Effect of Nb on the Continuous Cooling Transformation Rule and Microstructure, Mechanical Properties of Ti-Mo Bearing Microalloyed Steel

In recent years, with the fast development of automotive industry, more and more attention has been focused on developing high strength automobile steels with excellent formability. The microalloying elements, such as Nb, Ti, Mo, which can facilitate grain refinement and precipitation hardening, were added into steels to achieve high strength and good formability. The Ti-Mo and Ti-Mo-Nb microalloyed high strength ferritic steel were developed. In this work, the continuous cooling transformation curves (CCT) of Ti-Mo and Ti-Mo-Nb steels were obtained. And the effect of Nb on the microstructure and mechanical properties of Ti- Mo low carbon microalloyed steel was investigated by means of SEM, HRTEM and EDS. The results showed that Nb could raise the A(c1) and A(c3) temperatures, and restrain the ferrite-pearlite and bainite transformation. Moreover, Nb could also refine the microstructure and harden the matrix of steel which attributed to the strain-induced precipitation of nano-sized (Ti, Nb, Mo) C particles identified by HRTEM and EDS. It was also found that the strain-induced precipitation of (Ti, Mo) C was existed in the Ti-Mo steel. And both of (Ti, Mo) C and (Ti, Nb, Mo) C particles were NaCl type structure. The lattice constants/the average particle sizes of (Ti, Mo) C and (Ti, Nb, Mo) C were 0.432 nm and 0.436 nm / 12.11 nm and 8.69 nm, respectively.