Niobium (Nb) is a common component of microalloyed steels, especially ferritic and bainitic steels. It serves three main purposes: condition austenite grains, enhance steel properties via grain refinement and fine precipitate formation, and facilitate the decomposition of austenite to other phases, especially during cooling processes. Generally, niobium exists as a solution in austenite or carbonitride before phase transformation. There is a growing body of knowledge to provide a compressive understanding of the role of Nb in different microalloyed steels.
The state of niobium in austenite before phase transformation varies in different types of steels, making its influence a complex phenomenon. Although the role of niobium in both ferritic and bainitic steels has been extensively studied, there are still conflicting results that require further clarification. The contradiction mainly revolves around the influence of Nb on the strengthening mechanism and phase transformation kinetics in ferritic and bainitic steels, which are relatively complex and different. The complexity can be attributed to two main reasons: different transformation mechanisms and different niobium states between the two types of steels. Thus, more research is necessary to clarify these contradictions by comparing the role of niobium in different microalloyed steels.
In their paper published in the research journal, Materials Science and Engineering A, Dr. Feng Cai, Dr. Mingxing Zhou and Dr. Junyu Tian led by Professor Guang Xu from Wuhan University of Science and Technology conducted a detailed comparison of the role of niobium in ferritic and bainitic steels. Specifically, they studied its influence on the microstructure, transformation and mechanical properties of both ferritic and bainitic steels using different techniques, including dilatometry, optical microscopy and field emission scanning electron microscopy. This experiment was mainly carried out during the continuous cooling process and involved three different low-carbon steels with varying niobium contents.
The authors showed that the effects of niobium on the phase transformation kinetics of both the ferritic and bainitic steels were non-monotonic. In both cases, an increase in Nb content initially inhibited the transformation kinetics before accelerating it. This phenomenon was attributed to the stiff competition between two opposing factors: (1) the solute niobium decreased the carbon diffusion ability to inhibit the growth of ferrite and bainite, and (2) increasing the amount of niobium increased the nucleation rate due to the effects of refined prior austenite grains and formation of precipitates containing niobium.
However, the influence of niobium addition on the transformations of ferritic and bainitic steels was completely different. Addition of 0.046 wt.% Nb inhibited the bainitic transformation kinetics but significantly accelerated the ferritic transformation because the solute Nb in austenite before the ferritic transformation was less than that before the bainitic transformation. Other effects of adding niobium amount on the two types of steels included an initial increase in the strength before a little change, a slight reduction in the elongation and a reduction in the work hardening capacity. Furthermore, the two types of steel exhibited different strengthening mechanisms.
In a nutshell, Professor Guang Xu and co-workers studied the role of niobium in low-carbon ferritic and bainitic steels. The primary mechanism for increased strength in ferritic and bainitic steels was the precipitation and dislocation strengthening, respectively. Additionally, phase transformation strengthening was also dominant in the bainitic steels. In a statement to Advances in Engineering, Professor Guang Xu explained that their findings provide a better understanding of the metallurgical mechanism of niobium and its role in steels.
Cai, F., Zhou, M., Tian, J., & Xu, G. (2022). Comparative study of the role of niobium in low-carbon ferritic and Bainitic Steels. Materials Science and Engineering: A, 851, 143579.