Zn surfaces, blocks of layered platelet crystals were interspersed with smooth areas composed of fine crystals. The crystalline blocks were significant on the IF steel sheets (Fig. 2(b), (c), (d)). The 1% and 5% rolling scarcely affected the morphology of the Zn deposited on IF steel sheets. In the block areas, the Zn growth was apparently randomly regardless of the crystal orientation of the steel sheets. In contrast, the smooth area composed of fine deposits appeared to be formed by epitaxitial growth of Zn, as mentioned below.
SEM images of Zn deposited on Al-killed and IF steels without chemical polishing. [(a) Al-killed steel, (b) IF steel without rolling, (c) IF steel with 1% rolling, (d) IF steel with 5% rolling]
Figure 3 shows SEM images of the Zn deposited on Al-killed and IF steel sheets after chemical polishing for 10 s. The Zn deposited on both sheets was regularly layered and almost parallel to the sheets. This arrangement is seen when the deposited Zn preferentially orients along the {0001} basal plane of the hexagonal close-packed (hcp) structure. Therefore, we inferred that the Zn deposited on the chemically polished sheet preferred the {0001}-plane alignment.
SEM images of Zn deposited on Al-killed and IF steels after chemical polishing for 10 s. [(a) Al-killed steel, (b) IF steel without rolling, (c) IF steel with 1% rolling, (d) IF steel with 5% rolling]
Figure 4 shows the crystal orientations of the Al-killed and IF steel sheets after Zn deposition without chemical polishing. Both sheets were preferentially oriented in the {100} plane, although the degree of the preferred orientation was larger on the IF sheet than on the Al-killed sheet (Fig. 4(a)). The {100} orientation was further enhanced on the rolled IF sheets. In contrast, the Zn deposited on both surfaces preferred the {0001} orientation. On the rolled IF steel sheets, the orientation index of {0001} somewhat decreased while that of $\{ 10\bar{1}4\} $ slightly increased (Fig. 4(b)). The $\{ 10\bar{1}4\} $ orientation implies that the {0001} basal plane of the hcp structure inclined to the steel sheets by 28.2°. In other words, some of the Zn platelet crystals deposited parallel to the steel sheets were inclined on the rolled sheets.
(a) Crystal orientations of Fe on Al-killed and IF steels with rolling and without chemical polishing and (b) orientation of Zn deposited on the steels. IF steel was rolled.
Figure 5 shows the crystal orientation of the Al-killed and IF steel sheets after Zn deposition with chemical polishing for 10 s. Both steel sheets preferentially oriented in the {111} plane, but with different degrees of the preferred orientation. Specifically, the orientation index of {111} was larger on the IF sheets than on the Al-killed sheets (Fig. 5(a)). Rolling exerted negligible effect on the {111} orientation of the IF steel sheets. On the other hand, the preferred orientation of Zn was {0001} on both steel sheets, but was more significant on IF steel than on Al-killed steel (Fig. 5(b)). Moreover, the orientation of the deposited Zn was scarcely changed by rolling the IF steel. Chemical polishing altered the preferred orientations of the Al-killed and IF steel sheets from {100} to {111}. The degree of the orientation change was notably larger on the IF sheets than on the Al-killed sheets. As discussed below (Fig. 10), increasing the time of chemical polishing increased the {111} orientation index of both the Al-killed and IF steel sheets, indicating that the {111} orientation of Fe increased from the surface to the interior of the steel sheets. Chemical polishing decreased the preferred orientation {0001} of the Zn deposited on the Al-killed steel sheets, but increased the {0001} orientation of the Zn deposited on IF steel sheets. On the rolled IF steel sheets, the {0001} orientation of Zn was somewhat enhanced by chemical polishing.
(a) Crystal orientations of Fe on Al-killed and IF steels with rolling and chemical polishing for 10 s and (b) orientation of Zn deposited on the steels. IF steel was rolled before chemical polishing.
Figure 6 shows the crystal orientation mapping images of the Fe on the Al-killed steel sheets after chemical polishing for 10 s and removing the Zn deposit (panel a), and the deposited Zn on this surface before removal (panel b). The {110} and {0001} pole figures correspond to Fe and Zn, respectively. The Fe and the deposited Zn were analyzed at the same location. From the crystal orientation mapping images, the average crystal-grain size of Fe on the Al-killed steel sheets was measured as 7.8 µm (Fig. 6(a)), and the basal plane of the hcp structure of Zn was inclined to the steel substrate (Fig. 6(b)). From the {110} and {0001} pole figures of Fe and Zn respectively, the Burgers’ orientation relationship {110}Fe//{0001}Zn was obtained.
