Study On Precipitation Behavior In Bainite Region Of Vanadium Containing Titanium Microalloyed Steel And Development Of Bainitic Steel

In this paper, vanadium containing titanium microalloyed steels are used as research objects. The deformation behaviors of experimental steels at high temperature, the discipline of austenite continuous cooling transformation, the thermodynamic calculation of (V,Ti)(C,N) precipitation in austenite and the precipitation behavior of V(C,N) in bainite region was studied. Hot rolling experiments under lab condition were also conducted on the basis of the experiments mentioned above, and our main work and results are as follows:(1) By conducting single-pass compression tests, we studied the deformation behaviors of experimental steels under high temperature, drawn strain-stress curves.The dynamic recrystallization activation energy of V-Til, V-Ti2, V-T13,V-Ti4, V4 are 332.76kJ/mol, 326.34kJ/mol,314.41kJ/mol,266.353kJ/mol and 263.921kJ/mol respectively. It showed that the rising of vanadium content increased dynamic recrystallization activation energy, adding 1.15% manganese made dynamic recrystallization activation energy increase 15kJ/mol, adding 0.0189% titanium had little effect on dynamic recrystallization activation energy of vanadium microalloyed steels.(2) Static CCT curves of V-Til, V-Ti2, V-Ti4 tested steels were drew by austenite continuous cooling transformation experiment, the rising of manganese content narrowed the ferrite and pearlite transition region and promoted phase transition of bainite.(3) The precipitation thermodynamic calculation of (V,Ti)(C,N) in austenite showed that: at high temperature, the precipitation of TiN was predominantly, the amount of TiN reduced as temperature dropped. Vast of vanadium was dissolved in austenite, a little bit of VC and VN precipitated at this time. When it came to 900?, a large quantity of VC precipitated and the amount of VN was cut down. In austenite region, the precipitation of TiC was not obviously, temperature had little effect on the precipitation of TiC.(4) The isothermal precipitation experiment in bainite region was done by thermomechanical simulator and the results showed that:Abundant precipitates of nanoscale V(C,N) in the dislocated bainitic ferrite matrix was observed, the diameter of V(C,N) was 3-7nm. The volume fraction of V(C,N) precipitation grew with the increasing of holding temperature, when 600? held 40min, a large amount of V(C,N) precipitated.(5) V-Ti01,V-Ti02 and V-Ti03 microalloyed bainitic steel was made under the condition of laboratory. The final rolling temperature was between 800? and 840? and the final cooling temperature was between 550? and 580?. Ferrite and pearlite mixed phase would be obtained in V-Ti01, bainite and ferrite mixed phase could be observed in V-Ti02 and V-Ti03. Fine impact toughness from -70? to 20? was tested, and the strength of steel which had more vanadium was higher than the less one in the same process.