| Nb-containing microalloyed steels have been widely used in automobiles,ships,oil and gas pipelines,etc.It remains to be further studied on how to fully utilize the strengthening effect of Nb elements in microalloyed steels.In this paper,Mo element is added in Nb-containing microalloyed steel in order to control precipitation reaction of the steel and control the structure to achieve the purpose of improving the strength.In this paper,low carbon microalloyed steel containing 0.1% Nb was used as the experimental material,and the effects of Mo addition on its microstructure,precipitated phase structure and mechanical properties were studied.Accordding to the theory of strengthening,the phase nucleation theory?the phase behavior of precipitated phase and the perspective of the in-depth analysis of its impact mechanism,the main work have been carried out in the paper as followings:(1)The material design,preparation and process design of the materials studied were carried out by means of metal material performance analysis software JMat Pro and high vacuum arc melting furnace;Through metallographic microscope(OM)? scanning electron microscope(SEM),transmission electrons Microscope(TEM)and X-ray diffractometry(XRD)were used to analyze the effects of different Mo content and different annealing time on the microstructure of niobium microalloyed steel were analyzied.The microstructure and dislocation configuration of the precipitated phase were also discussed.The results show that the addition of Mo element leads to the microstructure of the annealed nitrile-containing microalloyed steel body matrix,and the steel of each component shows that a certain degree of microstructure coarsening with the prolonging of the holding time.The size of the precipitated phase(Nb,Mo)C in the steel where Mo is added at the same time is finer than that which is not added,and the distribution is more dispersed.Besides,the Mo atom can get into the crystal lattice of the precipitated phase NbC and replace part of the Nb atom,resulting in a smaller lattice constant.(2)The mechanical properties of the annealed state of the microalloyed steel with Mo or not were investigated by the MTS Criterion Model.The tensile fracture morphology was observed by SEM and anayzied with microalloyed steel.The theoretical strength model was analyzed in the contribution of different strengthening mechanisms to its theoretical yield strength.The results show that the addition of Mo can simultaneously increase the ultimate tensile strength and yield strength of niobium-containing microalloyed steel,but its elongation at break decreases slightly,and the port morphology shows typical dimple microporous characteristics.The theoretical yield strength analysis shows that precipitation strengthening and fine grain strengthening play a leading role in the strength increment of several strengthening mechanisms,and raising with the increase of Mo content.(3)According to the semi-coherent orientation relationship between the NbC precipitation phase and the ferrite matrix,With the help of the thermodynamic equilibrium solid solubility formula of the precipitation phase,precipitation kinetics model under different nucleation mechanisms of the niobium microalloyed steel with Mo or not is established.The results show that the interface energy between(Nb,Mo)C and ferrite is lower than that of NbC,and(Nb,Mo)C particles have lower critical nucleation work and critical nucleation size,Which has a higher nucleation rate and faster nucleation kinetics.(4)Based on the first-principles calculation method of density functional theory,the properties of adhesion,interfacial energy,electronic structure and bonding characteristics of Mo before and after entering ?-Fe/NbC interface were studied.The terminal interfaces of two different atoms were considered,and the influence of the displacement positions of six Mo atoms on the interfacial structure of ?-Fe/NbC was analyzed.The results show that the stability of the C-top interface structure is stronger than that of the Nb-top interface,while the ?-Fe/(Nb,Mo)C interface formed by the replacement of Nb atoms have a cleaner and better interface stability than the Mo atoms entering the interface NbC side is cleaner than Better interface stability.Therefore,Mo element has a certain promoting effect on the nucleation of NbC in ferrite.In addition,the differential charge density,density of states and Milliken population analysis of the interface also explain the reason why Mo enhances the ?-Fe/NbC interface,which provides a theoretical basis for analysising the precipitation behavior of carbide in the niobium-molybdenum composite microalloyed steel. |
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