Effect of Nb addition on the microstructure and mechanical properties of an 1800Â MPa ultrahigh strength steel

The microstructure-property relationship of Nb-free and Nb-bearing ultrahigh strength steels were studied in this work. Martensitic steels with 1800 MPa tensile strength were obtained by conducting reheated quenching and tempering after thermomechanical rolling. The microstructures of the steels were investigated by scanning electron microscopy, transmission electron microscopy electron backscattering diffraction technique. The results show that with the addition of Nb, the prior austenite grain growth was impeded, which leaded to the further refinement of final martensite microstructure. Randomly distributed Nb-rich (Nb,Ti)C in Nb bearing steel played an important role in the strengthening of the steel. As revealed by dilatometry and differential scanning calorimetry, with the increase of tempering temperature the decrease of mechanical properties mainly initiated by the decomposition of retained austenite during low temperature tempering, and the formation of harmful iron carbides was retarded when the steel microalloyed with 0.021% Nb.