005 Results Fig. 1 shows the microstructures obtained for the three steels after the reheating treatment at 1250ºC after etching in a picric based solution. In the case of the C2Mn2 and C2Mn2Al1 steels the etching process reveals that after soaking fully austenitic microstructures were obtained, with average grain sizes of 69±4 μm and 100±3 μm respectively. In the case of the C2Mn2Al2 steel, conversely, Fig. 1c) shows that two different constituents with very different grain sizes were present in the microstructure. After etching with a solution of 2%HNO3 in ethanol the coarse grains were identified as martensite, the product from the quenched austenite, and the smallest grains as ferrite. The results denote thus that after soaking at 1250ºC, the C2Mn2Al2 steel presents a duplex microstructure with an average austenite grain size of 65±2 μm and a low ferrite fraction. These results are in good agreement with the general trend attributed to Al, which is known to significantly raise the A3 temperature [2, 3]. Calculations performed with the Thermo-Calc software also corroborate this trend. It must be mentioned that the amount of ferrite present in this specimen (fa~2%) was considered too low to have a significant effect on the mechanical softening values. Fig. 1. Microstructures obtained after soaking for the a) C2Mn2, b) C2Mn2Al1 and c) C2Mn2Al2 steels. Fig. 2 shows the fractional softening experimental data obtained for the three steels studied at different deformation temperatures and pass-strains. In all cases the experimental data could be fitted to an Avrami type equation of the form: (1) where X is the fractional softening corresponding to a time t, t0. 5 is the time to reach a 50% fractional softening and n is the Avrami exponent. As usually reported, in all cases increasing the pass-strain or the deformation temperature leads to faster softening kinetics. From the figures it is also interesting to note that except for the case of the C2Mn2Al2 steel the Avrami exponent is approximately constant, with a calculated value of n~0. 8, and it is not affected by pass-strain or deformation temperature variation. As shown in Fig. 2c), only in the case of the C2Mn2Al2 steel did decreasing deformation temperature lead to a reduction of the Avrami exponent, from n=0. 73 for a deformation temperature of 1100ºC to n=0. 32 at 950ºC. Fig. 2. Effect of deformation temperature (a, b and c) and of pass-strain (d and e) on the softening curves obtained for the steels studied in this work. Fig. 3 shows the softening curves determined for the C2Mn2 and the C2Mn2Al steels at the same deformation conditions. Although the curves corresponding to the C2Mn2 and C2Mn2Al1 steels are not directly comparable due to the difference in the initial austenite grain sizes, comparison between those corresponding to the C2Mn2 and C2Mn2Al2 steels indicates that Al addition leads to a significant retardation in softening kinetics. Moreover, it is evident from inspection of Fig. 3c) that the retardation is enhanced in the case of the 2%Al steel at the lowest temperature of 950ºC. As already mentioned, in this latter case the n Avrami exponent of the fitting curve of the C2Mn2Al2 steel is significantly lower than the value obtained in the rest of the cases. Fig. 3. Effect of steel composition on the softening curves obtained for the steels studied in this work. Discussion In order to study the effect of Al addition, a normalised softening time, t0. 5 (), which excludes the effect of deformation conditions and initial grain size was calculated for all the tests, defined as follows [] A. I. Fernández, P. Uranga, B. López and J. M. Rodriguez-Ibabe: ISIJ Int. Vol. 40 (2000).
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7 Table 3. SRP values calculated for common microalloying elements [10]. Nb in… SRP Mo in.. SRP Nb steel 222 Mo steel 20 Nb-Mo steel 265 Mo-Nb steel 20 Nb-V steel 275 Mo-Nb-V steel 21 V in… Ti in… V steel 13 Ti steel 83 V-Nb steel 25 Summary The effect of Al addition on the static softening behavior of C-Mn steels has been characterized. It has been found that Al leads to a retardation of the static recrystallization kinetics in C-Mn steels. Its effect is equivalent to that exerted by 0. 026%Nb for 1%Al addition and by 0. 05%Nb for 2%Al addition. It has also been shown that for the highest Al level, 2%, and the lowest deformation temperatures, 1000 and 950ºC, austenite to ferrite phase transformation occurred concurrently with mechanical softening, leading to a significant retardation of the softening kinetics. Acknowledgments The authors acknowledge economical support from the European Union, Research Programme of the Research Fund for Coal and Steel (RFSR-CT-2009-00011). References.
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