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

In recent years,Ti-Mo microalloyed ferritic high strength steel has developed rapidly and has a broad application prospect in automobile,construction machinery and other fields.Compared with Ti element,V element has a more significant precipitation strengthening effect.Further adding V to Ti-Mo microalloyed steel is expected to achieve further improvement in strength,but at present,there is still little research on Ti-Mo-V complex microalloyed steel,which needs systematic research.Based on this,the precipitation behavior of the microalloy in austenite and ferrite is systematically studied in this paper for the Ti-Mo-V complex microalloyed steel,the isothermal transformation law is revealed,and the influence of coiling temperature on the structure and properties is analyzed,so as to provide a basis for ultra-high strengthening of Ti-Mo-V microalloyed steel.The theoretical calculation results show that the second phase of Ti-Mo-V complex microalloyed steel is?Ti,Mo,V?C.With the increase of V content,the volume fraction of precipitatation in Ti-Mo-V microalloyed steel increases,the precipitation kinetics of austenite slows down,and the precipitation kinetics of ferrite accelerates.In addition,increasing the deformation of austenite and increasing the energy storage of deformation will promote the precipitation of?Ti,Mo,V?C in austenite,and the critical size of the precipitation will be reduced,which is conducive to grain refinement.At the same time,the increase of deformation-induced precipitation in austenite will accelerate the precipitation of?Ti,Mo,V?C particles in ferrite.The results of the isothermal transformation study show that the microstructure of Ti-Mo and Ti-Mo-V-2 steel is transformed from ferrite to bainite within the temperature range of 700?-550?with the decrease of isothermal temperature,and the matrix hardness increases first and then decreases.Moreover,the hardness of Ti-Mo-V-2 steel is higher and the bainite transformation kinetics is slower.In other words,V can inhibit bainite transformation which is conducive to the acquisition of total ferrite structure.When the isothermal temperature of 550?,the bainite transformation kinetic models of Ti-Mo and Ti-Mo-V-2 steel areB_RF=1-exp(-0.03?t^1.71)and B_RF=1-exp(-0.01?t^{2.0 9}).The Ti-Mo-V-1 steel was produced by a laboratory mill.The results show that the microstructure of the steel is ferrite when coiling temperature is 650?.With the decrease of coiling temperature,the granular bainite in the test steel increases gradually,the ferrite grains refine continuously,and the average particle size of precipitates decreases.When the coiling temperature is 600?,the mechanical properties of the test steel are the best.Its tensile strength,yield strength and elongation are 917 MPa,856MPa and 21.5%respectively.The precipitation strengthening and fine grain strengthening increment reached 429 MPa and 237 MPa respectively,which were the most important strengthening mechanisms.