Phase Transformation and Precipitation Mechanism of Nb Microalloyed Bainite-Martensite Offshore Platform Steel at Different Cooling Rates

The phase transformation and precipitation mechanism of Nb microalloyed offshore platform steel during continuous cooling are studied by the thermal expansion method and transmission electron microscopy. The results show that a large amount of NbC is precipitated in the investigated steel during phase transformation and is classified into three types according to its size: Type I (>100 nm) is randomly generated in the austenitization stage; type II (20-100 nm) is precipitated by stress induction during thermal deformation; and type III (approximate to 10 nm) is formed by the combination of segregated C and Nb during the cooling process after the formation of bainitic ferrite matrix. In addition, herein, a model of phase transition and precipitation behavior at different cooling rates is designed. At a low cooling rate (0.5 degrees C s(-1)), the microstructure consists of a granular structure formed by diffusion and granular bainite formed by shearing, which is a diffusion phase transformation. At a medium cooling rate (3 degrees C s(-1)), the orientation relationship of ferrite and austenite is [001]alpha//[011]gamma and (110)alpha//(-1-11)gamma, which is semidiffusion and a half-shear mechanism. At a high cooling rate (15 degrees C s(-1)), there are a large number of intertwined dislocations in martensite, which is a shear mechanism.