Crystal orientation mapping images and {110} and {0001} pole figures of (a) Al-killed steel after chemical polishing for 10 s and (b) Zn deposited on the Al-killed steel.
Figure 7 shows the crystal orientation mapping images of Fe on the IF steel sheets after chemical polishing for 10 s and removing the Zn deposit (panel a), and the Zn deposited on this surface before removal (panel b). Again, the {111} and {0001} pole figures refer to Fe and Zn, respectively. From the crystal orientation mapping images, the average crystal-grain size of Fe on the IF steel sheets was measured as 15.5 µm (Fig. 7(a)), and the basal plane of the hcp structure of Zn was almost parallel to the steel substrate (Fig. 7(b)). From the {111} and {0001} pole figures of Fe and Zn respectively, the orientation relationship {111}Fe//{0001}Zn was obtained, which differs from the Burgers’ relationship.
Crystal orientation mapping images and {110} and {0001} pole figures of (a) IF steel after chemical polishing for 10 s and (b) Zn deposited on the IF steel.
Figure 8 shows the crystal orientation maps on the chemically polished, 5%-rolled IF steel sheets. Panels a and b are the crystal orientation mapping images of bare Fe on the rolled IF sheet and the Zn deposited on the rolled IF sheet, respectively. Owing to the high strain and roughness of the Fe and Zn surfaces, the orientations of Fe and Zn are not partially analyzable in the crystal orientation mapping images. On the 5% rolled IF steel, the strain was not removed by chemical polishing. From the crystal orientation mapping images of Zn (Fig. 8(b)) and the {111} and {0001} pole figures of Fe and Zn, respectively, the preferred orientation of Zn on the rolled IF steel sheets was determined as {0001}, and the orientation relationship was determined as {111}Fe//{0001}Zn, which again differs from Burgers’ relationship.
Crystal orientation mapping images and {110} and {0001} pole figures of (a) IF steel with 5% rolling and chemical polishing for 10 s and (b) Zn deposited on the IF steel.
The effect of surface strain and steel-sheet crystal orientation on the Fe–Zn orientation relationship was further investigated on Zn-coated steel sheets that were chemically polished for 60 s. Figure 9 shows the surface KAM maps of the polished Al-killed and IF steel sheets. The surface strain was lower after polishing for 60 s than after polishing for 10 s. The average KAM values of the Al-killed and IF steel sheets were 0.40 and 0.53 respectively (versus 0.97 and 0.86 respectively after chemical polishing for 10 s).
KAM maps of the (a) Al-killed and (b) IF steels after chemical polishing for 60 s. The IF steel was not rolled.
Figure 10 shows the crystal orientations of the Al-killed and IF steel sheets after chemical polishing for 10 and 60 s. Increasing the chemical polishing time from 10 s to 60 s increased the orientation index of the {111}Fe on both sheets. This indicates that the {111}Fe orientation increased from the surface to the interior of the steel sheets.
Crystal orientations of Fe on the Al-killed and IF steels after chemical polishing for 10 and 60 s. [Fe orientation ● 222, ▲ 110, □ 200, ◆ 211]
Figure 11 shows the crystal orientations of Zn deposited on the Al-killed and IF steel sheets with chemical polishing for 10 and 60 s. Increasing the chemical polishing time of the Al-killed steel sheets decreased the {0001}Zn orientation and increased the $\{ 10\bar{1}3\} $ orientation. In particular, the {0001} basal plane of Zn was inclined at 35.5° to the steel substrate after polishing for 60 s. In contrast, the orientations of Zn deposited on the IF steel sheets were almost identical after polishing for 10 s and 60 s, and were approximately {0001}.
