Abstract
Streaky mark defects on hot dip galvannealed Ti-IF sheets were analyzed using SEM, EBSD, FIB, and TEM. Results revealed that bright streak had slightly larger skin-passed area in correspondence to more uniform GA coating. After the removal of GA coating, more coarse grains on the substrate surface, primarily elongated grains along the rolling direction with the normal direction//<100>, were found in the bright streak area. These coarse grains accelerate the Fe-Zn diffusion reaction during galvannealing and result in a flatter coating surface and an even coating thickness. The source of these coarse grains at the uppermost surface could be traced back to the hot rolling.
Introduction
In order to prevent corrosion on the surface of steel sheet and extend their service life, it is often necessary to coat them with a layer of metallic zinc. This method is currently the most common and effective anti-corrosion measure. This galvanized thin steel sheet is called galvanized iron sheet. Because of its good corrosion resistance, it is widely used in home appliances, automobiles and other fields.1–5 In the actual production process, the steel sheet is immersed in a zinc pot at around 450°C. When the steel plate comes out of the zinc pot, the zinc liquid covers the surface of the steel sheet. By controlling the gas blowing speed of the gas knife edge, the thickness of the coating can be controlled. The steel sheet with coating is called a hot-dip galvanized iron sheet (GI sheet). In order to further improve the corrosion resistance and welding performance of the steel sheet, it is often treated by alloying furnace after hot galvanizing, and heat treatment at 500 ∼ 550°C, so that the iron and zinc in the steel substrates diffuse each other to form a zinc-iron alloy phase coating. This alloyed coated steel sheet is called a galvannealed sheet (GA sheet).6–8 After normal alloying of hot-dip galvanized sheet, the iron content (mass fraction) in the coating is about 10%, which provides GA sheets with better corrosion resistance, welding performance, coating performance and impact resistance compared to GI sheet.
Surface appearance of a galvannealed steel sheet is of prime importance for automotive outer panel applications. However, various types of surface defects can be found on the galvannealed steel sheet due to the inherent difficulty of the galvannealing coating process.9,10 The formation of the surface quality defects in these galvannealed sheets as mentioned in the literature is closely related to the surface state or defects of the substrate, such as inclusions, scratches, roll marks, color differences, warping and other defects.11,12 Among all common coating defects, the so called ‘streaky mark’ is a particularly common one that appears as dark and bright streaks parallel to the rolling direction (RD) and cannot be concealed after painting.
Bi 13 studied dark streaky edge defects on galvannealed 1180 MPa UHSS. He found that the variation in the amounts of internal oxides formed in the subsurface of different zones of the hot-rolled strip is the root cause of the formation of these dark streak defects. Zhong 14 also found that line streak on Ti-IF steel by using the hot dip simulator. These linear-streaky defects can be minimized by decreasing the bath Al content, GA temperature, and/or dew-point, or by increasing the snout entry temperature and/or coating weight. Its formation is also related to steel chemistry, surface properties of the incoming steels and coating line configurations. Jordan 15 observed the effect of substrate grain size on Fe-Zn phase formation and growth and pointed out that that the Fe-Al inhibition layer is first attacked by liquid zinc at sites corresponding to substrate steel grain boundaries. Jin 16 investigated a streaky defect formed on hot dip galvannealed Ti-added IF steel, and suggested that elongated grains at the uppermost surface are the main cause of this streaky defect. Friedel 17 observed dark streak defects on hot dip galvannealed Ti-added IF steel, noting that stretched grains with {100}<110 > texture promote the Fe-Zn diffusion reaction. Although both Jin and Freidel concluded that the elongated grains at the uppermost surface are the main cause of the streaky defect, but give different reason why the elongated grains formed at the uppermost surface.
In this paper, the detailed characterization and cause analysis of the bright and dark streaky defects distributed along the rolling direction on the GA coating surface of Ti-IF steel are carried out, which has certain guiding significance for the actual production of iron and steel enterprises.
