Effects Of Nitrogen Addition And Isothermal Transformation On Multi-phase Microstructure And Mechanical Properties Of Low Carbon Mo-V-Ti Steel

The multi-phase steel,with high strain hardening index and low yield-to-strength ratio,generally has higher overload resistance and uniform elongation.The application will be expected in the field of earthquake-and ductile deformation-resistant structures of long-span buildings and bridge.However,high strength and toughness were difficult to be considered in the design of multiphase steels based on low yield-to-strength ratio,and the comprehensive properties were difficult to be adjusted and controlled,which results in the limited application of multiphase steels.In recent years,nitrogen microalloying and isothermal transformation have been tried to solve this key technical problem.However,due to the complex microstructural factors,the mechanism of nitrogen microalloying and isothermal transformation have rarely been involved.In this paper,the effects of nitrogen and isothermal process on the ferrite matrix and M/A constituent of the low-C Mo-V-Ti steels,the relationship between the multi-phase microstructure and tensile and impact properties of low carbon Mo-V-Ti steel were systematically studied by scanning electron microscopy(SEM),transmission electron microscopy(TEM)and electron back-scattering diffraction(EBSD),combined with the mechanical properties.The effect of nitrogen on phase transformation thermodynamics and kinetics of low carbon Mo-V-Ti steels were studied.With the increasing nitrogen content,the thermodynamic equilibrium temperature A₃ of ferrite transformation decreased slightly,and the measured actual temperature Ar₃ of ferrite transformation kinetics increased gradually at each cooling rate.The phase zones of austenite and ferrite calculated by thermodynamics and the cooling rate optimized by kinetics,combined with the actual rolling and heat treatment processes,the austenitizing temperature,isothermal pre-accelerated cooling rate and the isothermal temperature in the middle temperature were selected as 1200?,50?/s,450?600?.The effect and mechanism of nitrogen on the multi-phase microstructure of low carbon Mo-V-Ti steels were studied.After a 5 min isothermal transformation at 500?,each microstructure was composed of acicular ferrite,granular bainitic ferrite and M/A constituent,irrespective of the N content.Increasing nitrogen content created a larger number of nano-sized particles as well as micro-sized particles.The former decreased the average diameter of prior austenite grains by pinning the austenite grain boundaries and the latter promoted the heterogeneous nucleation of intragranular acicular ferrite.Combined with decreased supercooling,all three together has led to the increased fraction of high angle grain boundary and decreased fraction of low angle grain boundary.The increased N content changed the M/A constituent from twin-type M/A to lath-type M/A constituent.The effect and mechanism of isothermal temperature and time on the multi-phase microstructure of low carbon Mo-V-Ti-N steel were studied.After a 5 min isothermal transformation at 600?450? and 0.5?10 min isothermal transformation at 500?,each microstructure was composed of polygonal or acicular ferrite,granular bainitic ferrite and M/A constituent,irrespective of the isothermal temperature and isothermal time.Quantificationally compared the calculated results with the experimental results at different quenching and isothermal temperatures,acicular ferrite transformation occurred preferentially during the cooling stage,followed by the granular bainitic ferrite transformation.The fraction of acicular ferrite and the degree of supercooling increased with the decreasing isothermal temperature resulting in decreasing the mean equivalent diameter and M/A constituent size.However,the degree of supercooling at 500? was high enough,and hence,the ferrite transformation was rapid and the phase transformation was completed in a relatively short time.The relationship between the multi-phase microstructure and tensile properties of low carbon Mo-V-Ti-N steel was studied.With the increasing N content,the contribution of boundary strengthening and dislocation strengthening decreased,while the contribution of precipitation strengthening and another strengthening increased.The contribution of boundary strengthening was 54%?57%of the total yield strength.With the decreasing isothermal temperature,the grain refinement can enhance the yield strength and the mean equivalent diameters of ferrite grain with the misorientation tolerance angles ranging from2°to 6°were effective in governing the yield strength due to their close Hall–Petch relationship.However,the change of strain hardening ability and yield-to-strength ratio were related to the fraction of M/A constituents.The relationship between the multi-phase microstructure and impact properties of low carbon Mo-V-Ti-N steel was studied.Increased N content and decreased isothermal temperature can obviously improve in the impact properties and change the fracture behavior from cleavage brittle fracture to microvoid ductile fracture at-20?.The decrease of the hardness and average size of the M/A constituent can weaken the stress concentration on or around the M/A constituent,enhancing the difficulty of crack initiation.The increase in the fraction of acicular ferrite and high angle grain boundary can increase the resistance of crack propagation,enhancing the difficulty of crack propagation.