低活化钢在高温高压和高能粒子流的多重作用下产生的高温蠕变和辐照损伤等缺陷，限制了其在可控核聚变领域的应用。低活化钢晶粒尺度和强化相尺寸及分布对其强韧性、高温蠕变性和辐照性能有重要影响。大塑性变形能够大大细化晶粒、有效改善强化相尺寸及分布，是提高低活化钢综合性能进而延长其使用寿命的有效手段。本研究将传统的往复挤压和挤扭工艺有机结合，开发出一种工艺塑性好、变形量大的闭塞式变截面往复挤-扭-镦工艺，实现工艺的最优化以制备高性能的大尺寸块体低活化钢。将理论分析与实验研究相结合，深入分析在大变形和热处理过程中低活化钢的晶粒细化、相转变、强化相回溶与析出规律，揭示主要工艺因素对低活化钢性能的影响机理，并通过位错、晶界与强化相的相互作用阐明低活化钢微观组织对其强韧性、高温蠕变和辐照性能的影响机理。这对改善低活化钢的综合性能，延长其使用寿命，进而促进低活化钢在核聚变领域的应用具有重要的理论意义和科研价值。

High temperature creep and irradiation damage caused by the high temperature, high pressure and energetic particle flux in the reduced activation ferritic/martensitic (RAFM) steels restrict their applications in the field of nuclear fusion. The grain size and strengthening phases of RAFM steels have important influence on strength and toughness, high temperature creep and radiation resistance. Severe plastic deformation (SPD) can significantly refine grains and redistribute strength phases, which is an effective way to improve the comprehensive performance of RAFM steels and prolong service life. In this study, a new type of blocking varietal cross section cyclic twist-extrusion-upsetting process is developed, which is based on cyclic extrusion compression (CEC) and twist extrusion (TE).The process has advantages of good process plasticity and large deformation, which is benefit to optimize the deformation process and fabricate large size bulk RAFM steels with high performance. At the same time, the theoretical analysis and experimental research are combined to analyze the mechanism of grain refinement, phase transition, dissolution and precipitation of strengthening phase during the processing of severe plastic deformation and heat treatment, and the influence of main process factors on the properties of RAFM steels will be investigated. In addition, the influence of microstructures on the strength and toughness, high temperature creep and radiation resistance will be discussed through the grain boundary evolution and the interaction between strengthening phases and dislocations in the processed RAFM steels. It has important theoretical significance and scientific research value for improving the comprehensive performance of RAFM steels, prolong service life, and promote the application of RAFM steels in the field of nuclear fusion.