Investigation of Continuous Cooling Transformations and Microstructures in a Low Carbon Ti-Nb Bearing Microallyed Steel by Means of Dilatometry and Metallography

In this investigation, continuous cooling transformations and microstructures have been studied in detail in a low carbon Ti-Nb bearing microalloyed steel by means of dilatometry, metallography and hardness measurements. The results indicate that when the cooling rate is greater than 1˚C/s, the dilatation curve is associated with a major dilatation at relatively low transformation temperatures, which is consistent with the martensitic phase transformation. For a cooling rate between 0.2-0.8˚C/s, the dilatation curve is also characterized with a uniform single transformation over the martensitic and bainitic phase transformation temperature range. However, as the cooling rate is reduced to less than 0.2˚C/s, the dilation curve is characterized with a quite variable double stage transformation which is associated with the ferrite and bainite transformation products. These results suggest that the bainitic and martensitic phase transformations are developed with a similar displacive mechanism, while ferrite is formed by a diffusional mechanism.