Research On Phase Transformation Behaviours And Corrosion Resistance Of High-Strength Low-Alloy Pipeline Steels

Pipelines have been mainly used to transport oil and gas for more thanhalf a century for their economic efficiency, safety, high transportationvolume, and uninterrupted transportation. With the rapid increase of thedemand for oil/gas, the research of pipeline steels have gainedunprecedentedly tremendous attention. As the exploitation of fossil fuelresources have been shifted to adverse environment, the properties ofpipeline steels need to be considerably improved. For the purpose ofacquiring pipeline steels with overall quality, the technologicalparameters should be strictly controlled to achieve ideal microstructure.Microstructure of pipeline steels acquired after intermediate phasetransformation is quite complicated, especially acicular ferrite, whichhas very favorable properties. However, the microstructure controllingparameters and phase transformation mechanism of acicular ferrite remainunclear. In addition, low pH and high-sulfur oil fields require thedevelopment of pipeline steels that are resistant to corrosion inhigh-sulfur environment.With respect to the situation mentioned above, the present studiesfocused on a type of high strength low alloy pipeline steel. Phasetransformation in the intermediate temperature range during continuouscooling, isothermal quenching, controlled rolling and corrosionresistance properties of the steel was investigated by using ThermalSimulator and Transformation Measuring Apparatus. The microstructuresafter continuous cooling or isothermal quenching were studied. The phasetransformation process was described by transformation kinetics. Theeffect of rolling in the austenite non-recrystallization range on thephase transformation of acicular ferrite was explored. Besides, means toimprove corrosion resistance were proposed. Several conclusions wereachieved as follows. (1) Cooling rate has significant influence on the phase transformationduring continuous cooling. As the cooling rate increased, the acquiredroom temperature microstructure changed from a mixture of polygonalferrite, acicular ferrite, and bainite to the dual phase of acicularferrite and bainite. By building JMAK kinetics model on the phasetransformation in the continuous cooling, it was demonstrated thatdiffusion-controlled growth phase transformation precededinterface-controlled growth mechanism. Austenization temperature alsoaffected the phase transformation during the continuous coolingprocedures. As the austenization temperature was elevated, significantgrowth of the crystal grains was observed, and the acquired roomtemperature microstructure shifted from a mixture of polygonal ferrite,acicular ferrite, and bainitic ferrite to the single phase of bainite.(2) Isothermal quenching process of the steel was systematicallystudied. It was found that the isothermal temperature as well as theaustenization temperature had a great effect on the phase transformationbehavior. The results showed that as the isothermal temperature decreased,the acquired room temperature microstructure gradually changed frompolygonal ferrite to acicular ferrite, and eventually to bainitic ferrite,resulting in an increase in the hardness of the microstructure. Using JMAK,autocatalysis, and shear-dominated phase transformation model todescribe the first and second stage of isothermal quenching process, itwas showed that the phase transformation rate of polygonal ferrite, whichwas governed by diffusion-controlled mechanism, gradually decreasedwhile the phase transformation rate of bainite increased, with thedeclining of isothermal temperature. With the increasing of austenizationtemperature, the acquired room temperature microstructure was coarsened.At the same temperature, slower isothermal transformation rate was morefavorable for the formation of bainitic ferrite, thus enlarging the biniteregion in the TTT curve.(3) Rolling in the Austenitic non-recrystallization zone hadimportant influence on the phase transformation of acicular ferrite. Theresults showed that after rolling at850oC to various deformation amounts,the achieved microstructure changed in the following order as the deformation amount increased: mixture of bainitic ferrite and acicularferrite, acicular ferrite dominated structure, mixture of acicularferrite and polygonal ferrite, and refined polygonal ferrite. As the rateof cooling after rolling increased, the microstructure became refined.With the increasing of rolling temperature, the achieved microstructurebecame coarsened, leading to a decrease of hardness. As the strain rateincreased, the microstructure was refined and thus the hardness increasedgradually; increasing the strain rate appropriately was beneficial to therefinement of microstructure.(4) The precise control of C content for the sulfur-resistantpipeline steels was realized by optimizing micro-alloying design, i.e.reducing C content and increasing Mn, and by adopting advancedmanufacturing technology. S content was significantly reduced by usinghigh-class steel craps and refining slag with proper composition. Throughstrict controlling of C, S, P and inclusions, hydrogen damage was highlyreduced at the source. A novel technology featured with Premier QualityFinishing (PQF) rolling and screw-expansion to manufacturelarge-diameter pipelines was proposed, which had the advantages of thePQF stable rolling and ensured quality of both the inner and outer surface.(5) Effect of heat treatment on pipe s size variation wasinvestigated. Outer diameter was controlled as per negative tolerance(-0.6mm～-1mm) while wall thickness according to positive tolerance(0.1mm～0.3mm), and thus dimensional accuracy of large diameterpipelines was further improved. External water dripping and internalinjection were combined in the heat treatment of pipes to ensure thehomogeneity of microstructure and properties of steels, as well as properhardness. This technique reduced the adverse effect of heat treatment onthe corrosion resistance of steels. Special treatment was applied to theinner surface of the pipes to further improve the corrosion resistance.In the manufacturing of large-diameter pipes for Puguang Gas Field,Tianjin Pipe Corporation made a number of technological innovations andapplied precise control to the whole manufacturing process. Therefore,dimensional accuracy of the products has reached the requirements ofrelevant standards, and the hydrogen-induced cracks and hydrogen blisters were largely restrained. The resistance to hydrogen damage of the finalproducts was considerably improved. The success indicated that fulllocalization of the manufacturing of large-diameter pipes used inhigh-sulfur corrosive environment has been realized.