October 2, 2008

Structural and Textural Characterization of the Substrate and Coated Layer in an Industrial Galvannealed Interstitial-Free Steel

By Chakraborty, A Ray, R K; Sangal, S

An attempt has been made to characterize the microstructural and textural aspects of the substrate and the coated layer on industrially produced galvannealed interstitial-free (IF) steel. A number of experimental techniques were used for this purpose. The major part of the coating was found to be made up of the delta layer, favorable for good formability. The texture of the substrate steel contained a sharp gamma component, which imparts high deep drawability. The offbasal texture {01.3}(uv.w) of the coating was also beneficial for this purpose. The favorable microstructural and textural characteristics led to satisfactory powdering resistance to the coated material. DOI: 10.1007/s11661-008-9589-z

(c) The Minerals, Metals & Materials Society and ASM International 2008


THE modern auto industry demands steels having excellent corrosion resistance, especially in those countries where corrosive calcium or sodium chloride spreads are used to prevent roads from freezing during winter.[1] Several technologies have been developed that employ products that use zinc-based coatings. Among them, the galvannealing process has received the most attention, because galvannealed coated steel exhibits superior corrosion resistance, better paintability, and good weldability.[2] In the galvannealing process, steel is first immersed in an aluminum containing zinc bath and then given a postcoating heat treatment. This heat treatment causes the zinc in the coating to interdiffuse with the substrate iron to form several Fe-Zn intermetallic phases that are stacked on the steel substrate.[3] Industrial galvannealed coatings contain approximately 10 wt pet iron. Though galvannealed layers have the preceding advantages, they suffer from an inherent drawback in the form of poor formability. The higher is the iron content in the coating, the poorer is its formability. The embrittlement of the coating mainly depends on the iron content and distribution of different Fe-Zn intermetallic phases.[4-6]

Several efforts have been made to correlate the microstructure of galvannealed coatings to their performance, especially the coating formability during press forming operations.[7-23] Most of the work has been carried out using galvanizing and galvannealing simulators, where the growth of the coating takes place under fully controlled experimental conditions. The relevance of the results of such investigations to the industrial galvanizing and gaivannealing processes, where there is practically very little control of the working parameters during operation, is questionable. That is why the present work has been undertaken to characterize the structure and texture of the substrate and the coated layer and also to study the mechanical behavior of an industrially produced galvannealed coating over interstitial-free (IF) grade steel to yield data that may be useful to the steel industry.

The present study involves several characterization techniques such as grazing incidence X-ray diffraction (GIXRD), cross- sectional optical microscopy, cross sectional scanning electron microscopy (SEM) with energy-dispersive spectrometry (EDS), glow discharge optical emission spectroscopy (GDOES), cross-sectional transmission electron microscopy (TEM), and also anodic dissolution. These techniques were employed to characterize the galvannealed coating and to identify the different Fe-Zn intermetallic layers such as the gamma (Gamma), gamma^sub 1^ (Gamma^sub 1^), delta (delta), and zeta (zeta). Thorough textural measurements were also carried out on the substrate steel and on the coating. Finally, an attempt has been made to correlate the formability properties of the coating with the microstructures as well as the textures of the steel substrate and of the coated layer.


The chemical composition of the steel is shown in Table I. The entire gaivannealing process on the steel strip (0.7-mm thickness) was carried out in the continuous galvanizing and gaivannealing line at Tata Steel. The bath temperature was maintained at 460 [degrees]C. The dissolved aluminum content of the zinc bath was kept at a constant level of 0.134 wt pct. The important industrial parameters for the gaivannealing process are given in Table II. The residence time of the steel strip in the heating zone of the galvannealing furnace was 12 seconds.

The GIXRD study of the galvannealed coating was carried out using an ARL X'tra X-ray diffractometer made by ThermoelecIron Corporation (Waltham, MA). For this purpose, a small piece of galvannealed coated steel having dimensions 3 cm x 3 cm was used. To minimize the depth of penetration of the X-ray beam within the sample, the parallel incidence X-ray beam was kept at an angle of 1 deg with respect to the coating surface. The scan rate of the XRD measurements was maintained at 3 deg/min. The GIXRD data were first collected from the top surface of the coating. For analyzing the subsurface layers of the coated sample, it was treated with 5 vol pet HNO^sub 3^ in distilled water solution for 30 seconds before GIXRD study.[12] For comparison purposes, GIXRD was also carried out over the substrate steel after removing the entire coating, using the same experimental conditions. The measured intensity vs angle (20) plots were indexed by matching the different peaks with the standard data obtained from International Centre for Diffraction Data (ICDD), 2005 edition.

