The Effect of Niobium on the Microstructure and Phase Transformation Kinetics in Low-Carbon Medium Manganese Steels

Medium-Mn steels are a promising candidate to fulfil the requirements of the 3rd generation of AHSS (Advanced High Strength Steels) which combine high strength and high ductility. A new approach is to add micro alloying elements, like niobium, to affect microstructural development during recrystallization and phase transformations. Using a quenching dilatometer, the kinetics of the phase transformations from austenite (810 °C and 840 °C) to its transformation-products by varying different cooling rates (100 K/s to 0.03 K/s) were investigated for two low carbon medium-manganese steels with different contents of niobium. Based on the dilatometric curves, the results of light optical microscopy (LOM) and quantitative metallography, measurements of the retained austenite (RA) content by X-ray diffractometry (XRD) and hardness measurements, continuous-cooling-transformation (CCT) diagrams were constructed. Furthermore, the influence of the element niobium on the microstructure after intercritical annealing was investigated by similar methods. Different etching-methods (Nital, Klemm-I) were applied to reveal different microstructural aspects. Besides considerable grain-refinement effects with increased niobium content, the phase-transformations were accelerated in the CCT-diagrams. The microstructure varies from 100 % martensite to certain amounts of bainite, but also ferrite at lower cooling rates. Intercritical annealing results in different amounts of retained austenite.