Influence Of Microstructure And Sulfer Content Of Pipeline Steels On Mechanical And H₂S-resistent Property

The demands for pipeline steels are being increased dramatically. It is necessary to develop high pressure resistant, long life and corrosion resistant pipeline steel. In order to obtain mechanical, H2S-resistant and some other excellent comprehensive properties, the ideal microstructure of pipeline steel should be acicular ferrite. Recently, most of the research and development of pipeline steels focus on component design and optimized Thermo-Mechanical Control Process (TMCP). However, the reports about the effects of detrimental element and inclusion shape on pipeline steels are very few, and the researchers held different views. It is necessary to research the effects of the detrimental element content and the inclusion shape on the mechanical property and H2S-resistant of pipeline steels.In the present work, the commercialized pipeline steels with different sulfur contents were adapted, and the transformation behaviors of super-cooling austenite in these steels were investigated by measuring continuous cooling transformation diagram (CCT). The results showed that acicular ferrite could be obtained under a cooling rate range of 7.5℃/s~20℃/s. Based on the above results, the TMCP schedule was applied to hot rolling process for the tested steels. The influences of compositions and microstructures on mechanical properties were discussed. The results indicated that pipeline steels with addition of micro-alloyed elements (Ni, V, Ti, Mo) are inclined to obtain acicular ferrite microstructure with high density dislocations, exhibiting excellent strength and toughness. Without micro-alloyed elements the microstructure are inclined to be polygonal ferrite plus pearlite. Especially, it is easy to form elongated MnS inclusions along the rolling direction once the sulfur content in pipeline steels reaches 100 ppm, which deteriorates the impact toughness.Finally, the influences of microstructures and sulfur content on the H2S-resistant behavior were analyzed. When the sulfur content in pipeline steels is 50ppm, the H2S-resistanit standard of pipeline steels can be satisfied. The pipeline steels of acicular ferrite with high density dislocation have poor sulfide stress corrosion cracking (SSC) resistance. However, the pipeline steels of polygonal ferrite+pearlite with low density dislocation have good SSC resistance.