Study On Microstructure & Property Of Pipeline Steel With Excellent Deformability Based On The Method Of Accelerated Cooling In The Critical Region

In this paper, material microstructure analysis, mechanical property testing and welding thermal simulation techniques were applied to established the relationship of excellent deformability pipeline of technology-organization-performance applied complicated geological conditions. The purpose of this study is to make ordinary steel on the basis of maintaining the previous original strength have the anti-the performance of excellent deformability, through the heat treatment process of the existing ordinary pipeline accelerated cooling in the critical region.The results shows that the three experimental steel X70, X80 and X100 by test methods of accelerated cooling in the critical can access to (B+F) dual phase structure; duplex steel in tension state have the continuous curve of stress-strain; When the heating temperature is 840℃, the test steel have high strength and plasticity, high strain hardening index and low yield ratio; the microstructure of the test steel is mainly made up of small bainite, supplemented small ferrite of a small amount and high density of dislocations. Therefore, material can access to higher mechanical properties and of the performance of excellent deformability.Through welding thermal simulation experiments of the two-phase pipeline shows that different heat input conditions and the different second thermal cycle peak temperature conditions, CGHAZ toughness of the Duplex X80 steel and general X80 steel have the same variation; with the increase of heat input, the strength of duplex X80 steel CGHAZ. reduces. In order to obtain higher tough level of duplex X80 steel CGHAZ, heat input should be controlled at 30kJ/cm the following; in the second thermal cycle process, the formation of carbon-rich MA-element of high contents, large size is the major cause of welding critical coarse grained region appears local brittleness.The studies to two-phase steel by different pre-strain has shown that for ordinary X80 and dual-phase X80, pre-strain make the strength and the yield ratio of the test steel is higher than the level of base metal, plastic and toughness is lower than the level of the base mctal; after strain, dislocation density of the test steel promotes,causes material strength increased, ductility and toughness decreased; compares with the normal X80, the trend of duplex X80 strength increasing, ductility and.toughness decreasing is slightly.The research by strain aging of duplex steel indicates that strain aging makes the strength and the yield ratio of test steel higher than the level of base metal, plastic and toughness lower than the level of the base metal. With the strain aging temperature increasing, strength and hardness increases, plasticity and toughness decreases, yield ratio has the ascending trend, hardening index decreases. Compares with traditional X80 steel, Duplex X80 steel exists less tendency of strain aging.