Nano-precipitates Control And Characterization Of Low Carbon Microalloyed Steels

Low carbon microalloyed steels are widely used for the excellent strength-toughness combination and weldability.Niobium,vanadium,titanium,and molybdenum were often added to optimize the mechanical properties based on precipitation strengthening.Under the same alloy addition,the precipitation strengthening is mainly controlled by the precipitates size.Three method can be used to control the precipitates size,reducing the interfacial energy and elastic strain energy,increasing the nucleation driving force and the site for nucleation.In this paper,different thermal mechanical control process(TMCP)parameters were investigated to obtain high density nano-precipitates distributing uniformly in the expected matrix,then the strength of steel can increase significantly with less microalloys addition.The complex precipitation formation mechanism,and crystallographic characteristics of precipitates were systemically investigated.The main innovative work and conclusions are as follows:(1)Co-precipitation strengthening of TiC and Fe3CA small amount of Titanium(0.08 mass%)addition based on the composition of Q345 grade steel can improve the yield strength of steel to 650 MPa.The volume fraction and average size of the precipitates was measured by small angle X-ray scattering(SAXS)and small angle neutron scattering(SANS).The results indicated that Fe3C and TiC can be formed simultaneously.The precipitation strengthening contribution of nanoscale precipitates can achieve to 350 MPa.Fe3C had a stronger precipitation strengthening effect than that of nanoscale TiC,contributed to 2/3 of the total precipitation strengthening.Together with solid solution strengthening and grain refinement strengthening,the theoretical calculated values matched well with the experimental values.The traditional comprehensive strengthening theory was revised for the reason that "ultrafine grain strengthening can not be directly added with dislocation strengthening or precipitation strengthening".(2)Interphase precipitation mechanismThe precipitation behavior of low carbon Ti-bearing steel during isothermal quenching process have been investigated.The result indicated that the sheet spacing,inter-particle spacing,and size of the interphase precipitation carbides decreased with temperature.The precipitation morphology changed into planer interphase precipitation(PIP)from curved interphase precipitation(CIP)with the decrease of temperature.Besides,the sheet plane of interphase precipitation was parallel with neighbor ferrite-cementite lamellar in pearlite.The formation mechanism of interphase precipitation is same with that of pearlite,which is "eutectoid decomposition mechanisms".(3)Complex carbides formation kinetic calculation and mechanismThe chemical composition and lattice parameter of the nanometer-sized carbides in ferrite of Nb-V bearing steel isothermally treated at 650? for different time were studied by high resolution transmission electron microscopy(HRTEM)and energy dispersive X-ray spectroscopy(EDX).The results indicated that the average size of the carbides increased with isothermal holding time.Vanadium was found to be gradually incorporated into NbC lattice to form a complex(NbxV1-x)C particle and the atomic ratio of vanadium gradually increased during coarsening,which leads to a decrease in the lattice parameter of carbide and reduces the misfit strain between the carbide and ferrite matrix.Precipitation-Temperature-Time(PTT)diagram of carbides formed in ferrite of Nb-V-bearing steel calculated by Avrami equation was the typical "C" shape,with the optimum precipitation temperature of 650?.The formation mechanism of complex carbides in Nb-V microalloyed steel has been investigated by using 3-DAP and HRTEM.The results revealed that most of the precipitates formed according to replacement precipitation mechanism,which can enhance the nucleation rate and stability of carbides for the reducing interfacial energy.(4)Crystallographic characterization of precipitatesCrystallographic characterization of precipitates have been investigated by HRTEM,including precipitates structure,lattice parameter,and the orientation relationship(OR)with matrix.The shape factor,interfacial dislocation,interfacial energy of precipitate and matrix were calculated in specific ORs between precipitates and ferrite.The results indicated that the carbide with FCC structure and matrix with BCC structure can satisfy different ORs,including one new OR,named NEU-OR which can be described as[110]MC//[101]ferrite and(002)MC//(101)?.The transformation matrix calculation confirmed that NEU-OR is not variant of known ORs.According to shape factor calculation,we can get the conclusion that the carbides obeyed BN or NEU-OR with matrix showing disk shape,while for the carbides obeyed other ORs with matrix showing spherical shape.The interphase precipitation carbides strictly obeyed BN and NEU-OR with ferrite matrix,while for the precipitates obeyed NEU-OR with ferrite had lower interfacial energy,is favorable to improving the nucleation rate and stability.