Experimental material and methods
Experimental material
A GA coating sample with bright streaky defects as shown in Figure 1 was obtained from continuous galvanizing line (CGL). The streaky defects occurred on both sides of the entire roll and exhibit an alternating bright/dark distribution pattern. The streaks were observed moving across the strip surface along with the steel strip's motion. No obvious streaks were detected on the surface after hot rolling, pickling, cold rolling, or cleaning. However, they became visible after galvanizing and before skin-passing. The surface inspection system showed low sensitivity to such defects. The chemical composition of the base steel was 0.0012 mass% C, 0.003 mass% Si, 0.2–0.3 mass% Mn and 0.04–0.06 mass% Ti and the coating weight was 40–50 g/m2 per side. The steel strip was produced via a series of processes. It was continuously cast, hot rolled to a thickness of 3 mm, pickled, and cold rolled to a thickness of 0.65 mm. The hot rolling finish temperature was 920–930 °C and the coiling temperature was 710–720 °C. The annealing temperature was 800–810 °C. Galvannealing temperature was maintained at around 490–500 °C. Around 1.0% skin pass elongation was applied on the galvannealed coated surface.
Macrostructure of streaky defects.
Experimental method
Comparative characterizations were conducted step by step on the specimens cut from the
Results and discussions
Morphological characteristics of streaky defects
Figure 2(a) shows the typical surface morphology of the bright streaky defects. The skin pass marks are light gray, which are brighter than the areas without skin pass marks. The skin pass marks in Figure 2(a) are green-colored in Figure 2(b) for quantitative analysis using digital micrograph data processing software. The total green area of the skin pass marks in Figure 2(b) is 34% for the bright streak. In comparison, the typical surface morphology of the dark streaky defects was shown in Figure 2(c). The green-colored image of Figure 2(c) is shown in Figure 2(d). The total green area of the skin pass marks in Figure 2(d) is 24% for dark streak. More than 5 fields were taken for each streak to compare. As a result, the total area of the skin pass marks of bright streak is larger than that of dark streak. Therefore, more light can be reflected from the bright streaks, which is why the bright streak appear brighter than the dark streak.
The surface SEM morphology of bright and dark streaky defects under low magnification 200× (a) bright streak; (b) skin pass marks (green color) in bright streak; (c) dark streak; (d) skin pass marks (green color) in dark streak.
The surface morphology of GA coating was further enlarged to a higher magnification, 2000 ×, as shown in Figure 3(a) for bright streak and Figure 3(b) for dark streak, respectively. Both coarse rod-like phases and fine granular phases were found in the surface of GA coating. The EBSD Kikuchi patterns were given in Figure 3(c) and (d), for the coarse rod-like phase P1 in Figure 3(a) and the fine granular phase P2 in Figure 3(b), respectively. It was proved that the coarse rod-like phase was the ζ phase (FeZn13), which has monoclinic crystal structure, C12/m1(12) space group. Its lattice constant is a = 13.394 Å, b = 7.5980 Å, c = 5.066 Å, β = 127.23. The fine granular phase was the δ phase (Fe13Zn126), which has hexagonal crystal structure, P63/mmc(194) space group. Its lattice constant is a = 12.8297 Å, c = 57.286 Å. As could be observed from Figure 3(a) and (b), the bright streak had slightly more coarse rod-like ζ phases. But the dark streak had slightly more fine granular δ phases.
The surface SEM morphology and Kikuchi pattern of GA coating (a) bright streak; (b) dark streak; (c) Kikuchi pattern of P1 point in Figure 3(a); (d) Kikuchi pattern of P2 point in Figure 3(b).
Microstructure and orientation of substrate surface
The ferrite grains of the uppermost surface of the steel substrate were revealed using nital etchant after the removal of the GA coating, as shown in Figure 4. Three fields of view for both bright and dark streak were given. The coarse grain boundaries are outlined with red dotted lines. As can be seen from Figure 4, there are more coarse grains stretched in the rolling direction in bright streak than that of the dark streak. EBSD orientation analysis was performed on one of the fields of the bright streak. The results are shown in Figure 5. The simulated orientation of P1 and P2 coarse grans in Figure 5(a) are insets in the IPF map of normal direction (Figure 5(b)). The normal direction of these two coarse grains, which are elongated in the rolling direction, is nearly perpendicular to {100}. Slight variation of orientation in the grains are observed.
Substrate ferrite grains of of the uppermost surface (a, b, c) bright streak (d, e, f) dark streak.
Substrate microstructure of the uppermost surface of the bright streak (a) morphology (b) IPF map of normal direction.
