Dynamic CCT Curves and Precipitation Sensitivity of Nano-sized Carbonitride Against Cooling Rate in the High-V,N Micro-Alloyed Steel
In order to achieve precise control of the structure and precipitates during the rolling-cooling process of the high V,N micro-alloyed steel,the Gleeble-1500D thermal simulated test machine was used to study the thermal expansion curve and phase transformation rules of the experimental steel at different cooling rates after rolling.In particular,the dynamic continuous cooling transformation curve(dynamic CCT curve)of supercooled austenite,and the microstructure evolution behavior,microhardness and sensitivity of the nano-sized carbonitride precipitation behavior to the cooling rate were studied.The results show that when the cooling rate is lower than 3 ℃/s,the microstructure of the experimental steel is composed of ferrite and pearlite.When the cooling rate is 3 ℃/s,bainite transformation occurs,and the matrix structure is composed of ferrite,pearlite and bainite.On the other hand,the pearlite structure disappears,and the martensite structure begins to be formed at the cooling rate of 8 ℃/s,and the matrix structure is composed of intergranular ferrite,bainite and martensite.When the cooling rate reaches 20 ℃/s,the matrix structure is dominated by martensite and it is mixed with a small amount of proeutectoid ferrite and bainite.Furthermore,the cooling rate also has a significant influence on the precipitation behavior of nano-sized carbonitrides.When the cooling rate is within 1 ℃/s,the diameter and number density of the nanoprecipitates in the polygonal ferrite a show strong sensitivity to the cooling rate.The diameter of the nanoprecipitates is apparently decreased with the increase in cooling rate,and it is inverse for the change in number density.When the cooling rate increases from 1 ℃/s to 3 ℃/s,the diameter of the nanoprecipitates further decreases,and the number density tends to be stable.As the cooling rate is further increased from 3 ℃/s to 5 ℃/s,the diameter of the nanoprecipitates keeps constant,and the number density is decreased.It is also found that less nanoprecipitates are contained in the bainite structure and the bainite is not helpful for the precipitation.Based on the above research on the structure evolution and precipitation law,a high V,N micro-alloyed steel with a yield strength of more than 700 MPa and meeting seismic requirements has been industrialized and trial-produced.
high V,N micro-alloyed steelcooling ratedynamic CCT curvephase transformation lawnano-sized precipitates
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