The Study Of Microstructure And Mechnical Properties For Pipeline-Steel X65

The demands for pipeline steels are being increased dramatically with the national economy development. In the fierce market competition, driven by the cost considerations, gas&oil transportation companies have readily adopted higher strength pipeline steels with thicker gauges to permit gas&oil transportation at higher pressures. As a result, recent construction of pipelines has shown a trend towards higher operation pressure with greater wall thickness and higher strength pipe. At the same time, acicular ferrite pipeline steels are becoming accepted as the prevalence for the new construction because of their high strength, excellent toughness, good weldability and satisfied hydrogen sulfide resistance.This thesis, applied controlled rolling and controlled cooling technology on the production of micro-alloyed pipeline steel X65 which was provided by Anshan Iron and Steel Group Corporation, by means of laboratory simulation test. Then we did research on the characteristics of the change of microstructures and properties at various deformation conditions, and clarified the relations between microstructures and mechanical properties of X65 and the strengthening mechanisms of acicular ferrite pipeline steels. The experimental aim was to offer the important theoretical foundation and technological basis for the improvement on microstructures and the enhancement on mechanical properties of acicular ferrite pipeline steels, which would be brought into effect through the technology of TMCP in the industrial manufacture. The main contents are given as follows:1. According to the process system designed by ourselves, the practical controlled-rolling and controlled-cooling tests were performed by using the rolling mill in the laboratory. And the improved TMCP schedules containing a two-stage multi-pass controlled rolling coupled with moderate cooling rates were applied to the hot rolling experiments.2. The metallographic specimens of X65 pipeline steel were prepared after the process of TMCP. The microstructures of the specimens were examined using optical microscope (OM). And the effects of different technological parameters of controlled rolling and controlled cooling on the microstructures of X65 pipeline steel were completely studied. 3. The metallic film specimens of X65 pipeline steel were prepared after the process of TMCP. The microstructures of the specimens were analyzed using transmission electron microscope (TEM). And the strengthening mechanisms of acicular ferrite pipeline steels were investigated comprehensively.4. The mechanical properties of X65 pipeline steel were tested after the process of TMCP. And the actions of different technological parameters of controlled rolling and controlled cooling on the mechanical properties of X65 pipeline steel were researched on exhaustively.5. Comprehensive analyzing all the experimental results, the effects of technological parameters of controlled rolling and controlled cooling on the microstructures and mechanical properties of X65 pipeline steel were generalized, and the relations between microstructures and mechanical properties of X65 pipeline steel were clarified thoroughly. Thus, the flexible rolling of pipeline steels would be obtained only by optimizing the process of TMCP, so as to achieve the improvement on microstructures and the enhancement on mechanical properties of pipeline steels, without changing their existing chemical compositions.