In order to explore the distribution depth of the coarse ferrite grains and obtain good quality EBSD orientation distribution maps, a series of EBSD analysis were conducted. First, one specimen was taken from the bright streak area. Then the GA coating was removed using electro-polishing. Extra care was taken for just the uppermost surface of the substrate exposed to the outside for further EBSD analysis. As shown in Figure 6(a) and (c), the stretched coarse grains, whose normal direction is nearly perpendicular to {100}, can be found in the uppermost surface of the steel substrate. But when about 15μm layer was removed from the uppermost surface of the steel substrate by mechanical polishing, only globular grains were observed on the subsurface of the steel substrate. It means that the section below the surface designated as bulk structure does not feature any stretched coarse grains (Figure 6(b) and (d)).
Band contrast maps (a, b) and orientation distribution of normal direction (c, d) of substrate of the bright streak the uppermost surface (a, c) and the sub-surface 15 μm below the uppermost surface (b, d).
One of the fields of view of the electro-polished uppermost surface in the bright streak area was further enlarged to observe more details of the elongated coarse grains. As shown in Figure 7(a), the surface of the coarse grain is quite rough, which is in a good correspondence to small angels of IPF map of normal direction in Figure 7(b) and deformation bands in the KAM map in Figure 7(c). It is further proved that the coarse grains, whose normal direction is nearly perpendicular to {100}, are stretched in the rolling direction.
EBSD analysis of the uppermost surface of the steel substrate in the bright streak (a) SE morphology, (b) IPF map of normal direction, and (c) KAM map.
Morphological characteristics of cross section
The cross section specimen of bright streak area was prepared by FIB technology. The cross section covers areas both with skin pass mark and without skin pass mark, as shown in Figure 8(a). The GA coating of the area without skin pass mark is rough. In comparison with the GA coating of the area without skin pass mark, that of the area with skin pass is much smoother Figure 8(a) and (b). The thickness of the GA coating of the area with skin pass is significantly greater than that of the area without skin pass mark. It was found that the ferrite grain is globular underneath the GA coating of the area without skin pass mark, but that of the area with skin pass mark is a stretched grain. Therefore, it can be concluded that the stretched coarse grain greatly accelerate the average growth rate of the Fe-Zn phase. A coating with a flatter surface and a more even thickness in microscopic scales forms on the stretched grains.
Schematic of cross section specimen preparation by FIB (a) surface morphology before Pt deposition, (b) overview, and (c) enlarged overview.
EBSD orientation analysis was performed on the Map1 and Map2 areas, outlined with red dotted lines, which correspond to parts of the globular and stretched grain, respectively. The normal direction of the globular grain is nearly perpendicular to {110} and no deformation band was observed in the grain (Figure 9(a) and (c)). However, the normal direction of the stretched grain is nearly perpendicular to {100} and many deformation bands were observed in the grain (Figure 9(b) and (d)).
IPF maps (a, b) and KAM maps (c, d) of the bright streak area, showing the globular grain (Map1) and stretched grain (Map2) regions respectively.
Two TEM specimens were taken from the TEM1 and TEM2 areas, outlined by blue dotted lines, respectively. The TEM1 specimen is the cross section in the area without skin pass mark, while the TEM2 specimen is the cross section in the area with skin pass mark. TEM observation results are shown in Figure 10. It was found that there is a thin layer on the surface of the substrate. After EDS composition and diffraction pattern analysis, this thin layer was confirmed to be the Γ layer, and its phase was identified as Fe3Zn10. The maximum thickness of the Γ layer on the globular grain substrate is about 712 nm, as shown in Figure 10(a). However, the maximum thickness of the Γ layer on the stretched grain substrate is about 1.44 μm, as shown in Figure 10(b), which is much thicker than that on the globular grain substrate. It iwas further proved that the stretched gains can facilitate the Fe-Zn diffusion reaction.
TEM microstructure of cross section in bright streak area (a) the area without skin pass mark (TEM1), (b) the area with skin pass mark (TEM2).
