The proposed methodology consists of five steps:l.

It is normally accepted that for hot-dip galvannealed coatings best properties are obtained for a coating iron content between 10-11 mass%. In a series of works [1][2][3][4][5][6][7][8][9][10] both the isothermal and non-isothermal kinetics of iron enrichment of the zinc coating have been quantitatively modeled taking into account factors such as coating mass and aluminum content. That modeling was combined with the time-temperature path followed by the moving sheet during the galvannealing cycle. This time-temperature path depends on the line velocity and the galvannealing furnace. An example of a time-temperature path is given in Fig. 1 obtained by passing a sheet through a galvannealing furnace with a velocity of 1 m/s. This combination of the kinetic modeling with the time-temperature path produced the "processing windows" proposed by Lopes et al. 10)Those previous papers focused exclusively on hot-dip zinc coated sheets in which an interstitial free, IF, steel was the substrate. However it is well-known 11) that the steel substrate has a profound effect on the subsequent kinetics of iron enrichment of the zinc coating: low carbon substrates result in a considerably slower iron enrichment kinetics when compared with IF steel substrates. This fact has in itself not only a fundamental but also a significant practical importance.In this work the effect of the steel substrate on the kinetics of iron enrichment and on the processing window of hot-dip galvannealed coatings on steel sheets is investigated.Two hot-dip galvanized steel sheets were used. Both were produced in zinc baths with similar Al content, 0.20 mass% (nominal) and similar coating weight, 80 g/m 2 (nominal). On one sheet the substrate was a Ti-IF steel and on the other a low carbon steel. 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 100ϫ 100ϫ0.85 mm were taken from the same side of each sheet and 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 specimens 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. Figure 2 shows a comparison between the isothermal kinetics of iron enrichment of the IF and low carbon steel zinc coated sheets. The data are plotted as a time, temperature, transformation, TTT, curve. The time necessary to reach a coating iron content of 11 mass% at a given temperature is plotted. The difference in kinetics is quite substantial: the kinetics of the iron enrichment of the low carbon sheet was much slower than that of the IF sheet. It is wor...

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 1-12) 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" [9][10][11][12] 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. [9][10][11][12] 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.12) Both were produced in zinc baths with similar Al content, 0.20 mass% (nominal) and similar coating weight, 80 g/m 2 (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...

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