Crystal orientations of Zn deposited on the Al-killed and IF steels after chemical polishing for 10 and 60 s. [Zn orientation ● 0002, ▲ $10\bar{1}3$, □ $10\bar{1}4$, ◆ $10\bar{1}0$]
Figure 12 shows the crystal orientation mapping images of Fe on the Al-killed steel sheets after polishing for 60 s and removing the deposited Zn (Fig. 12(a)), and the Zn deposited on this surface before removal (Fig. 12(b)). In the crystal orientation mapping images, the basal plane of the hcp structure of the deposited Zn was inclined to the steel substrate, as observed after chemical polishing for 10 s (c.f. Figs. 12(b) and 6(b)). From the {110} and {0001} pole figures of Fe and Zn respectively, the Burgers’ orientation relationship {110}Fe//{0001}Zn was identified on the Al-killed steel sheets after chemical polishing for 60 s.
Crystal orientation mapping images and {110} and {0001} pole figures of (a) Al-killed steel after chemical polishing for 60 s and (b) Zn deposited on the Al-killed steel.
Figure 13 shows the crystal orientation mapping images of the Fe on the IF steel sheet after chemical polishing for 60 s and removing the Zn (Fig. 13(a)), and of the Zn deposited on this surface before removal (Fig. 13(b)). From the crystal orientation mapping images, the orientation of the deposited Zn was almost {0001}, but the degree of the preferred {0001} orientation was significantly changed from that of polishing for 10 s (c.f. Figs. 13(b) and 7(b)). From the {111} and {0001} pole figures of Fe and Zn respectively, the orientation relationship on the IF steel sheets after chemical polishing for 60 s was {111}Fe//{0001}Zn, identical to that after chemical polishing for 10 s (Fig. 7).
Crystal orientation mapping images and {110} and {0001} pole figures of (a) IF steel after chemical polishing for 60 s and (b) Zn deposited on the IF steel.
Figure 14 shows the surface SEM images of the Zn deposited on the Al-killed and IF steel sheets after chemical polishing for 60 s. The smooth areas on both sheets indicate that the Zn preferentially oriented in the {0001} basal plane. The Zn was especially smooth on the IF steel, but the Zn deposited on both steel sheets was smoother after polishing for 60 s than after polishing for 10 s (Fig. 3).
SEM images of Zn deposited on (a) Al-killed steel and (b) IF steel after chemical polishing for 60 s.
As the crystal orientation of Zn affects the appearance and press formability of electrogalvanized steel sheets,1–5) understanding how the surface textures of steel sheets influence the Zn orientation on the sheets, and elucidating the orientation relationship between Zn and Fe, are worthy undertakings. The results of these analyses are summarized in Table 1. The grain size of the IF steel sheets was approximately twice that of the Al-killed steel sheets. Chemical polishing was expected to reduce the surface strain on the steel sheets, and the reducing effect was expected to enhance with increasing polishing time. The indices of the {111}Fe orientations on the Al-killed and IF steel sheets increased after chemical polishing for 10 s, and further increased after chemical polishing for 60 s.
The deposited Zn preferentially oriented in the {0001} plane on both unpolished steels (see Fig. 4(b)). The crystal orientation of deposited Zn reportedly depends on the deposition overpotential, and changes from {0001} to $\{ 10\bar{1}1\} $, $\{ 11\bar{2}0\} $, and $\{ 10\bar{1}0\} $ as the overpotential increases.33,34) On the unpolished steel sheets, the Zn grew randomly as evidenced in the SEM images (Fig. 2), indicating that the deposition overpotential was low under the applied electrolysis condition; consequently, the deposited Zn easily preferred the {0001} orientation.
On the Al-killed steel sheets, the {0001}Zn orientation significantly decreased after chemical polishing for 10 s (Fig. 5(b)), and further decreased after chemical polishing for 60 s. On the other hand, the {0001}Zn orientation on the IF steel sheets was only moderately increased by 10-s chemical polishing (Fig. 5(b)). Chemical polishing decreases the surface strain of the steel sheets, favoring the epitaxial deposit of Zn (Figs. 3, 14). That, is, the crystal orientation of deposited Zn is easily affected by the crystal orientations of steel sheets.