Discussion
Through detailed analysis of the GA coating surface, cross-section, and substrate surface of the GA coating streaky defects, it was found that the microscopic manifestation of the streak defects primarily lies in the size of the skin-passed regions. The results show that the skin-passed area in the bright streak is larger than that in the dark streak. After pickling to remove the GA coating, a larger skin-passed area corresponds to more stretched coarse grains on the substrate surface. And the stretched coarse grains only appear on the outermost surface of the steel substrate. The section 15μm below the uppermost surface is the same as the bulk of the substrate, consisting of equiaxed globular grains. Further analysis of these stretched coarse grains revealed that their normal directions are nearly perpendicular to {100} and that retained deformation bands exist in them to some extent, which means that full recrystallization had not yet occurred. Most of the these elongated coarse grains are unrecrystallized and just recovered.
About how the unrecrystallized grains accelerate the growth rate of the Fe-Zn phase, thereby promote faster GA coating growth leading to a coating with a uniform thickness in microscopic scale, and ultimately affect the reflection of light, both Jin 16 and Friendel 17 all explained very clearly in their papers. A schematic (Figure 11) has also been given to explain the formation mechanism of the streaky defect caused by unrecrystallized grains. Although both of Friendel and Jin's research material are Ti-IF steel, they give different reasons why the stretched grains remained after annealing. Jin 16 mentioned the source of the unrecrystallized grains at the uppermost surface could be traced back to the hot rolling. He assumed the temperature of the surface layer was a little lower than that of the inner side of the hot strip, especially at the strip edges. When finish rolling temperature was not high enough, it is possible for the surface layer to drop into the mix region of austenite and ferrite. Those grains which are hot rolled below Ar3 temperature could result in the formation of stable end orientation of ferrite which is hardly to recrystallize during annealing. But Friendel 17 believe that higher hot strip coiling temperature might cause significant internal oxidation, which is not removed by the pickling of the hot rolled strip. The fine internal oxides are capable of obstructing and even fully suppressing recrystallization. Titanium oxides are particularly effective in this way. Thus a fine grain or completely recovered microstructure is produced. Friendel 17 also think that the internal oxidation may occur only when the cold strip is annealed before being galvanized due to a high dew point in the annealing gas. Bi 13 also found that the difference in the amounts of internal oxides formed in the subsurface of different zones of the hot-rolled strip influenced the pickled steel surface, cold-rolled steel surface, annealed steel surface and the galvannealed coating surface, thus forming streaky defects in ultra-high strength steel. He thought that the internal oxides mainly affect the supply of diffusible alloy elements in the subsurface during the annealing process, thus promoting the growth of the GA coating. However, he did not mention that internal oxides might suppress recrystallization, and therefore produce unrecrystallized grains, although the stretched grains can be seen in his schematic of the formation mechanism of the dark streaky edge defect.
A schematic diagram of the effect of coarse grains on the growth of the GA coating.
Combined with the previous analysis of streaky defects, we speculate that the unrecrystallized grains at the uppermost surface could be caused by the finish hot rolling in the mix region of austenite and ferrite due to the slight low hot rolling finish temperature. However, we cannot exclude the possibility of internal oxidation because of a high hot strip coiling temperature.
Conclusion
In this paper, the microstructure of streaky defects in GA sheet was characterized in detail by SEM, EDS, EBSD, FIB and TEM, and the formation mechanism of streaky defects was discussed in depth.
The size of the skin passed region of the bright streak in GA coating is larger than that of the dark streak. The bright streak has more coarse rod-shaped ζ phase and fewer fine granular δ phase than the dark streak. There are more stretched coarse grains in the uppermost surface of the substrate in bright streak than in dark streak. The normal direction of these stretched coarse grains are nearly perpendicular to {100} and retained deformation bands exist in them to some extent. The stretched coarse grains only appear on the outermost surface (less than 15 μm deep to the upper most surface) of the steel substrate. The thickness of the GA coating above the stretched grains is significantly greater than that above the globular grains. The thickness of the Γ layer above the stretched grain substrate is also much thicker than that above the globular grain substrate. It thus proves that stretched coarse grains can accelerate the Fe-Zn diffusion reaction. The streaky defects are closely related to the hot rolling process. It might be caused by the finish hot rolling in the mix region of austenite and ferrite due to the slight low hot rolling finish temperature.
Footnotes
Author contribution(s)
Funding
The authors received no financial support for the research, authorship, and/or publication of this article.
Declaration of conflicting interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.