The Study Of Phase Transformation During Continuous Cooling And Deformation Of Nb Micro-alloyed Steel

By using the thermecmastor-Z thermomechanical simulator, one-way hot compression test of 20MnSiNb and 20MnSi steel was performed. The static and dynamic continuous cooling transformation curves (CCT) were established by means of thermal expansion technology and microstructure observation, and the effects of cooling rate, deformation and addition of Nb element on the γ→α transformation of Nb microalloyed steel were studied. The results showed that:For the undeformed and deformed specimens, as the supercooling degree increased with an increasing cooling rate, the phase transformation was retarded, and the transformation temperature Ar₃ decreased greatly, thereby refining ferrite grain dramatically. As the cooling rate increased from 0.1°C/s to 10°C/s, the refinement of ferrite grain was remarkable, however, when the cooling rate was above 10°C/s, it decreased and the size of ferrite grain tended to be stable.The predeformation of specimens caused deformation stored-energy, on the one hand, the deformation stored-energy resulted in the increase of the transformation temperature of γ→α, thereby deteriorating the influence of transformation on grain refinement; on the other hand, it also promoted the nucleation rate of ferrite grain, and moreover, predeformation can effectively enhance the effects of transformation on grain refinement. Due to the interacting of two aspects, the predeformation had less influence on the transformation temperature Ar₃ and the size of ferrite grain.When the austenitizing temperature was 1000℃, the element Nb existed mainly in the form of Nb(C,N) compound, and it had low solubility in austenite, thereby decreasing hardenability of steel, the actual transformation temperature increased. The main effects of Nb(C,N) compound in steel were to restrain the austenite recrystallization, and raise the recrystallization ending and austenite grain coarsening temperature, which refined austenite grain, thereby refining ferrite grain.The results of this paper provided a valuable theory for determining proper process parameters of cooling after rolled and obtaining excellent iron and steel materials with close grain.