Softening Kinetics in High Al-Nb Microalloyed Steels

Abstract:

The combined effect of Al and Nb additions on the static softening behavior of C-Mn steels was investigated. The compositions of the steels studied in this work are representative of the recently developed TRIP-assisted steels: a base composition of 0.2wt.%C, 2wt.%Mn, 50ppmN, three different Al levels, 0.03 (base steel), 1 and 2wt.%, and two Nb contents of 0.03 and 0.07wt.%. Double-hit torsion tests were performed at different deformation temperatures (925-1065oC) and pass-strains (=0.2 and 0.35). It was found that solute Al produced a significant retardation on the static softening kinetics, this effect being enhanced by the addition of Nb. Additionally, below 1000oC the addition of 2 wt.%Al promotes the γα phase transformation to occur concurrently with softening. For the Nb microalloyed steels strain induced precipitation also occurred, resulting in a complex interaction between softening, phase transformation and strain induced precipitation.

• Periodical: Materials Science Forum (Volume 762)
• Pages: 140-145
• DOI: https://doi.org/10.4028/www.scientific.net/MSF.762.140
• Online since: July 2013
• Authors: Z. Aretxabaleta, Beatriz Pereda, S.V. Parker, Beatriz López
• Keywords: Aluminium Alloying, Nb Strain Induced Precipitation, Static Recrystallization, TRIP Steel

References

[4] where t0.5 is the normalized recrystallization time of the Al alloyed steels (C2Mn2Al1, C2Mn2Al2 and C2Mn2Al2Nb3) and t0.5REF is the normalized time calculated for the reference steels (i.e. C2Mn2 for the first two steels and C2Mn2Al1Nb3 for the other steel). An average value of SRP = 6 was obtained for Al. This value is significantly lower than those reported in the literature for common microalloying elements such as Nb (SRP=222) [[] J. Jonas: High Strength Low Alloy Steels Proc., D.P. Dunne and T. Chandra eds., Wollongong University Press (1984), 80. ]. However, it must be noticed that the amount of Al considered in this work is nearly one order of magnitude higher than the Nb contents in the range 0.02-0.07 wt.% typically found in microalloyed steels. The retardation exerted by 1 wt.% and 2 wt.%Al is equivalent to that exerted by 0.027 wt.% and 0.054 wt.%Nb, respectively. Taking into account the above results the equation proposed by Fernandez et al. [10] for the time to reach 50% recrystallization was implemented to include the effect of Al in solid solution as follows:

[5] where [Nb], [Ti] and [Al] represent the wt.% concentrations of dissolved Nb, Ti, and Al, respectively. Fig.5 illustrates the good correlation between experimental data and the values calculated using Eq.(5). Fig. 4. Normalised t0.5 for the different steels. Fig. 5. Comparison between t0.5 experimental data and the predictions of Eq.(5). Summary and Conclusions It has been found that the addition of high amounts of Al produces a considerable delay in softening kinetics, this effect being enhanced by further addition of Nb. At temperatures above 1000ºC this retardation is attributed to a solute drag effect of both Al and Nb, whereas at lower temperatures in addition to this solute drag effect, strain induced precipitation of niobium and concurrent g®a phase transformation in the case of 2 wt.%Al additions are also contributing. The effect of Al in solid solution was quantified by the SRP parameter and a pre-existing equation for determining the recrystallization time, t0.5, was modified to take the Al effect into account. Acknowledgment The authors acknowledge financial support from the European Union, Research Programme of the Research Fund for Coal and Steel (RFSR-CT-2009-00011). References