Study On Microstructures And Properties Of High Strength And High Toughness Offshore Platform Steel

High strength, high toughness, fatigue resistance, corrosion resistance, lamellar tearing resistance and welding performance are all needed to produce heavy gauge and high strength platform steel.In present paper, low carbon and adding such alloying elements as Nb, V, Ti and Ni is the main route of the component designing of test steels. Effects of finished cooling temperature, cooling methods, cooling routes and content of Ni on the microstructures and properties of test steels were investigated, and heat treatment after rolling is also studied. The conclusion that the low-temperature flexibility of test steel can be proved obviously and at the same time the strength is not reduce rapidly while using technology of TMCP+L+T is presented. Chief works as follows:(1) The continuous cooling transformation (CCT) curve after no deformation and25%deformation of austenite at900℃is determined by MMS-300thermal simulation machine. Results show when there is no deformation the bainite can be obtained in the cooling rate range of0.5℃/s～30℃/s and the martensite will appear in microstructures while the cooling rate is more than10℃/s. But after25%deformation of austenite at900℃the ferrite phase will appear in microstructures while the cooling rate is not more than2℃/s and the bainite can also be obtained in the cooling rate range of0.5℃/s～30℃/s. Therefore, making deformation at900℃can accelerate ferrite phase transformation and bainite transformation will be rejected at the same time.(2) Effects of control rolling and control cooling technologies on microstructures and properties of test steels according to hot rolling test for two times in the laboratory are studied. Results show that there have a best content of nickel exist in the test steels, when the percent of nickel is in the range of0.53%-0.8%, M/A islands are small and disperse in the steels, therefore, the toughness of test steels is best at this time.(3) Effect of heat treatment on microstructures and properties is studied according to TMCP+T and TMCP+L+T technology routes. It was found that the two treatments can improve toughness of test steels obviously while the yield strength is more than500MPa and the tensile strength is more than650MPa, the impact energy can reach more than150J at-60℃and-80℃test temperature. The microstructure after heat treatment is made up of polygonal ferrite and tempered martensite or tempered bainite. A stronger precipitate hardening was observed, and the precipitate, which was responsible for hardening and increasing strength of test steel, was identified to be (Nb,Ti)(C,N).