Research On Microstructure Control And Refinement Technology For X70 Pipeline Steel

Long pipeline is the most economical, convenient and primary way for transporting oil and natural gas. With the resource structure adjustment, the demands of the oil and natural gas have increased drastically. Therefore, the technique and technology of producing pipeline steel have been developed rapidly. To pipeline steels for long distance oil and gas transportation, the excellent integration properties microstructure is the main objective in the process of morden R & D and production.Continouse cooling transformation law, influences of controlled rolling and controlled cooling parametres on microstructure of X70 pipeline steel of Tangsteel were researched by using thermal simulator, metalloscope, canning electron microscopy and transmission electron microscopy and so on, and the microstructure and properties were also analyzed in the paper. The main conclusions are as follows.Hot deformation accelerates the acicular ferrite phase transformation and polygonal ferrite phase transformation, heightens the starting transformation temperature and the finishing transformation temperature significantly, and shifts the CCT diagram moving upward to the left side corner. Acicular ferrite can be obtained in wider range of cooling rates, and the microstructure is finer under hot deformation compared with the conditions without hot deformation.The influence of rolling and cooling parameters on microstructure and refinement of X70 pipeline steel has been studied by using thermal simulation test. The results show that the best acicular ferrite can be obtained by accurate control of begaining finished temperature, finished rolling temperature, cooling speed after rolling and final cooling temperature. With the decreasing of cooling speed after rolling and final cooling temperature, the volume fraction of acicular ferrite in the microstructure increases obviously.According to the processing technology of controlling and refining microstructure for acicular ferrite pipeline steels, X70 pipeline steel had been rolled by trial in 1700 production line in Tang Steel. Fine, homogeneous mixed structure with the acicular ferrite as the main structure was obtained. The inclusions in steel are mainly sphericity, subspheticity of silicates and sulfuret, and the size of most inclusions is smaller than 5μm. X70 pipeline achieved excellent comprehensive mechanical properties:σbis 645～685MPa,σsis 565～585MPa, A5 is 21.5%～25%, the impact toughness values are all very high above-60℃.