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 3^rd generation of AHSS (Advanced High Strength Steels) which combine high strength with high ductiliy. A new approach is to add micro alloying elements, like niobium, to affect microstructural development. Using a quenching dilatometer, the kinetics of the phase transformations from austenite to its transformation-products at 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. With the results of optical microscopy (OM), quantitative metallography and scanning electron microscopy (SEM) continuous-cooling-transformation (CCT) diagrams were developed. Furthermore the influence of niobium on the microstructure after intercritical annealing was investigated. The amount of retained austenite (RA) was determined by X-ray diffractometry (XRD). 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 higher amounts of bainite, but also ferrite at lower cooling rates. Intercritical annealing results in different amounts of retained austenite.