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

I. INTRODUCTION

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.

II. EXPERIMENTAL PROCEDURE

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.