Study On The Nano-precipitation Behavior,microstructure And Mechanical Properties Of Ti-V-Mo Complex Microalloyed Steel

High strength steel is the material of choice for the automotive and engineering machinery industry.In recent years,the development of the ferrite matrix precipitation strengthening high strength steel has attracted a great deel of attentions.Among all the strengthening mechanisms,grain refinement hardening and precipitation hardening are main mechanisms,that affect the ferrite matrix precipitation strengthening high strength steel.Applying TMCP technology refine the ferrite grain size to much finer than 2～3 μm is more difficult to achieve further technological breakthroughs.Precipitation hardening can simultaneously increase the tensile strength and yield strength of high strength steel,it is the smallest damage on the ductility except grain refinement hardening.Therefore,the combination of the complex microalloyed with TMCP to comprehensively control the effects of grain refinement hardening and precipitation hardening,and develop a high strength steel with significant mechanical properties and stability which has a excellent theoretical and partical value in industrical application.In this work,the Ti-V-Mo complex microalloyed is applied to study the relationship between mechanical properties with strain-induced precipitation and isothermal precipitation and microstructure by reasonable control(Ti,V,Mo)C particles of austensite and ferrite matrix precipitation.The main research contents and conclusions of this study are as followed:The effect of deformation temperature on the(Ti,V,Mo)C strain-induced precipitation behavior and microstructure evolution in austenite of Ti-V-Mo complexed microalloyed steel was studied.The results show that the strain-induced precipitation PTT curve of Ti-V-Mo complexed microalloyed steel has a typical "C" curve shape.The nose temperature is about 1000 ℃,and the corresponding(Ti,V,Mo)C particle precipitation start time and end time are 5.1 s and 472.4 s.The precipitated phase is(Ti,V,Mo)C particles with face-centered cubic structure with a size greater than 10 nm,and the precipitation strengthening effect is weak.The original austenite grain size is about 60 μm,and the grain size is the smallest at950 ℃,about 53 μm.In order to clarify the isothermal precession in the ferrite mitrix on the microstructure,precipitation behavior and hardness of Ti-V-Mo complexed microalloyed steel are discussed.The results show that the TTT curves of different isothermal temperatures are in the shape of "C".When the isothermal temperature is lower,the size of the polygonal ferrite becomes smaller.As the isothermal temperature increases,the time required to complete the transformation from austenite to ferrite first decreases and then increases.The phase transformation speed of Ti-V-Mo steel is the fastest at 690 ℃.At different isothermal temperatures,the precipitated phases are all(Ti,V,Mo)C particles with a face-centered cubic structure of 4～5 nm.The 3DAP results show that the carbides are interphase precipitation and dispersion precipitation in the matrix,as the center Ti-rich(Ti,V,Mo)C particles.At630～660℃,the carbide nucleation rate is the largest,and the hardness rise is the most significant.The hardness of the experimental steel has a larger increase of about 70 HV.The isothermal time is prolonged,ferrite gradually becomes equiaxed polygonal ferrite with uniform size,and the precipitates are evenly distributed in the matrix,and the microhardness inside the ferrite grains is more stable.Effect of the austenite deformation process on the isothermal treatment of the Ti-V-Mo complexed microalloyed steel ferrite mitrix was discussed.The results show that the complete ferritization time of Ti-V-Mo complexed microalloyed steel after the 920 ℃ deformation process is changed by 3600 s is advanced to 600 s,the average ferrite grain size is about 4 μm,the structure is relatively stable in the subsequent isothermal process,and the ferrite grain size is stable.The lower the austenite deformation temperature,the smaller the structure and the similar precipitation strengthening effect,which is about 340～370 MPa,which is beneficial to further improve the mechanical properties of the steel.The experimental steel was deformed at 890 ℃,and the hardness was the highest at 630 ℃ isothermal 1200 s,which was 490 HV,and the comprehensive control of the effect of grain refinement hardening and precipitation strengthening reached the best.