| Titanium microalloyed steel has high strength and excellent toughness and is widely used in machinery,vehicles,construction and other industries.However,with the light weight requirements of various industries,the development of higher strength titanium microalloyed steel has become a research hotspot.In recent years,it has been found that the strength of titanium microalloyed steel is obviously improved after isothermal treatment,and the nano-carbide particles precipitated on the substrate after isothermal phase transformation are important factors for strength improvement.In order to explore the relationship between phase transition and nano-carbide precipitation during isothermal process and the influence of phase transformation and precipitation on strength during isothermal process in titanium microalloyed steel.In this study,the microstructure changes of titanium microalloyed steel during isothermal process and the precipitation behavior of nano-carbides were studied by Gleeble-3800thermal simulator,optical microscope and transmission electron microscopy.The main contents and conclusions of this paper are as follows:(1)The TTT curve of the experimental steel was studied by Gleeble-3800 thermal simulator and the precipitation behavior during the isothermal process was observed by transmission electron microscopy.The TTT curve of the experimental steel showd"C"shape.The matrix microstructure is polygonal or quasi-polygonal ferrite at isothermal temperature of 650-750°C,and the transformation speed is the fastest at 675°C;EDS and Fourier transform diffraction analysis confirmed that the nanoparticles precipitated during isothermal process were TiC,with face-centered cubic structure and B-N relationship with ferrite matrix.(2)The intensity increment of the experimental steel under different isothermal processes was studied by Gleeble-3800 thermal simulator and a Vickers hardness tester,and obtained the precipitation-temperature-time curve(PTT)of TiC in the isothermal process.The results show that the PTT curve of the isothermal precipitation of the experimental steel is roughly"C"type,and the precipitation rate is the fastest at 700°C,and the time required for the completion of precipitation is about 180 s.The precipitates were observed by TEM with different isothermal processes,and it was found that the volume TiC particles increased significantly at the P_S time,and the particle size did not increase significantly.When the isothermal time was extended,the precipitated particles were coarsened and the strengthening effect was weakened.It can be considered that the precipitation and the intensity increment match(3)The precipitation form changes during the isothermal process were studied by isothermal compression test,Vickers hardness test,transmission electron microscopy.The results show that the precipitation of TiC in the isothermal process of the experimental steel has two stages:The precipitation of TiC in form of interphase precipitation and random precipitation at the phase transformation process;The TiC particles are precipitated in the form of random precipitates in the ferrite after the phase transformation.(4)The effects of phase transformation and precipitation on strength were analyzed by TTT curve,PTT curve,isothermal compression test and Vickers hardness test.The results show that the yield strength of experimental steel under different isothermal conditions is affected by phase transformation and precipitation.The phase transition occurs at the initial stage of isothermal.The yield strength increment mainly comes from phase transformation strengthening,and the increase of yield strength is mainly due to phase transformation strengthening in the early stage of isothermal transformation.The precipitation strengthening effect is gradually increased,and the peak of the yield strength is reached at the Ps time of the PTT curve;while the isothermal time is extended,the TiC particles are coarsened and the precipitation strengthening effect is weakened.(5)The strength control of experimental steels was investigated by OM?TEM and relative nucleation rate curve(NrT curve).The results show that the average grain size of ferrite decrease with the decrease of isothermal temperature,the average grain size of ferrite decreases due to the effect of fine grain strengthening and precipitation strengthening enhanced.Therefore,a larger yield strength can be obtained by lowering the isothermal temperature and holding it to the precipitation Pf time... |
PDF Full Text Request