Interphase precipitation behaviors of nanometer-sized carbides in a Nb-Ti-bearing low-carbon microalloyed steel

High strength low alloy steels utilize chemical composition design of low carbon content and are microalloyed with Nb, V and Ti, or other additions, such as Mo and B, etc. The increase of strength is attributed to grain refinement strengthening, solid-solution strengthening, dislocation strengthening and precipitation hardening. Moreover, the precipitation hardening attracts more and more attentions. However, the detailed results on the sheet spacing, inter-particle spacing, crystallography, composition and the nucleation site of the interphase precipitation carbides in Nb-Ti containing steels have not been reported as yet. In this work, the microstructure, mechanical properties and precipitation behaviors in a low carbon Nb-Ti microalloyed steel were investigated using the dilatometer and TEM. The results show that the interphase precipitation can be observed for different isothermal temperatures and the sheet spacing, inter-particle spacing and size of the interphase precipitation carbides are refined by lowing isothermal temperature. The characteristic sheets of interphase precipitation carbides were identified as planar and curved. The planar sheets of interphase precipitation carbides have been analyzed and found to be parallel with {011},{012},{013} and {035} planes of ferrite. Moreover, the interphase precipitation carbides which have been determined to be (Nb, Ti)C have a NaCl-type crystal structure with a lattice parameter of 0.434 nm and obey the Baker-Nutting (B-N) orientation relationship with respect to ferrite matrix. The contribution of the interphase precipitation hardening to the yield strength of the experiment steel has been estimated above 300 MPa, based on the Orowan mechanism.