Investigation On Precipitation Control Of Nano-sized TiC In Ti Microalloyed Steel

Hot rolled strip with yield strength over 700 MPa was successfully developed by Zhujiang Steel Co.Ltd through a single Ti microalloying technology on CSP production line,a large number of nano-sized TiC particles precipitated in steel is one of the main reasons for its high strength.Meanwhile,rolling and isothermal process is the key to promoting the precipitation of microalloyed carbides.In order to understand the strengthening effect of Ti in steel and improve the performance stability of Ti microalloyed steel,the strain-induced precipitation,isothermal precipitation and precipitation control of nano-sized TiC in Ti microalloyed steel were investigated by Gleeble 3800 thermal simulator and transmission electron microscope.The main contents and results of this paper are as follows:(1)The strain-induced precipitation behaviors of titanium carbide in steel during the deformation from 900? to 1000? were studied.The experimental results showed that after deformation at temperatures below 1000?,strain-induced precipitation occurred as the isothermal time prolongs,static recrystallization was inhibited and a precipitation platform appeared on the softening rate curve.Selected area electron diffraction and EDS analysis confirmed that the strain-induced precipitated nano-sized carbides were TiC with face-centered cubic structure.The precipitation-temperaturetime(PTT)curves of TiC precipitation exhibited a typical “C” shape,the tip temperature was 925°C,which has the fastest precipitation kinetic.At 925°C and below,recrystallization was co-suppressed by temperature and strain-induced precipitation,the softening rate of deformation austenite decreased significantly and no recrystallization occurred basically.The temperature range of 900-925? is suitable for the non-recrystallized region controlled rolling of Ti microalloyed steel to refine grains.The strain-induced precipitated TiC particles were slightly larger and the average size was above 10 nm,and its precipitation strengthening effect can be ignored.(2)The isothermal precipitation behavior of TiC in steel at isothermal temperatures of 550-650°C was studied.The experimental results showed that the isothermal treatment was the key step of TiC precipitation,after isothermally holding 600 s at 600?,the yield strength of the experimental steel increased by 120 MPa,and the yield strength reached the maximum value of 715 MPa.A series of experimental data of “process-precipitation-strength increment” can be directly applied to the improvement of isothermal or coiling process.Based on the analysis of fast Fourier transformed(FFT)diffraction pattern,it is determined that the precipitated particles during the isothermal process were TiC and obeyed Baker-Nutting Orientation relationship with ferrite matrix.The PTT curve of TiC isothermal precipitation can be quickly determined based on the strength increment curve,and the curve was rough "C" shape.The fastest precipitation temperature of TiC was 650?,and the maximum nucleation rate temperature was 600?.The best precipitation strengthening effect can be obtained through isothermal at the maximum nucleation rate temperature.(3)The relationships between strain-induced precipitation and isothermal precipitation of TiC and its control were studied.The experimental results showed that there was a significant competitive relationship between strain-induced precipitation and isothermal precipitation,the number of isothermally precipitated TiC particles decreased obviously with the increase of the strain-induced precipitation,leading to the weakening effects of the precipitation strengthening effect.Although precipitation strengthening effect was somewhat reduced by strain-induced precipitation when the deformation at 900?,the deformation structure were accumulated because of the inhibition of austenite recrystallization,which made grain refined and produced better fine grain strengthening effect;The experimental steel was deformed at 900? and isothermally relaxed for 100 s,after this,it was fast cooled to 600? and remained for 600 s,and the yield strength of the steel at room temperature reached 730 MPa.Compared with the experimental steel that was deformed at 950? and hold for 200 s and then fast cooled to 600? without holding,the strength was increased by 130 MPa;(4)The process control mechanism of TiC precipitation was studied.On the basis of mastering the TiC precipitation behavior,it was possible to simultaneously achieve better fine-grained strengthening and precipitation strengthening effect by controlling the precipitation of TiC in the austenite and the ferrite region by proper processes,which can further improve the comprehensive properties of the steel.It is suggested that the rolling temperature is controlled at 900-925? when the non-recrystallized rolling is used in the actual hot rolling process,and the isothermal treatment should be added after the fast cooling process,and the isothermal temperature is controlled at about 600 ?.