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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 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. 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 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 (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 1003 10030.85 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 worthy of note that in the low carbon steel sheet at 450°C after 120 s the coating iron content was still about 5 mass% and of course is not represented in Fig. 2. Another interesting point is that for the IF sheet one finds a decrease in transformation time as the transformation temperature is increased. On the other hand the low carbon sheet behavior is ISIJ International, Vol. 40 (2000), No. 10, pp. 1048–1050

Effect of the Steel Substrate on the Kinetics of Iron Enrichment and on the Processing Variables of Hot-dip Galvannealed Coatings on Steel Sheets.