| Ferritic steels have been widely used in the automotive,construction machinery and other fields,due to its excellent plasticity,formability and weldability etc.,moreover,production process of which is simple and cost is low.However,the strength of ferritic steels is too low to meet the needs of the development of related industries.How to increase the strength of ferritic steels has become one of the hotspots and difficulties in the research of steel materials.Therefore,in this study,the effect of Nb on the microstructure,mechanical properties,and precipitation behavior of the second phase of Ti-Mo low-carbon microalloyed steels was systematically investigated,which can provide a reference for the dvevlopment of microalloyed ultra-high strength steels.The effect of Nb on the continuous cooling transformation of Ti-Mo steel was systematically investigated.The results showed that the Nb element can shift the bainite phase zone to the left and bottom zone,namely,the bainite transformation was inhibited.Furthermore,the microstructure was refined to improve the strength of the Ti-Mo steel.The average size of(Ti,Nb,Mo)C particles in Ti-Nb-Mo steels was 8.69nm at 50 ~oC/s,which was finer than that in Ti-Mo steel.An analytical model for describing the precipitation thermodynamic and kinetics of(Ti,Mo)C and(Ti,Nb,Mo)C in austenite,showed that(Ti,Nb,Mo)C particles are easier to precipitate in austenite than(Ti,Mo)C particles.The main reason is that Nb addition enchance the precipitation driving force.The results of stress relaxation showed that the precipitation-time-temperature(PTT)diagrams exhibit a typical?C?shape curve,the nose of which were located at 950 ~oC and 975 ~oC in Ti-Mo and Ti-Nb-Mo steels,respectively.Nb was found to be incorporated into(Ti,Mo)C lattice to form a complex(Ti,Nb,Mo)C particle during precipitation,increasing the lattice content of(Ti,Nb,Mo)C.Ti-Mo and Ti-Nb-Mo complex microalloyed steel were obtained in labtoratory.The results showed that the volume fraction of bainite increased with the increasing coiling temperature from 550 ~oC to 600 ~oC in Ti-Mo steel,while only ferrite and carbide were found in Ti-Nb-Mo steel,which has excellent properties.When the coiling temperature was 550°C,the tensile strength,yield strength,and elongation were 755 MPa,712 MPa,and 22%,respectively. |
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