Study of Transformation Characteristics and Nano-Scale Precipitation in Nb Microalloyed Steel

Abstract

In order to control the microstructure and nanoscale Nb(C, N) precipitation in a Nb microalloyed steel, three deformation schedules were employed, followed by cooling process to study the transformation and precipitation behaviors of the tested steel. It was found that plastic deformation in the non-recrystallization region of austenite accelerates the onset of transformation from austenite to ferrite, while the progress of ferrite transformation is retarded. The deformation also refines the ferrite grain size. The transformation behavior for deformation temperature of 850 and 910 °C are almost the same due to the similar deformed austenite. Although the latter deformation temperature is 60 °C higher, the deformation can also significantly refine the transformation microstructure. With the decrease in deformation temperature and increase in cooling rate, the precipitation start temperature decreases, and the nucleation zone of precipitation is transformed from austenite to ferrite or bainite under continuous cooling condition. When the cooling rate reaches 10 °C s−1, the nucleation of precipitation is totally inhibited during cooling process. The precipitate size is significantly refined by increasing cooling rate due to the increase of precipitation driving force.