ZINC/IRON PHASE TRANSFORMATION STUDIES ON GALVANNEALED STEEL COATINGS BY X-RAY DIFFRACTION

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1 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume ZINC/IRON PHASE TRANSFORMATION STUDIES ON GALVANNEALED STEEL COATINGS BY X-RAY DIFFRACTION S. Wienströer 1, M. Fransen 2, H. Mittelstädt 1, C. Nazikkol 1, M. Völker 1 1) ThyssenKrupp Stahl AG, Kaiser-Wilhelm-Str. 100, Duisburg, Germany 2) Philips Analytical B.V., Lelyweg 1, EA 7602 Almelo, The Netherlands ABSTRACT X-ray diffraction (XRD) is an analytical method which can be used for the characterisation of crystalline steel sheet coatings. A novel X-ray detection system enables an extreme reduction of measuring time, which allows XRD techniques to be an alternative fast analytical method for surface investigations. In combination with a heating chamber, fast surface phase transformations on metallic specimens can be investigated. The results of a zinc/iron (Zn/Fe) transformation study on galvannealed steel sheets are described in detail. The requirements for using XRD as an online process control tool for this material are discussed. INTRODUCTION The properties of flat rolled steel can intentionally be influenced by surface treatment. The coating of flat rolled steel sheets is one of the most applied methods of surface treatment. The analytical determination of these coatings is important to understand the correlation between coating composition and material properties, e.g. corrosion resistance and forming quality. Annealing of galvanised steel is one of the methods for forming a high-quality coating. In this galvannealing process, cold-rolled steels are coated with zinc on both sides by a continuous hotdipping process [1-2]. Immediately as the strip exits the coating bath, the zinc coating is subjected to an in-line heat treatment that converts the coating by interdiffusion to an approximately 10 µm zinc/iron layers system. Depending on annealing temperature, annealing time, steel grade, aluminium content in the zinc pot and other parameters, different intermetallic Zn/Fe phases are formed in the coating which determine significantly the quality of the product, e.g. formability, paintability or weldability. Due to an increased demand for this material within the last years covering the production of inner and outer automobile panels, quality control techniques became a key issue for this product. X-ray diffraction is an analytical method for characterisation of crystalline phases. Its main advantage over other analytical techniques is that it can distinguish between different crystallographic forms (phases) of the same elements. A disadvantage of the technique is that it is slow and does not allow the observation of fast phase transformations. Meanwhile, however, a significant reduction of measuring time is possible by use of a novel detection system based on a multi-strip semi-conductor technique. This new technological system allows XRD to be used as a fast, non-destructive measurement technique for the determination of crystalline coatings on steel sheets. High-temperature XRD investigations of the galvannealing process will be described and compared with SEM results of cross-sectional cuts. Additionally, the roadmap towards on-line production control of galvannealed steel sheets will be described.

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

3 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume EXPERIMENTAL The basic equipment is an X`Pert PRO MPD diffractometer (Philips Analytical, The Netherlands). Measurements were carried out by the use of coupled Theta-Theta-geometry with a 240 mm radius goniometer circle. A copper x-ray tube in combination with a fixed 2 divergence slit, a fixed 4 anti-scatter slit and 15 mm mask was used. The diffracted beam optic consists of a secondary monochromator (graphite 002) and X`Celerator detector which is based on multi-strip technology. This new detector enables to lower the measuring time by a factor of approximately 100 in comparison with a normal scintillation counter to get the same signal intensity/noise ratio. Additionally, a sample spinner (2 spins/s) was used to minimise texture effects in the plane of the specimen holder. If the sample has to be heat treated during investigation the basic equipment was combined with a non-ambient chamber HTK 1200 (Anton Paar, Austria). Specimen material was generally hot-dip galvanised cold-rolled steel with a layer thickness of approximately 7.5 µm. For investigation the specimens were degreased by acetone. The measurements were run within a temperature range between 300 C and 550 C with annealing times from a few seconds up to several minutes. ZN/FE PHASE TRANSFORMATION STUDY ON GALVANNEALED STEEL SHEETS The alloy formation on a galvannealed steel sheet depends on the annealing temperature, annealing time, steel grade, aluminium content in the zinc pot and other parameters. The knowledge of all process parameters is important to control the production of the coating and to form the correct alloy layers on the surface. On a galvannealed sample s surface, three different intermetallic phases with a different Fe/Znratio can be observed by XRD. These phases are denoted with Zeta (ζ), Delta (δ) and Gamma (γ). The Zeta-phase is the layer on top of the multi-layer system. Gamma is the layer next to the steel substrate. The Delta-phase is the thick layer in between. The chemical formulas and structure data are given in Table 1 [3]. Table 1: Chemical formula and structure data of Zn/Fe phases Phase Formula Iron Space Lattice content group parameter Zeta (ζ) Delta (δ) Gamma (Γ) FeZn ,1 % C 2/m monoclinic FeZn 11 FeZn 6.67 Fe 5 Zn 21 Fe 4 Zn % P63mc hexagonal % F43m (I43m) cubic a = Å b = 7.61 Å c = 5.06 Å a b 12.8 Å c Å a = Å (a Å)

