Study On Precipitation Behavior In Bainite Region And Properties Of V-N-Mo Microalloyed Steel

Microalloying technology has been one of the most important technologies in the steel industry.Due to its rich resources and excellent performance,VN microalloyed steel has received increasing interest.Some studies have shown that the addition of Mo can optimize the microstructure of VN microalloyed steel and improve the precipitation strengthening effect.For the systematic study of V-N-Mo microalloyed steels,five different components of microalloyed steel were designed.The high temperature deformation behavior,the continuous cooling transformation process,the low temperature bainite precipitation behavior were studied.According to the results of the study,a laboratory trial was conducted,and its structure performance were observed.The main findings are as follows:(1)The results of high temperature austenite deformation behavior shows that the addition of N elements and Mo elements increases the peak stress of the test steel.N element and Mo element can promote the dynamic precipitation at the condition of low deformation temperature and slower deformation rate,thus inhibiting the dynamic recrystallization behavior.The results show that the increase of N content and Mo content increases the recrystallization activation energy of steel,which makes the dynamic recrystallization process more difficult.(2)The results of austenite continuous cooling transformation shows that the N element is beneficial to expand the ferrite phase region and pearlite phase region,inhibit the bainite transformation,make the bainite phase region shift left,and can refine the low cooling rate under the ferrite grains.The addition of Mo element can significantly delay the starting transition temperature of the ferrite phase transformation and pearlite phase transformation,expand the bainite phase region,increase the bainite transformation of the cooling rate range,shift the bainite phase area to the right.When the Mo content reaches 0.4%,the bainite structure can be obtained at the cooling rate of 0.5?/s.The Mo element can also refine the microstructure and improve the hardness of the tissue.(3)The study of bainite precipitation behavior shows that most of Mo elements is present in the form of solid solution,and only a small part of it is present as precipitates in the steel.As the content of Mo increases,the amount of equilibrium precipitationcan significantly increase,About 75%of Mo elements was precipitated in the form of MoC,the rest was into the V(C,N).MoC can inhibit the formation of cementite,Mo incorporating into the V(C,N)can refine the precipitate size.The HRTEM comparison test showed that the relationship between the precipitation phase and the matrix phase is Baker-Nutting relationship,with Mo elements incorporating into the VC,the VC lattice constant from 4.157(?)increased to 4.19A.The error rate between the precipitation phase and the matrix phase also increased from 2.5%to 3.3%.There by increasing the interfacial structure of the precipitated phase and the matrix phase,increasing the resistance at the early stage of precipitation.Under the known three temperature conditions,the precipitation at 600? was more than 700? and 500?,With the extension of isothermal time,the advantage would be further expanded.(4)The results of V-N-Mo steel rolling test shows that Mo element is beneficial to improve the strength of microalloyed steel,With the furnace cooling for 48 hours,the tensile strength of VNMo3 experimental steel up to 800 Mpa,which increased by 120MPa compared VN steel.The V-N-Mo microalloyed steel has excellent impact toughness at room temperature,and the addition of Mo and N has a slight negative effect on the toughness of microalloyed steel.With the extension of the holding time,the structure of VN steel was coarsened and the number of sub-grain boundaries decreased.As for Mo-added VNMo microalloyed steel,with the insulation time inceasing,the structure had no obvious coarsening phenomenon,the number of sub-grain boundaries in the structure increased.The results of electron probe experiments shows that bainite,ferrite and retained austenite are present in VNMo3 experimental steel.V element and Mo element in a uniform dispersion state in the structure,there is no obvious segregation phenomenon.