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In order to produce galvannealead coatings, the hot-dip galvanized sheet enters a galvannealing furnace as soon as it emerges from the zinc bath. This furnace normally comprises three stages: a heating stage, a soaking stage and a cooling stage. The length of each stage varies from plant to plant and depend on several factors such as heating and cooling method and sometimes on the space available on the galvanizing line. As is well known the main objective of such furnace is to deliver an alloyed coating with a coating iron content between 10–11 mass%, which is normally accepted as the optimum iron content for best properties. In previous works a quantitative study was carried out to describe the isothermal kinetics of iron enrichment of the Fe–Zn coating. Furthermore, in industrial galvannealing practice, the zinc coating is subjected to a nonisothermal heating cycle. In those works it was shown how one can predict the nonisothermal kinetics of iron enrichment from the isothermal kinetics which can be fairly easily measured in the laboratory. More recently the concept of “processing window” for the galvannealing process has been introduced. A processing window is essentially an area in a soaking temperature vs. line velocity plot within which the galvanealing cycle will result in a coating iron content between 10 and 11 mass%. Each particular galvannealing furnace will have its processing window. In those works it was shown how to calculate “processing windows” for the galvannealing process and the effect of Al content of the Zn bath, of the furnace characteristics and of the steel substrate were examined. Finally, the effect of small changes in processing variables in the controlling parameters was discussed with reference to the processing windows. Details can be found in those papers. Those previous papers essentially focused on the “direct” problem, namely, for a given furnace and galvanized coating what would be the adequate processing window. However it might be of some use to study also the inverse problem. For a certain processing condition and galvanized sheet it might be interesting to determine the furnace configuration that could be successfully used to achieve the goal of producing a coating iron content between 10–11 mass%. In this work the determination of the furnace configuration, more specifically of the length of each stage of the galvannealing furnace as a function of the kinetics of iron enrichment of hot-dip galvanized coatings is investigated. The two hot-dip galvanized steel sheets used in this work were the same used in a previous work. Both were produced in zinc baths with similar Al content, 0.20 mass% (nominal) and similar coating weight, 80 g/m (nominal). On one sheet the substrate was a Ti–IF steel and on the other a low carbon steel. In what follows the former will be referred to as the IF steels sheet whereas the latter will be referred to as the low carbon steel sheet. The substrate chemical analysis were (in mass%): C 0.0035; Mn 0.14; P 0.01; S 0.007; Si 0.006; Ti 0.07; N 0.003; Al 0.05; Fe balance and C 0.04; Mn 0.15; P 0.01; S 0.01; Si 0.003; N 0.004; Al 0.04; Fe balance, respectively. Specimens measuring 100 100 0.85 mm were taken from the same side of each sheet and isothermally annealed in salt bath at 450, 475, 500, 525 and 550°C for holding times ranging from 5–120 s and water quenched (cooling rate about 90°C/s). The heating rate was about 40°C/s and the annealing times were measured from the instant the specimen reached the required temperature. From the center of the specimen disks with 60 mm in diameter were taken for the determination of iron content. This was done separately on each side of the disk using a sulfuric acid solution to dissolve the coating. The isothermal experiments described above were the only experimental data generated in this work. In what follows calculations were made using the results from those experiments. These calculations employed the same methodology described in a previous work that is repeated below in condensed form for convenience: (1) First, the isothermal iron enrichment kinetics of the IF and low carbon steel sheets are obtained. This was done by isothermal annealing of the samples within the temperature interval of 450–550°C for times ranging from 0–120 s and analyzing the resulting iron content in each case. (2) The data obtained in step (1) are fitted by a simple isothermal model: W W0 (WS W0)(1 exp( kt)) (1) where W is the coating iron content, WS is a saturation coating iron content and W0 is the initial coating iron content, all in mass%. From such a fitting k and WS were obtained and are summarized in Table 1 of Ref. 12). (3) The third step consists in defining at least approximately the temperature vs. time cycle undergone by the galvanized sheet within the galvannealing furnace. For the purposes of the present study the temperature profile experienced by a galvanized sheet inside a galvannealing furnace can be represented as in Fig. 1. It can be seen that this temperature profile consists of three distinct stages. In the heating stage the temperature is increased from the galvanizing bath temperature up to the soaking temperature. In the soaking stage the temperature is kept constant and finally during the cooling stage the temperature is fairly quickly decreased. The temperature at which the sheet enters the galvannealing furnace was taken in this study to be equal to 450°C. A soaking temperature equal to 500°C, typical for galvannealing processing was chosen in this study. The cooling rate was also kept fixed in this study and equal to 10°C/s that is representative of forced air cooling. This furISIJ International, Vol. 42 (2002), No. 7, pp. 803–805

Determination of the Heating and Soaking Stage Length of a Galvannealing Furnace.