On the Al-killed steel sheets with chemical polishing for both 10 and 60 s, the orientation relationship between Fe and Zn was the Burgers’ orientation relationship {110}Fe//{0001}Zn. During Zn deposition on an α-Fe substrate, the initial Zn deposits are known to grow epitaxially according to Burgers’ orientation relationship.11,24) In this study, the Zn deposited on Al-killed steel sheets exhibited similar epitaxial growth. On the Al-killed steel sheets, the {0001}Zn orientation decreased after chemical polishing for 10 s (Fig. 5(b)), which is attributed to that Zn deposited epitaxially according to Burgers’ orientation relationship of {110}Fe//{0001}Zn on the steel sheets with preferred orientation of {111}. With increasing chemical polishing time to 60 s, the orientation of {0001}Zn further decreased (Fig. 11), which is due to that the orientation of {111}Fe further increased (Fig. 10) and Zn deposited epitaxially according to Burgers’ orientation relationship on the steel sheets with preferred orientation of {111}.
On the IF steel sheets with chemical polishing for both 10 s and 60 s, the orientation relationship was {111}Fe//{0001}Zn, which differs from the Burgers’ relationship. Epitaxial Zn deposits obeying the {111}Fe//{0001}Zn relationship have been reported in previous studies.25–27) The misfit ratios of {110}Fe to {0001}Zn and of {111}Fe to {0001}Zn are 10.4% and 13.9%, respectively.25) Both misfit ratios are small, but {111}Fe and {0001}Zn are less well matched than {110}Fe and {0001}Zn. As the IF steel sheets have a larger grain size than the Al-killed steel sheets, epitaxial Zn deposition is easier on the IF steel sheets. Moreover, the {111}Fe orientation index was more significant on IF steel than on Al-killed steel (Fig. 10). The {0001} orientation of Zn is known to become prevalent under electrolysis conditions that favor epitaxial Zn deposition.8,26,27,35) On IF steel sheets, where Zn inherently deposits epitaxially and the {111}Fe orientation is strong, the orientation relationship is {111}Fe//{0001}Zn despite the higher misfit ratio between {111}Fe and {0001}Zn than between {110}Fe and {0001}Zn.
In the X-ray diffraction evaluations (Fig. 11), the crystal orientations of the Zn deposited on IF steel sheets were insensitive to the chemical polishing time, but the orientation of {0001}Zn was more significant with chemical polishing for 60 s (see the EBSD crystal orientation mapping images of Zn; Figs. 7(b) and 13(b)). The surface of the deposited Zn was smoother after polishing for 60 s than after polishing for 10 s (c.f. Figs. 3 and 14), because the increased {111}Fe orientation and reduced surface strain promoted the epitaxial Zn deposition. Consequently, the {0001}Zn orientation increased as the {111}Fe//{0001}Zn relationship strengthened.
On the rolled steel sheets without chemical polishing, the {0001}Zn orientation slightly decreased (Fig. 4(b)). After a seemingly epitaxial initial deposition, the Zn deposited randomly on the unpolished sheets (Fig. 2). Rolling suppressed the epitaxial growth by increasing the surface strain of the steel sheet, thereby reducing the {0001}Zn orientation. On IF steel sheets, the orientation of {0001}Zn was slightly higher with chemical polishing than that without, which is attributed to that the epitaxial growth of Zn with relationship of {111}Fe//{0001}Zn increased due to increase in orientation of {111}Fe with chemical polishing. On the chemically polished surface for 10 s, rolling exerted little effect on the orientation of the deposited Zn (Fig. 5(b)), because the polishing removed much of the surface strain imposed by the rolling.
This study investigated the effect of the surface texture of steel sheets on the crystal orientation of the deposited Zn and the orientation relationship between the Fe and Zn crystal grains. Chemically polishing the steel favors the epitaxial deposition of Zn by decreasing the strain on the steel surface. Zn deposited on polished Al-killed steel satisfied the Burgers’ orientation relation {110}Fe//{0001}Zn. In this case, the {0001}Zn orientation reduced as the {111}Fe orientation was promoted and the Zn deposition increasingly followed the {110}Fe//{0001}Zn relationship. In contrast, Zn deposited on IF steel preferred the {111}Fe//{0001}Zn relationship. Epitaxial growth of Zn is easier on IF steel (with larger crystal-grain size) than on Al-killed steel (with smaller grain size). The {111}Fe orientation was more prominent in IF steel than in Al-killed steel. Because the proportion of {0001}Zn increases under conditions that favor epitaxial growth of Zn and the {111}Fe orientation was prominent in IF steel, this appears to be the cause of the relation of {111}Fe//{0001}Zn. The orientation relation between the deposited Zn and steel, and hence the crystal orientation of the deposited Zn, depends on the strain, crystal orientation, and grain size of the steel.