The crystallographic texture of the coating surface was determined using a PaNalytical X'pert PRO XRD machine with a texture goniometer. For this purpose (143), (054), (330), (241), and (249) pole figures for galvannealed coating were determined from which orientation distribution functions (ODFs) were calculated by the method of Bunge[24] using Labotex software.

For cross-sectional scanning electron microscopic study, a sample (1 cm x 0.5cm) was cut from the galvannealed sheet. During polishing, there was a distinct possibility of the edges of the coating falling off the substrate or getting damaged. To prevent this, a 200-[mu]m thin copper strip was used as the supporting material. Then, the entire assembly was polished using 0.1-[mu]m fine diamond paste, and further etched in an etchant made up of a mixture of 1 pet picric acid in amyl alcohol and 1 pet HNO^sub 3^ in amyl alcohol along with a few drops of HF. An SEM study was carried out using an scanning electron microscope operated at 20 kV (FEI, Hillsboro, OR, Model No. Quanta-200).

The quantitative depth profiling (QDP) was carried out using a LECO* GDS-850A GDOES. For this purpose, a small piece of galvannealed coated sample having dimensions 5 cm x 5 cm was cut out and cleaned thoroughly using acetone followed by ethanol and then placed in the sample holder.

* LECO is a trademark of LECO Corporation, St. Joseph. MI.

An attempt was made to confirm the GDOES results by means of anodic dissolution study, carried out using an EIS-300, Gamry Instruments (Warminster, PA), DC 105. The electrolyte contained 250 g/L NaCl and 50 g/L ZnSO^sub 4^. The pH of the electrolyte was 4. During the preceding study, a current density of 0.5 mA/cm^sup 2^ was maintained.[25,26]

The percentage thicknesses of the different Fe-Zn intermetallic phases were determined by superimposing the compositional ranges for the different phases obtained from the standard Fe-Zn phase diagram on the preceding QDP-GDOES profiles.

The Fe content of the coating was determined by the ASTM standard gravimetric method (A90/A 90M-01).[27] For this purpose, a small piece of galvannealed coated sample was cut using a low-speed isomet diamond cutter. The edges of the test piece were covered using lacquer so that, during exposure of the sample in acidic solution, Fe should not come out from the edges. The total coated surface area and weight of the sample were measured before exposure into the acidic solution. The test piece was then immersed into a 50 vol pct HCl solution in distilled water containing a 0.2 vol pct Baychem inhibitor. During reaction, large bubble formation took place due to the evolution of H2 gas. The sample was taken out from the solution immediately when the bubble formation stopped. In this method, it was expected that only the Fe-Zn intermetallic compounds would be dissolved into the acidic solution. The weight of the sample after chemical exposure was measured. The Fe content of the solution was determined using an inductive coupled plasma spectroscope, Spectro Analytical Instrument, GmbH version 2.0e/8/88 (Kleve, Germany). The amount of Fe so determined was the actual Fe content in the coating. The Fe content of the coating was also determined from the QDP- GDOES profiles. The details of the calculation are reported in the Appendix.

A cross-sectional TEM study was carried out using a JEOL** 2000FX transmission electron microscope with an operating voltage of 160 kV. The sample for the TEM study was prepared by mechanical polishing followed by dimpling and the ion milling route.

