Effect of Niobium Addition on Microstructure and Mechanical Properties of Fe–7Al–0.35C Low-Density Steel

Abstract

The present work reports the effect of niobium addition on a Fe–7wt.%Al–0.35wt.%C-based low-density steel. A hot rolling temperature of 1100 °C was selected to carry out rolling in the intercritical (ferrite + austenite) region. The phases formed on adding Nb were predicted by thermodynamic calculations using ThermoCalc. All the alloys exhibited a significant (20% or more) tensile elongation. Niobium addition also resulted in a significant increase in strength and hardness of the hot-rolled steel. About 80% increase in the yield strength is observed as the Nb content increases from 0.2 to 1.0 wt.%. Nb carbides present at rolling temperature resist grain growth and lead to improved mechanical properties. This study demonstrates the advantages of adding Nb to Fe–Al–C-based low-density steels. Earlier attempts to add Nb to low-density steels resulted in very low ductility. This may be related to their susceptibility to hydrogen embrittlement. Melting of high aluminum low-density steels in a controlled atmosphere may lead to a considerable improvement in mechanical properties. Further, it is also important to maintain a high C/Nb ratio to avoid the formation of Laves phases.