PROPERTIES AND MICROSTRUCTURE OF THIRD GENERATION X90 PIPELINE STEEL

The mechanical properties and microstructure of the TO (third generation) X90 pipeline steels which have been industrially trial-produced were investigated. The results showed that the microstructure of the 18.4 mm thick X90 pipeline steels consist of quasi-polygonal ferrite, lath bainite and M/A (martenite/austenite) island. The yield strength was between 615 and 660 MPa, and the tensile strength was above 720 MPa, and the yield ratio was below 0.9, the impact absorbed energy at -30 degrees C was over 200 J, and the shearing area of drop-weight tear test (DWTT) at -15 degrees C was more than 80%. The ratio of the soft phase (quasi-polygonal ferrite) and the hard phase (lath bainite) was about 3 : 2, and the dislocation density in the lath bainite was higher, while that in the quasi-polygonal ferrite was lower. The EBSD results showed that the lath of the hard phase met the shear transformation characteristics, indicating that the effect of microstructure control on hard and soft phase has been achieved.