**JEOL is a trademark of Japan Electron Optics Ltd., Tokyo.

Because the Fe-Zn reaction is expected to depend on the orientations of grains and the grain boundary characteristics of the substrate steel, detailed characterization of the substrate was also carried out. For studying the texture of the substrate steel, the galvannealed coating was removed by exposing the coating to a 50 vol pct dilute HCl solution in distilled water followed by mild polishing. After that, electropolishing was carried out in a mixture of 80 pct acetic acid and 20 pct perchloric acid solution with platinum electrode at 20 V. The electropolishing was carried out for a period of approximately 50 seconds when a mirror polished surface was obtained. The substrate steel texture was determined using an OIM camera attached to an FEI, Quanta-200 scanning electron microscope operated at 20 kV. Using a step size of 1.5 [mu]m, a number of different areas were selected for texture measurement. The data were then analyzed by the TSL-OIM software to construct the Phi^sub 2^ = 45 deg section (Bunge notation) of the ODF. To measure the formability of the galvannealed coating, a Double Olsen test was carried out. In this test, a 10 cm x 10 cm galvannealed coated steel sample was first washed with petroleum ether. The weight of the sample was measured before testing. After that, a hardened steel ball was punched onto the top surface of the coating to produce a hemispherical cup. The sample surface was then reversed, and the same steel ball was again punched onto the top of the hemispherical cup. After thorough cleaning by petroleum ether, the weight of the sample was measured, and the weight loss was calculated. The average of three such tests has been reported as the weight loss.


Figure 1 shows the GIXRD plots obtained from the substrate steel, the top surface of the coating, and after partial removal of material from the coated layer by chemical treatment. Clearly, the major part of the coating is made up of the delta phase, although the presence of some zeta phase can also be seen.

The cross-sectional scanning electron micrograph of the coated sample after etching is shown in Figure 2. The different phases in the coated layer have been marked on the basis of the EDS results.

Quantitative depth profiling of the galvannealed coating was also carried out using GDOES (Figure 3). The percentage thicknesses of the different Fe-Zn intermetallic phases were determined by superimposing the compositional ranges of the phases (Table III) obtained from the standard Fe-Zn binary phase diagram (Figure 4) on the preceding profile, and the summary of the results is reported in Table IV. The GDOES results clearly indicate that the major phase present on the top surface of the coating is delta. There is some zeta phase also present here, and this has been indicated by the GIXRD results also. The anodic dissolution results shown in Figure 5 clearly corroborate the GIXRD and GDOES findings.

The Fe content of the coating is one of the parameters that control the formability of the galvannealed coating. The coating Fe content was determined by the wet chemical method and from the QDP- GDOES profiles. Table V compares the coating Fe content obtained by two different methods. In the wet chemical method, the coated sample is immersed in an acidic solution for dissolving the entire galvannealed coating. In this case, there is a chance of arriving at an excess Fe content value in the coating due to the possibility of dissolution of at least some of the substrate steel in the acidic solution. On the other hand, it is also possible that the entire coating may not be dissolved in the acidic solution, and in that case, less Fe content is obtained in the coating. The accuracy of the Fe content, determined by the wet chemical method, therefore suffers from the preceding limitations. The GDOES results are expected to be reasonably accurate, where the analysis is carried out in the atomic layer. Because the coating layer during galvannealing is made up of a number of Fe-Zn intermetallics and because the Gamma layer lies just above the substrate steel, the coating layer thickness has been taken to extend right up to the Gamma layer. In other words, the entire thickness extending from the top of the coating to the layer containing 27 wt pet of Fe (the highest amount of Fe that the Gamma layer may contain) in the GDOES plot was considered for the purpose of measurement of the coating Fe content. The necessary calculations for determining the Fe content of the galvannealed coating from QDP-GDOES profiles are shown in the Appendix.

Figure 6 shows a typical cross-sectional TEM micrograph of the galvannealed coating. The circled region in part (a) has been further magnified in (b) of the figure. Selected area diffraction patterns (SADPs) were taken from the different regions of the coating, marked by arrows. The SADPs were indexed and the phases were identified after comparing with the standard ICDD. Clearly, the a phase is present at the top part of the coating, below which the Gamma and Gamma^sub 1^ phases form.

