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

Abstract

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 (≈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 °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 °C s−1), the orientation relationship of ferrite and austenite is [001]α//[011]γ and (110)α//(−1−11)γ, which is semidiffusion and a half‐shear mechanism. At a high cooling rate (15 °C s−1), there are a large number of intertwined dislocations in martensite, which is a shear mechanism.

Archives of Metallurgy and Materials

“…4 is very difficult without selected area electron diffraction (SAED) pattern. However, in the absence of SAED, the aforesaid two phases have been distinguished from the width dimension, carbide precipitate and dislocation density of the lath constituents of the microstructure which is in agreement with the earlier study [31]. The finer lath along with high dislocation density and no carbide precipitation indicate the martensite lath.…”

“…In the traditional hot rolling or austenitising temperature range, a strong suppression for grain growth after recrystallisation could be obtained due to fine NbC particles smaller than 10 nm in Nb-microalloyed steels [28,29]. The numerous fine carbides of Nb and Ti led to remarkable precipitation strengthening effect [30,31] in the present steel. Thus, the strength and toughness of the present steels are improved by the combination of grain refinement and precipitation strengthening mechanisms.…”

Effect of Ce on inclusion evolution and HAZ mechanical properties of Al-killed high-strength steel

“…Second-phase strengthening is an important step in steel production. Adding an appropriate amount of V, Nb, Ti, and other alloy elements into steel forms carbides and nitrides, which pin grain boundaries and dislocations to improve steel strength [18][19][20]. To further analyse the effect of second-phase particles in HAZ during welding process, the evolution of carbides and nitrides in Ce-free steel was evaluated using the Thermo-Calc software, as shown in Figure 6.…”

Hot Deformation Behavior of V–Ti Microalloy Steels

“…

Microalloy steel has become a multipurpose steel due to its high strength, high toughness, and excellent welding performance. This type of steel is designed with a low-carbon composition, and the three microalloying elements of Nb, V, and Ti are added together to strengthen the effect, [1] but there are few reports on the research of V-Ti microalloyed steel. V-Ti compound addition can greatly reduce the cost and facilitate the development and application of large precipitation strengthened high-strength steel.

…”