Solid solution and precipitation behavior of Nb in Al-bearing ferritic steels

With the rapid development of the global economy, problems in energy production and environmental protection are becoming severe. To reduce fuel consumption and CO₂ emissions, it is essential to reduce the weight of automobiles and other huge construction structures. Recently, a number of studies have been conducted on the use of low-density steels for automobile applications by incorporating aluminum in steel. The light elements can increase the lattice constant of steel while reducing the density of steel to achieve a lower atomic weight. Aluminum as a light element replaces the iron atoms in the unit cell, increasing the volume while reducing the weight, thereby reducing the density of steels. In this regard, ferritic Fe-8%Al steels indicated a 10% reduction in density compared with the conventional steels. To clarify the solid solution and precipitation behavior of Nb in Al-bearing ferritic steels, heat treatment tests were carried out under a series of temperature. The precipitates of NbC and the dissolved Nb solute in ferrite matrix with high Al content were studied using electrolytic dissolution technique, X-ray diffraction technique, and inductively coupled plasma-atomic emission spectrometry (ICP-AES). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were also applied. The experimental results show that the precipitates are NbC and also some Al₂O₃ and AlN inclusions. It is also found that with increase in the solution temperatures, the NbC in the as-cast samples becomes fewer and the coarsening behavior occurs. Moreover, when the temperature was over 1100℃, almost all the precipitates were dissolved. Furthermore, using Thermo-Calc software, the thermodynamic properties of Nb and relevant compounds were studied, and the interaction coefficient between Al an Nb was calculated. The results indicate that Al decreases the activity of Nb, and the solubility of NbC increases. Finally, the solid solubility formula of NbC was deduced, which can provide a basis for further application of ferritic steels with a high Al content.