Figure 7 represents the Phi^sub 2^ = 45 deg ODF section of the substrate steel. A reasonably strong gamma (gamma) fiber can be detected in this figure. The gamma fiber is not very uniform in nature and shows maxima at {111}{112} locations. The sharp gamma fiber indicates satisfactory formability of the substrate steel. The grain boundary character distribution (GBCD) of the substrate steel is shown in Figure 8. This figure shows the presence of a nearly equal number of fractions of low-angle ( 15 deg) grain boundaries. The coincidence site lattice number fraction is rather small (0.08). The {111}, [113}, {313}, {001}, and {101} planes of the substrate steel are known to control the growth rate of the zeta phase.[30,31] The color-coded map of crystal planes of the substrate steel along with the densities of the preceding planes parallel to the rolling planes is displayed in Figure 9. It has been found that the higher the fraction of {111}-{113}-{3I3} planes on the surface of the steel, the higher is the amount of pillarlike ordered C crystals formed, which have lower growth rate. On the other hand, randomly oriented zeta crystals with higher growth rate are formed on {001}-{101} substrate planes.[30] Figure 9 clearly indicates that in this material the density of the former type of planes is much higher than the latter type, and this ensures that zeta crystals with lower growth rate only can form. In this case, these crystals can be easily converted into the favorable delta phase during the short galvannealing time.

The Phi^sub 2^ sections of the ODF of the coated layer (Figure 10) clearly indicate the presence of a reasonably strong fiber of the type {01.3} [left angle bracket]uvw[right angle bracket]. The fiber is not very uniform in intensity, as can be seen from a magnified view of the Phi^sub 2^ = 0 deg section (Figure 11). In the hexagonal delta phase, the primary slip planes are the basal (0002) planes. If the basal planes of delta crystals are aligned in such a way that under the applied stress a significant amount of resolved shear stress exists on these planes, then plastic flow will be facilitated.[31] The {01.3} planes make an angle ~60 deg with the basal planes {00.2}. Therefore, in these galvannealed samples, the basal planes are inclined at -60 deg with respect to the sheet planes. Shaffer et al.[32] observed that in electrogalvanized materials no cracking takes place when the basal plane makes an angle between 40 and 70 deg with respect to the substrate surface.

As mentioned earlier, a Double Olsen test was carried out to determine the formability characteristics of the galvannealed coating. It essentially measures the powdering resistance of the coating. The weight losses were measured before and after each test, and the value of the weight loss (from the average of three tests) was found to be 5.8 mg. This value is comparable to the value obtained from other steels of similar chemistry and is definitely on the lower side.[33] These results, coupled with the textural results on the coated layer, clearly demonstrate that, as far as the coating is concerned, its formability is quite satisfactory.

The textural results show that the substrate steel has a strong {111} fiber texture (Figure 7), and near perfection of the texture can be visualized from the rather high number fraction (-0.45) of low angle boundaries present (Figure 8). These will ensure a high level of formability for the substrate steel.

X-ray and TEM studies (Figures 1 and 6) have revealed that most of the coating layer is made up primarily of delta crystals. A galvannealed coating can fracture or get detached from the substrate during the sheet forming operation due to the presence of hard, brittle, and ordered Fe-Zn intermetallic phases, including the delta. The brittleness of these phases can further increase with the increase of the Fe content of the coating.[6] The average Fe content of the coating lies within the range 11 to 12 wt pet (Table V), and this is not substantially higher than what the most favorable delta phase contains, namely, 9.35 wt pet (average). It therefore appears that the Fe content of the coating is not high enough to jeopardize the otherwise good formability of the coating.

Careful analysis of the coated layer by GDOES and by anodic dissolution studies (Figures 3 and 5) has shown the presence of the zeta phase in this material. The OIM mapping (Figure 9) studies have indicated that these C crystals have a lower growth rate, which helps in converting them easily into the favorable delta phase during the short time of galvannealing.

Thus, the results obtained from the structural and textural characterization of the substrate and the coated layer clearly indicate that the material under consideration will have a high degree of formability.


The key findings of this investigation can be summarized as follows. 1. The coating contained mostly the delta phase at the top.

2. The texture of the substrate steel showed a sharp y fiber, favorable for imparting good deep drawability.

3. The {01.3} planes of delta phase in the coating are parallel to the substrate steel surface. Such alignment of the (0.13} planes of delta is beneficial for the coating formability.

4. Overall, the substrate steel, along with the coated layer, will have a high degree of formability.


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A. CHAKRABORTY, Researcher, and R.K. RAY. Visiting Scienlisl, are with the Research and Development Division. Tata Steel. Jamshedpur, PIN-831 007. India. Contact e-mail: [email protected] yahoo.com S. SANGAL, Professor, is with the Indian Institute of Technology, Kanpur PIN- 208 016, India.

Manuscript submitted October 30, 2007.

Article published online July 19, 2008


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