Precipitates in X70 pipeline steel

The precipitates in X70 acicular ferrite pipeline steel were investigated by transmission electron microscope. There are two typical precipitates in X70 pipeline steel. One is cubic shaped compound precipitates Ti(Nb)NC with the size of 50 nm-1 μm, in which TiN plays the main role and the ratio of Ti/Nb reaches 5-12. The other is spherical shaped Nb(Ti)C with the size less than 20 nm, in which NbC is the main phase with the ratio of Nb/Ti within 1-6.37. The spherical shaped precipitates exist in multi-grains proved by electron diffraction. The cubic shaped precipitates formed above 1150°C, and varied scarcely during the process afterwards. The spherical (ellipsoidal) shaped precipitates emerged almost totally in the range of 900°C to 1100°C, and kept coherent or semi-coherent relationship with the matrix. The precipitate of V was not obviously observed reminding the weak effect of V in steel. The results from the high-temperature ductility and strength curves show that the precipitation started at 1050°C and the amount of the precipitates reach the highest within the temperature range of 900°C-50°C in the condition of no deformation.