| With the development of oil industry, submarine oil exploitation, which originally located at shallow-sea shelf, has extended to the deep ocean. Requirements, not only high strength, but also high toughness, lamellar tearing resistant property and low costs, have been proposed for offshore platform steels. In present paper, microstructure evolution and precipitation behavior of low-cost offshore platform plate produced with TMCP technology were investigated. Industrial trials were conducted and results showed that each index of mechanical properties has satisfied the requirements. The whole research work was supported by the project "Development of offshore platform plate", and state863project "Key technology and theory of the production of high strength, thick offshore platform plate". Chief original work is as follows:(1)Effects of deformation temperature, strain and strain rate on austenite dynamic recrystalization behavior and microstructure evolution were investigated. Result showed that low strain rate and high deformation temperature were advantageous for austenite dynamic recrystalization. Activation energy of dynamic recrystalization and mathematical model for deformation resistance were obtained. The static recrystalization behavior for C-Mn steel, Nb-Ti steel, Nb-V-Ti steel and high Nb steel was studied by double-hit test. Flatform appeared on the recrystalization curve of microalloyed steel due to the induced precipitate. Activation energy of static recrystalization was determined and the static recrystalization kinetic model was established.(2)Thermal expansion method was employed to determine the CCT curves and transformation behavior with continuous cooling for C-Mn steel, Nb-Ti steel, Nb-V-Ti steel and high Nb steel. Results showed that the ferrite transformation has been suppressed by grain boundary segregation of Nb, polygonal ferrite fraction of Nb-Ti steel was lower than that of C-Mn steel under the same cooling rate and deformation condition. When cooling rate was below5℃/s, polygonal ferrite transformation was promoted and acicular ferrite was suppressed by austenite deformation for C-Mn steel, Nb-Ti steel and Nb-V-Ti steel. However, granular bainite transformation nucleated inside the grain was promoted and bainite transformation nucleated on grain boundary was suppressed for high Nb steel. When the high cooling rate was applied, upper bainite transformation was suppressed and acicular ferrite was promoted.(3)The precipitation behavior in different stages of TMCP was studied. The effects of cooling route and temper process on precipitation for C-Mn steel、 Nb-Ti steel、 Nb-V-Ti steel and high Nb steel was revealed. It was found that the undissolved cuboid precipitates after reheating process remained unchanged during rolling process. The strain-induced precipitates obtained a few seconds after rough rolling were found to be spherical shaped and austenite recrystalization during finish rolling was suppressed. Large quantity of precipitates appeared after laminar cooling, which was favorable for precipitation strengthening. Compared with the slow cooling mode, the precipitates obtained by two-step cooling mode in which first fast and then slow cooling rate were applied increased, sparsely distributed and of advantage for precipitation strengthening.(4) Chemical composition and TMCP processing parameters of low-cost offshore platform plate E36-Z35and E40-Z35were determined by laboratory hot rolling experiments. Industrial trials were conducted in Shou Qin field. Measured mechanical properties showed that favorable low temperature toughness, jointing property and lamellar tearing resistant property have been obtained for offshore platform plate. And it has been admitted by nine countries ship classification society.(5) Effects of Ni content and cooling mode on the microstructure and mechanical property were studied by laboratory hot rolling experiments. Industrial trials of offshore platform F460-Z35were conducted. Results showed that with the increase of Ni content and the transition of cooling mode from laminar cooling to ultra fast cooling, precipitation behavior has been transferred from interphase precipitation to dislocation precipitation and uniform precipitation and the strength has been increased.The effect of Ni content on low temperature toughness was related to cooling mode. Fine M/A was favorable for low temperature toughness. Offshore platform plate had excellent toughness and anti-lamellar tearing property. |
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