4 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume Analytical methods for determination of the properties of the galvannealed coating: Different analytical methods enable one to study the layer formation after production. Chemical and electro-chemical dissolution can be used to determine the Fe/Zn-ratio and the thickness of the multi-layer coating. The thickness of each single layer can be directly measured by scanning electron microscopy (SEM). These analytical methods are destructive and time-consuming. Another analytical method for galvannealed steel sheets which is neither destructive nor timeconsuming is X-ray fluorescence spectrometry, but it is limited to the determination of the Fe/Znratio. X-ray diffraction Each single phase can be determined by X-ray diffraction in an angle area between 72 and 80 2Theta [4]. The different signal intensities correlate with the thickness of the single phase. A typical diffraction pattern of a galvannealed steel sheet is depicted in figure 1. Figure 1: X-ray diffraction pattern of a typical Galvannealed steel sheet with ζ-, δ- and Γ-phase Single peak position and intensity depends on each Zn /Fe alloy to be characterised. Signal intensity and layer thickness is not a linear function. Mass absorption and layer geometry influence the signal intensity. Influence of heat treatment on the galvannealing process Phase formation on the surface depends on the temperature and residence time in the annealing furnace. In figure 2, a SEM study of cross cuts as a function of increasing heat treatment is shown.

5 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume Figure 2: Influence of heat treatment on phase transformation At the beginning there is only a zinc layer on the steel substrate. When heat treatment starts, the iron diffuses into the zinc layer and vice versa. At first, the multi-layer consists of a lot of Zetaand Delta-phase material. Additionally, a very thin Gamma-layer is visible at the interface. During heat treatment, the amount of Zeta-phase decreases and the amount of Gamma-phase increases. The multi-layer coating on the surface transforms again into a double-layer system which consists only of Delta and Gamma. The product at the end of heat treatment is overalloyed and a thick layer of Gamma-phase (approximately 2 µm) can be observed at the steel interface. During the forming process, the use of this material can possibly lead to powdering. Transformation study With an X`Pert PRO MPD system with X`Celerator and oven chamber, we observed the formation of the multi-layer in situ. For this measurement, fast symmetrical scans with a duration of only 51 seconds were repeated at a temperature of 370 C over a small angular range in which all the phases gave a reflection. A diffracted beam monochromator was used with the X`Celerator in order to suppress secondary sample fluorescence. The 3D-view diffraction pattern of the investigation of a sample with low aluminum content is shown in figure 3. The transformation of zinc and iron into the Zn/Fe alloys can be observed in situ. During this time the zinc signal decreases. Before the zinc layer is completely transformed, a Zeta-phase is observed. Two minutes later the Delta-phase is also formed. After approximately 20 min annealing time the amount of Zeta-phase reaches a maximum and decreases afterwards. During the decrease of the amount of Zeta-phase, the amount of Delta-phase increases. After about 35 min a Gamma-phase is also observed.

6 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume 46. Figure 3: Phase transformation of galvannealed (0.12 % Al) at constant temperature of 370 C Additionally, a sample with high aluminum content was also investigated by XRD. For this analysis, the temperature of the sample was rapidly increased from room temperature to 300 C (heating rate 60 K/min). At 300 C the heating rate was lowered (5 K/min) and the sample was heated and scanned alternately up to 550 C. Every 10 C-step the temperature was held constant and a short symmetrical scan with a duration of 51 seconds was made. A bird s eye view intensity diagram as a function of the temperature is shown in figure 4. Figure 4: Phase transformation of galvannealed (0.32 % Al) between 300 C and 550 C 295

7 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume The Zn/Fe transformation of the sample with high aluminum content at increasing temperature differs from transformation of the sample with low aluminum content at constant temperature. Approximately at 400 C, the zinc layer melts, and during this process the Delta-phase is directly formed on the surface. The formation of a Zeta-phase is more or less suppressed. Between 400 C and 470 C only the Delta-phase can be observed. At higher temperatures the Delta-phase transforms completely into a Gamma-phase. At the end of the observation only the Gamma-phase remains on the surface. All Zn/Fe alloys at the galvannealing process can be identified precisely by the use of X-ray diffraction. The measured data correlate with the expected theoretical phase transformation and the results of SEM studies. QUANTITATIVE INVESTIGATION OF ZINC/IRON ALLOYS To quantify the thickness of each layer on the galvannealed steel surface (multi-layer system), the absorption of upper layers has to be considered. Reference data of the SEM study and X-ray diffraction are combined with the data of the investigated steel. Also self-absorption of the determined layer has to be taken into account. An approximation of the layers thickness has to be made by comparison of the intensities I and layer thickness d of the reference specimen. In further studies this approximation has to be improved regarding layer roughness, crystal orientation, etc.. With the help of empirical data from a SEM-characterised reference sample data set, the calculation of correction coefficients of each alloy is in process. CONCLUSION Zinc/Iron transformation can be observed in situ during heat treatment by use of a novel detector system in combination with a non-ambient chamber. The results of the investigation correlate with the expected data of the galvannealing process. The process parameters, such as annealing temperature or aluminum content, influence the alloy formation. Problems of quantitative determination up to now: - Suitable SEM-characterized reference sample data set - The developed mathematical model (intensity I as a function of thickness d ) has to be improved regarding layer roughness, crystal orientation, etc. The quantitative determination of zinc/iron alloys is the basis for the development of an online-detection system in a manufacturer s facility. REFERENCES [1] J. Mackowiak, N. R. Short; International Metals Reviews, Review 237, No. 1, (1979) p 1-19 [2] D. C. Cook, R. S. Tuszynski, H. E. Townsend, Hyperfine Interactions, 54, (1990) p [3] International Centre for Diffraction Data, PDF-2 data base, , , , , , , , (JCPDS-ICDD 2000) [4] K. Chohata, M. Saito, T. Kittaka, T. Nagatani, Y. Hirose; System for making an on-line determination of degree of alloying in Galvannealed steels sheets, European patent, ,