Understanding the effect of aluminium on the microstructure on low level nitrogen steel

Aluminium has been used as a de-oxidant and grain refiner element for more than 100 years, however, the use of aluminium as a deliberate alloying addition in steels has attracted increased attention recently as a possible replacement for Si in Transformation Induced Plasticity (TRIP) steels. Although the effect of substitutional elements such as manganese and chromium has been investigated in detail in the last few decades, there has been little research concerned with the effect of Al as a substitutional element in steel in amounts higher than 0.1 wt%. This could be due to the previous lack of industrial interest and also technological concerns over the production of high Al-content steels. Work was carried out on three low carbon (0.02 wt%) manganese (1.4 wt%) steels with very low levels of nitrogen (10 ppm) which have been alloyed with very different aluminium contents (0.02, 0.48 and 0.94 wt%). Electron back scatter diffraction (EBSD) was employed to study the effect of excess aluminium (apart from aluminium nitride) on the final ferritic microstructure. In order to have a better understanding in relation to the role of excess aluminium in ferritic microstructure it required an investigation of the austenite to ferrite transformation. Prior to investigation of the influence of aluminium on austenite to ferrite transformation, attempts were made to reveal the role of excess aluminium in austenite formation. The results obtained from the latter part of the research enabled the author to better understand the role of excess aluminium in austenite grain formation and growth. From this study, it may be concluded that excess aluminium has a significant influence on as rolled ferritic structure which could be the result of changes in austenite to ferrite transformation kinetics. In addition, the results obtained from this research show a significant effect of excess aluminium on austenite formation and growth.