Study On Microstructure Control And Performance Of X70 Grade Anti - Sulfide Corrosion Pipeline Steel

At present, X70 grade corrosion-resistant pipeline steel are the highest level pipeline steel used in the domestic engineering field.On account of the demand of X70 grade anti-H2S corrosion pipeline steel in Southwest, through the research of the effect of TMCP rolling process and the coiling temperature on microstructure evolution and properties and the factors of hydrogen induced cracking produced in the pipeline steel,this paper provide theoretical guidance for the industrial production of anti HIC-performance in X70 grade pipeline.The research object of this paper are mainly two kinds steel which have different Nb content.The microstructure evolution of X70 anti-H2S corrosion pipeline steel contain 0.1% Nb in TMCP process was studied. The results reveal that, with thickness specifications decreasing, the microstructure grain refined, strength increased.Reduceing the plate thickness also helps the formation of banded structure. The polygonal ferrite were got when final rolling temperature was 820℃ with deformation induced ferrite transformation. The results of thermal simulation experiment showed that polygonal ferrite transformation occurred in y+a two-phase region, and polygonal ferrite nucleated and then grew up in 450℃ simulated coiling process; Carbon enriched bainite was mainly composed of carbide precipitates and lath bainite gained by phase transformation in 450℃ simulated coiling process.The effect of coiling temperature on microstructure,toughness and the second phase precipitation in X70 grade anti-H2S corrosion pipeline steel has been systematically studied. Studies have shown that, after finish rolling at 820℃,with the coiling temperature increasing,the tensile strength and yield strength increased, but the low-temperature impact toughness decreased in 0.1% Nb steel, while the 0.05%Nb steel strength have little change, low-temperature toughness decline. At different coiling temperature The second phase precipitation are (Nb, Mo)C particles, mainly random precipitate on dislocations, interphase precipitation are observed in partial region. With the coiling temperature dropping, the precipitation rate decreased, the amount of precipitation reduced, the average particle size increased.The anti-HIC properties was studied according to the GB 8650-2006 in X70 grade pipeline steel which have good resistance to H2S corrosion. The results show that,0.1% Nb test steel and 0.05%Nb test steel have good corrosion resistance, in addition, the crack sensitive rate, crack length rate and crack width rate were within the control range. Martensite/Austenite island, alumina (Al2O3) and AlSiCa composite inclusions were the main factors leading to HIC cracks.